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net-craft · 2 years ago

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12 Top Kotlin Features to Enhance Android App Development Process

Kotlin is a modern and concise programming language that is designed to run on the Java Virtual Machine (JVM). Kotlin is officially supported by Google as a first-class language for Android app development since 2017. Kotlin has many features that make it a great choice for Android app development, such as interoperability with Java, null safety, coroutines, extension functions, data classes, and more. In this article, we will explore 12 top Kotlin features that can enhance your Android app development process and make your code more readable, maintainable, and efficient.

1. Interoperability with Java: Kotlin is fully interoperable with Java, which means you can use existing Java libraries, frameworks, and tools in your Kotlin code without any hassle. You can also mix Kotlin and Java code in the same project and call Kotlin code from Java and vice versa. This makes it easy to migrate your existing Java code to Kotlin gradually or use Kotlin for new features in your existing Java project.

2. Null Safety: Kotlin has a built-in null safety feature that prevents null pointer exceptions, which are one of the most common causes of crashes in Android apps. Kotlin distinguishes between nullable and non-null types and enforces you to check for null values before using them. You can also use the safe call operator (?.) or the Elvis operator (?:) to handle null values gracefully.

3. Coroutines: Coroutines are a way of writing asynchronous and non-blocking code in Kotlin. Coroutines allow you to perform multiple tasks concurrently without blocking the main thread, which improves the responsiveness and performance of your app. You can use coroutines to handle network requests, database operations, background tasks, etc. Coroutines are also easy to write and read, as they use a sequential and suspending style of coding.

4. Extension Functions: Extension functions are a way of adding new functionality to existing classes without modifying them or creating subclasses. You can define extension functions for any class, even if you don’t have access to its source code. Extension functions are useful for adding utility methods or customizing the behavior of existing classes.

5. Data Classes: Data classes are a way of creating classes that mainly hold data and provide some basic functionality, such as getters, setters, equals, hashCode, toString, etc. You can create data classes by adding the keyword data before the class declaration. Data classes are useful for representing model objects or data transfer objects in your app.

6. Destructuring Declarations: Destructuring declarations are a way of unpacking multiple values from a single object or collection into separate variables. You can use destructuring declarations to simplify your code and make it more readable. For example, you can use destructuring declarations to assign the properties of a data class or the elements of a list or a pair to separate variables.

Continue reading 12 Top Kotlin Features to Enhance Android App Development Process

#Kotlin Features #scottsdale #arizona #phoenix #app development company #mobile app development company #app development #android app development

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samanthablake02 · 3 months ago

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Elevate Your Craft: Mastering Kotlin Programming in 2025

Did you know that companies adopting Kotlin report up to a 30% reduction in codebase size and significant gains in developer productivity?

Are you ready to benefit? Whether you are just entering the world of coding, or a veteran trying to evolve your skill set, Kotlin could be exactly what you have been looking for—especially if you aim to work in an Android app development company that prioritizes efficiency and modern programming solutions.

This blog post offers a robust guide that focuses on optimizing Kotlin programming. It highlights core concepts, avoiding typical mistakes, tools of trade and guidance from field leaders. Let us help you confidently maneuver the landscape in 2025.

Grasping Essential Kotlin Concepts

Comprehending the building blocks is key to mastering Kotlin programming. Variables, data classes and null safety features represent just a few essential pillars you must possess. Without proper understanding of this essential syntax and features, you will create a shaky foundation with inefficient work flow.

Variables: Unlike other popular coding languages such as Java, Kotlin features 'val' and 'var.' Val marks an immutable, read-only variable, and 'var' marks a mutable variable whose content may be changed at will. Properly grasping these fundamental qualities contributes to writing stable, well structured, maintainable code.

Data Classes: Kotlin excels through its concept of data classes; structured around the idea of concisely carrying data. The compiler, for you, creates functions that provide 'equals', 'hashCode', 'toString' making boiler-plate less intense. The result simplifies object modeling duties considerably.

Null Safety: One prevalent problem in development appears as "NullPointerExceptions." But fear not! Kotlin aims for resolution: at the language level the nulls get handled directly to increase application reliability! You might also benefit from its nullable type declaration; type marking might have a potential lack.

Navigating Common Pitfalls

Everyone makes mistakes during their work no matter if it's a job or school work, here are the potential problems that might come up when using Kotlin. Even with a modern programming language as elegant as Kotlin, it’s easy to stumble. Being aware of common pitfalls enables more effective problem solving capabilities along your coding adventures.

1. Overusing `!!` Operator: The not-null assertion operator (`!!`) forcibly unwraps a nullable type; use only when you are absolutely positive that your variable must hold value. A crash event results for you the programmer for sure, anytime a nullable expression occurs unexpectedly during process running as opposed by when you're testing, such exceptions turn against user interface performance issues from happening!

2. Neglecting Extension Functions: Many miss Kotlin's extensibility feature as something special beyond their immediate object; extension function benefits help streamline legacy classes using add-ons; avoiding redundancy can promote efficient code arrangement easily managed between users.

3. Ignoring Code Routines: Kotlin harnesses asynchronous concurrency without deeply entangled callback patterns utilizing light threaded concurrency that results effectively within high loads handled gently; avoid old, slow threads entirely utilizing code routine capabilities whenever your use scenario requires asynchronous activity processing patterns like when dealing user response in background, thereby sustaining system application interface reactivity despite simultaneous loads by processes or tasks!

Building a Kotlin Class Method: A Step-by-Step Guide

Creating a new class method (or function) in Kotlin is foundational to structuring your code. Consider this example involving an `Account` class and a `deposit` method. Step 1: Defining the Class First, define the `Account` class with any necessary properties: ```kotlin class Account(var balance: Double) { // Class content goes here } ``` Step 2: Creating the `deposit` Method Next, create the `deposit` method within the `Account` class. This method will take an `amount` parameter of type `Double` and add it to the account's balance. ```kotlin class Account(var balance: Double) { fun deposit(amount: Double) { balance += amount println("Deposited: $amount. New balance: $balance") } } ``` Step 3: Using the `deposit` Method Now, create an instance of the `Account` class and call the `deposit` method: ```kotlin fun main() { val myAccount = Account(100.0) myAccount.deposit(50.0) } ``` Output: ``` Deposited: 50.0. New balance: 150.0 ``` Explanation 1. Class Definition: `class Account(var balance: Double)` declares a class named `Account` with a constructor that takes an initial `balance` as a `Double`. 2. Method Definition: `fun deposit(amount: Double) { ... }` defines a method named `deposit` within the `Account` class. This method takes a `Double` parameter `amount`. 3. Logic Inside Method: `balance += amount` adds the `amount` to the `balance`. The line `println(...)` then prints a confirmation message with the new balance. 4. Usage Example: In the `main` function, we create an instance of `Account` and call the `deposit` method on that instance.

Learn More About Full-stack developers.

Guidance and Insights From Leading Kotlin Professionals

Learning comes through diverse resources; experience through guidance benefits by Kotlin master programmers or experts from prominent businesses! Take note through perspectives described henceforth that empower abilities or approaches; become the next programming rockstar!

Maintainability: Industry recognized developers always place value on code clarity through concise readability through self documentation via naming conventions. Keep your class clear for efficient future modification or change handling within projects managed successfully.

Testing: Consider thoroughly covering software via tests constantly - specifically automated test scripts using complex logics from different use contexts; it prevents the problems! Use edge context considerations always during unit script drafting too!

Upgrading: Regularly checking Kotlin's latest updates with their distinct novel syntax abilities will benefit via optimal integration; also regularly evaluating updated integration or build assistance enhances tool functionality.

Elevating Code Quality via Pragmatic Tips

Refine the abilities and style that will facilitate exceptional Kotlin code creation from conceptual stage until post production software runs well during its complete software cycle - using techniques provided within tips which enable better outputs than average code outcomes at each step

Adopt functions using scopes from enhanced organization throughout projects – make use the `apply`, `let`, `run`, etc for minimizing unnecessary object initialization setups to sustain a clear logic around operations across files inside programs that help team programmers or individual persons managing complex coding solutions over time without experiencing chaos across functions related inside your main algorithm flow which sustains neat function blocks facilitating collaborative improvements along software updates at scheduled production targets over lifespan after release version deployment for user access within its software functionality specifications during end implementation across teams from inception until users install and integrate through program release.

Employ `Sealed Classes`: Sealed classes, which fall under advanced category functions that define strict sets; they represent limited hierarchies through value or code constraints using inheritance limitations, with predefined instances which enhance data processing predictability avoiding future cases due potential exceptions; enabling stronger algorithm constructions and better overall fault handing from conception via deployment; thus sealed structures promote both cleaner system architectures or fault prediction with ease than open ended object sets – for better solutions created for better security on run without risks due external anomalies from unanticipated issues that need remediation along project maintenance after rollout when running.

Code Example Showing Scope Functions for Refined Data Processing

```kotlin data class Person(var name: String, var age: Int? = null) fun processPerson(person: Person?): String { return person?.let { // Use let to safely operate on a non-null Person it.age?.let { age -> // Nested let to safely operate on age if it's not null "Name: ${it.name}, Age: $age" } ?: "Name: ${it.name}, Age: Not Available" // Handle case where age is null } ?: "Person is null" // Handle case where person is null } fun main() { val person1 = Person("Alice", 30) val person2 = Person("Bob") val person3: Person? = null println(processPerson(person1)) // Output: Name: Alice, Age: 30 println(processPerson(person2)) // Output: Name: Bob, Age: Not Available println(processPerson(person3)) // Output: Person is null } ```

Benefits of using scoped function let here

By carefully nesting them in such ways where potential vulnerabilities can be effectively removed which can lead less chances runtime program breaking problems;

Frequently Asked Questions

Answering queries frequently encountered, clarifies misconceptions alongside enabling broader absorptions. These answer key concepts for learners as the progress on journey using Kotlin programming.

Is it complicated to grasp this modern language?

If you're acquainted using OOP and another coding such as java or c ++ using Kotlin does provide easy seamless transition experience; new comers usually take time adopting its distinct functions versus more well known syntax abilities such as C/ Java style syntaxes or procedural script structures though the learning pace picks rapidly across stages!

Does this Kotlin programming language support code interoperability?

Certainly it works interoperatively from JVM with complete backward similarity between each class available that empowers seamless implementation between all projects existing throughout application environments especially leveraging through present-day architectures already in execution during coding shifts!

How does it help mobile-app design?

Primarily Kotlin plays dominant during development via OS through benefits by easier coding rules syntax combined via reduced code resulting fast processing and therefore enhancing productivity! Better overall features than before too!

Can software created work well enterprise structures at businesses?

Totally Kotlin becomes greatly preferable in scalable programs, applications requiring constant upkeep like e commerce services at massive enterprises needing seamless integrations across wide structures plus reliable runtime operations ensuring customer success due by advanced type protections & concurrency qualities provided!

What resources enable expertise building at beginner programmers using Kotlin language design features from basic starting point onwards?

Web tutorial sessions or dedicated platforms using instructional guidelines in books online training series from well reviewed companies facilitate skill enhancements - especially that are coupled by involved personal projects done gradually across different levels! Experiment always as practice helps !

Conclusion

This post covers major foundations relating directly with learning more or getting skills within areas relating toward enhancing usage in current technologies especially pertaining throughout ecosystem related that supports different devices with seamless connection or information retrieval - ultimately, your skills in this sphere shall expand during coding periods leading ahead so prepare via dedicated approaches through regular engagements and exploration toward making innovations relating coding overall – it will benefit eventually due effort sustained over timeline by applying the skill across more projects going to contribute positively to the growth & efficiency associated from programs built either using group member assistance and individually – to build excellence upon programming techniques continuously moving along in all related stages till completion.

#Kotlin programming

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praveennareshit · 3 months ago

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Exploring Record Classes in Java: The Future of Immutable Data Structures

A record in Java is a special type of class designed specifically for holding immutable data. Introduced in Java 14 as a preview feature and made stable in Java 16, records eliminate the need for writing repetitive boilerplate code while still providing all the essential functionalities of a data model.

Key Characteristics of Java Records

Immutable by Default – Once created, the fields of a record cannot be modified.

Automatic Methods – Java automatically generates equals(), hashCode(), and toString() methods.

Compact Syntax – No need for explicit constructors and getters.

Final Fields – Fields inside a record are implicitly final, meaning they cannot be reassigned.

How to Define a Record Class in Java

Defining a record class is straightforward. You simply declare it using the record keyword instead of class.

Example: Creating a Simple Record

java

Using the Record Class

java

Notice how we access fields using methods like name() and age() instead of traditional getter methods (getName() and getAge()).

Comparing Records vs. Traditional Java Classes

Before records, we had to manually write constructors, getters, setters, and toString() methods for simple data structures.

Traditional Java Class (Without Records)

java

This approach requires extra lines of code and can become even more verbose when dealing with multiple fields.

With records, all of this is reduced to just one line:

java

When to Use Records?

Records are ideal for: ✔ DTOs (Data Transfer Objects) ✔ Immutable Data Representations ✔ Returning Multiple Values from a Method ✔ Reducing Boilerplate Code in Simple Models

Customizing Records: Adding Methods and Static Fields

Though records are immutable, you can still add methods and static fields for additional functionality.

Example: Adding a Custom Method

java

Now you can call circle.area() to calculate the area of a circle.

Using Static Fields in Records

java

Limitations of Java Record Classes

While records are powerful, they do have some limitations: ❌ Cannot Extend Other Classes – Records implicitly extend java.lang.Record, so they cannot inherit from any other class. ❌ Immutable Fields – Fields are final, meaning you cannot modify them after initialization. ❌ Not Suitable for Complex Objects – If your object has behavior (methods that modify state), a traditional class is better.

Conclusion: Are Java Record Classes the Future?

Record classes offer a modern, efficient, and elegant way to work with immutable data structures in Java. By removing repetitive boilerplate code, they improve code readability and maintainability.

If you’re working with data-heavy applications, DTOs, or immutable objects, adopting records is a great way to simplify your Java code while ensuring efficiency.

What’s your experience with Java records? Share your thoughts in the comments! 🚀

FAQs

1. Can I modify fields in a Java record?

No, records are immutable, meaning all fields are final and cannot be changed after object creation.

2. Are Java records faster than regular classes?

Performance-wise, records are similar to normal classes but offer better readability and maintainability due to their compact syntax.

3. Can a record extend another class?

No, records cannot extend any other class as they already extend java.lang.Record. However, they can implement interfaces.

4. How are records different from Lombok’s @Data annotation?

While Lombok’s @Data generates similar boilerplate-free code, it requires an external library. Java records, on the other hand, are built into the language.

5. What Java version supports records?

Records were introduced as a preview feature in Java 14 and became a stable feature in Java 16. For more Info : DevOps with Multi Cloud Training in KPHB

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surajkumasblog · 4 months ago

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How to Convert Character to String in Java

In Java, converting a character (char) to a string (String) is a common operation. Java provides multiple ways to achieve this, each suitable for different use cases. This article how to convert character to string in java a character into a string efficiently.

Using Character.toString(char c)

Java provides a built-in method Character.toString(char c) that converts a character into a string.

Example: char ch = 'A'; String str = Character.toString(ch); System.out.println(str); // Output: A This method is simple and recommended for converting a single character to a string.

Using String Concatenation

You can concatenate an empty string ("") with a character to convert it into a string.

Example: char ch = 'B'; String str = "" + ch; System.out.println(str); // Output: B This approach is widely used because of its simplicity and readability.

Using String.valueOf(char c)

The String.valueOf() method is another way to convert a character to a string in Java.

Example: This approach is widely used because of its simplicity and readability.

Using String.valueOf(char c)

The String.valueOf() method is another way to convert a character to a string in Java.

Example: This approach is widely used because of its simplicity and readability.

Using String.valueOf(char c)

The String.valueOf() method is another way to convert a character to a string in Java.

Example:

Using Character Wrapper Class and toString() Method

Java allows using the Character wrapper class with the toString() method to convert a character to a string.

Example:

Character ch = 'D'; String str = ch.toString(); System.out.println(str); // Output: D This method is useful when dealing with Character objects instead of primitive char values.

Using StringBuilder or StringBuffer

If you are dealing with multiple character conversions, using StringBuilder or StringBuffer can be efficient.

Example: char ch = 'E'; StringBuilder sb = new StringBuilder(); sb.append(ch); String str = sb.toString(); System.out.println(str); // Output: E This approach is useful when working with dynamic strings that require multiple modifications.

Conclusion

Converting a character to a string in Java is straightforward and can be achieved using various methods, including:

Character.toString(char c)

String concatenation ("" + char)

String.valueOf(char c)

Character.toString()

StringBuilder or StringBuffer

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codingchica · 2 years ago

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Not For Me, Thanks! Overriding in Java

Need to take a different approach than your parent class when it comes to some method? In Java, a child class can often override the logic from an inherited method. #java #tdd #objectOriented #inheritance

💚 TIP: References Quick List Java: Overriding Methods Example Code Example UML Class Diagram Source Code Table of Contents Table of ContentsIntroductionDiagramUnit Test for Object’s toString() ImplementationTDD CycleUnit TestsRuntime UpdatesMaven BuildCommit Introduction When inheriting method logic from a parent, we may need to alter that logic in the child. When we do so, this is called��

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#Java #ObjectOriented #Overriding #step-by-step #tdd

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kranthicynixit · 5 years ago

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selenium online training | selenium online courses

Apache Ant with Selenium and flash testing overview

When producing a full software product, numerous third-party APIs, their class route, the cleaning of previous binary executable files, the compilation of our source code, the execution of source code, the development of reports and the deployment code base, etc. You need to take care of everything. It would take an immense amount of time if these tasks are performed one by one manually, and the process will be vulnerable to errors. selenium online training

Apache ANT with Selenium

The meaning of a building tool like Ant comes here.

The benefit of creating an Ant

The application life cycle is generated by Ant, i.e. clean, compile, dependency set, run, report, etc.

The third-party API dependency can be set by Ant, i.e. the classpath of another Jar file is set by the Ant build file.

For End to End Delivery and deployment, a full application is developed.

It is a simple build tool where you can use the XML file to make all configurations and which can be executed from the command line.

As the configuration is separate from the actual application logic, it makes the code safe.

How to install Ant for installation

The following steps are taken to mount Ant in Windows

Step 1)

Download the .zip file from apache-ant-1.9.4-bin.zip to http:/ant.apache.org/bindownload.cgi

Step 2)

Unzip the folder and go to the root of the unzipped folder and copy the path.

Step 3)

Go to Start -> Machine -> right-click here and choose 'Properties' and then click Advanced Device Settings

Step 4)

This opens a new window. Click on the 'Factor Environment...' icon.

Step 5)

Click 'New...' and set the name of the variable as 'ANT HOME' and the value of the variable as the root path to the unzipped folder, and then click OK.

Step 6)

Now select the 'Path' variable from the list, and then click Edit' and add;%ANT HOME%\bin.

You can restart the machine once and now you are ready to use the Ant build tool.

Step 7)

Use the command line to verify your version of Ant:

Ant-Version Build.xml comprehension

The most critical component of the Ant compilation tool is Build.xml. For a Java project, all tasks related to cleaning, setup, compilation, and deployment are specified in this XML format file. If we use the command line or any IDE plugin to execute this XML file, all the instructions written in this file will be executed sequentially.

Inside a sample build.XML, let's understand the code.

The project tag is used to define a project name and an attribute-based on it. The foundation is an application's root directory.

● Property tags are used in the build.xml file as variables to be used in further steps.

● Goal tags are used as steps to be performed sequentially. The name attribute is the target's name. In a single build.xml, you can have different targets

● The path tag is used to logically bundle all files that are in the commonplace

● Path element tag sets the generic location root path where all files are stored.

● The path convert tag is used to convert all common file paths within the path tag to the classpath format of the framework.

● Used to set classpath for various third party jars in our project files tag

● The echo tag is used on the console to print text.

● The Delete tag cleans data from the designated folder

● A new directory will be created with the mkdir tag

● Used to compile java source code and transfer .class files to a new folder, javac tag

● The jar tag creates a jar file from the .class files

● The manifest tag will set your key execution class to

● The attribute 'depends' used to make one goal depend on another destination

● The java tag executes the main function from the jar created in the target compile portion.

Run Ant using the plugin Eclipse

Go to build.xml file to run Ant from eclipse -> right-click file -> Run as... -> click Build file -> right-click file -> Run as... -> click Build file

EXAMPLE:

We will take a small sample program that will very clearly demonstrate the features of Ant. Our project architecture is going to look like follows.

We have 4 goals here in this example.

Class route setting for external jars,

Clean code, previously complied with,

Compile current Java code for yourself

Run the code, run it

AntClass.class

TestAnt's package;

Java.util.Date import;

AntClass Public Class {

Static public void main(String...s){

System.out.println('HELLO PROGRAM ANT');

System.out.println("DATE IS TODAY->"+ currentDate());;"

}

Public static String currentDate(){

Fresh Date().toString() returns;;

}

Construct.xml

How to execute code for TestNG using Ant

Here we will construct a class using the TestNG method and set the Testing classpath in build.xml.

Now we'll create another testng.xml file to run the test method and call this file from the build.xml file.

Step 1)

In the testing kit, we build the "AntClass.class"

TestAnt's package;

Java.util.Date import;

Org.testng.annotations.Test imports;

AntClass Public Class {

The @Test

AntTestNGMethod(){{ Public Void

System.out.println('HELLO PROGRAM ANT');

System.out.println("DATE IS TODAY->"+ currentDate());;"

}

Public static String currentDate(){

Fresh Date().toString() returns;;

}

Step 2)

Construct a target for this class to be loaded into Build.xml

Step 3)

Create testng.xml

Testng.xml for checking

rg/testng-1.0.dtd" ">

Step 4)

In Build.xml, create a Target to run this TestNG code.

Step 5)

Absolute Build.xml Complete

Selenium Ant with Web driver:

We have discovered so far that we can place all third-party jars in a specific place in the system using ANT and set their direction for our project. Using this approach, we set all of our project's dependencies in a single place and make it more stable for compilation, execution, and deployment.

Similarly, we can easily discuss selenium dependency in build.xml for our testing projects using selenium, and we don't have to manually add a classpath to our program.

So now you can disregard the conventional way to set classpaths for the project listed below.

EXAMPLE:

The previous example we are going to change is

Step 1)

In the resource folder, set the selenium. jars property to a selenium-related container.

Step 2)

Add the selenium files to the set classpath target

Step 3)

Build.xml Com

Step 4)

Update the previously generated AntClass.java class to a new code.

TestAnt's package;

The java. util.List import;

Org.openqa.selenium.By import

Org.openqa.selenium.WebDriver Import;

Org.openqa.selenium.WebElement Import;

Org.openqa.selenium.firefox.FirefoxDriver importation;

Org.testng.annotations.Test imports;

AntClass Public Class {

The @Test

AntTestNGMethod(){{ Public Void

Driver for WebDriver = new FirefoxDriver();

driver.get;

List listAllCourseLinks = driver.findElements(By.xpath("/div[@class='canvas-middle']/a"));););

For(WebElement WebElement: listAllCourseLinks){ WebElement WebElement: listAllCourseLinks

System.out.println(webElement.getAttribute("href"));

}

Step 5)

The production after successful execution looks like this.

Flash testing varies from other component components

Flash is a technology that is obsolete. Capturing a flash-object is challenging as it is distinct from HTML. Flash is also an embedded SWF file that is (Small Web Format). Accessing Flash artifacts on a mobile device is often challenging.

Flash creation is more complex than SEO (Search Engine Optimization) HTML page development because flash is not completely readable by the search engine. Advanced technologies such as HTML 5 are, however, applied to solve problems such as performance and security.

Check it with the flash application.

There are two methods of testing Flash Applications:

Manual

By running test cases manually, you can test the Flash object as it is quick and easy to test. You ensure that the flash works properly as planned after bug fixation and provide sign-off.

Automation:

Use any automation method such as Selenium, SoapUI, TestComplete, etc to write a script and execute the script.

Conclusion

The key difference between flash and other elements, as described above is that Flash is embedded in SWF files, while other elements are embedded in HTML files. This is why, compared to flash, HTML is simple to catch. You can learn more about Ant build and flash test in selenium through Selenium online training.

#selenium online training #selenium online courses #selenium online training Hyderabad #selenium online training India #selenium with java online training #best selenium course online #selenium testing course online

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javascript83-blog · 5 years ago

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Guidelines for Using Exceptions in Java

Note: Whitespace has been automatically removed by this site for all the code examples.

You may want to read and adopt the guidelines that help you get the most out of Java's exception handling mechanism. You can write powerful and effective Java code by adopting most, or all, of the things recommended here.

Remember that exception handling provides a way to process synchronous errors such as divisions by zero and out-of-range array indexes. It's not meant for handling asynchronous events such as disk I/O completions and mouse clicks and keystrokes.

When to use Exceptions You may be wondering as to how you actually decide to create and throw an exception. As a rule of thumb, create exception classes for all common exceptions that are likely to occur in multiple classes of an application. In order to deal with simple errors that are likely to occur only in individual methods, try not to create an exception class. You can use an if statement in most cases to trap these types of errors.

Here's a simple example that shows when not to use a try-catch block in your code. The code throws an error when there's an exception of the type NullPointerException.

try {

System.out.println(refvar.toString()};

}

catch (NullPointerException e) {

System.out.println("refVar is null!");

This is pure overkill. You can instead use the much simpler if statement to handle these types of simple errors.

if (refVar!=null)

System.out.println(refVar.toString()};

else

System.out.println("refVar is null");

If your code can perform a simple logic test, as shown here, to handle an error, do so, rather than using an exception object to handle the error.

When you neither catch nor throw exceptions When you call any method that throws a checked exception, you must either throw the exception or catch it. If you decide that you can't handle the exception in the method where it occurs, write a throw statement to send the exception up to the calling method, which must then either handle it with an exception handler, or throw it up to its calling method.

The following set of examples show how you must either catch or throw a checked exception, in order to avoid compiler errors.

The first example shows code that throws an IOException. The statement in the method getFileLength may throw an IO exception when it calls the constructor of the RandomAccessFile class. In addition, the statement that calls the length method of the RandomAccessFile object may also throw an IOException. You thus specify the throws clause in the declaration for the getFileLength method to trap IOException.

public static long getFileLength() throws IOException

{

RandomAccessFile in = new RandomAccessFile("mytext.dat", "r");

long length = in.length();

return length;

}

Our next example shows code for a method that calls the getFileLength method. Since we already know that the getFileLength method call throws an IOException error, our new method must either handle the IO Exception or throw it. Our example here shows how to handle the IOException by catching it with a try statement:

public static int getRecordCount()

{

try

{

long length = getFileLength(); // this may throw an IOException

int recordCount = (int) (length / /RECORD_SIZE);

return recordCount;

}

catch (IOException e) // this will catch the IOException

{

System.out.println("IO Exception!");

return 0;

}

The third example shows how a method can call the getFileLength method without catching IOException. Instead, it throws the exception.

public static int getRecordCount() throws IOException

{

long length = getFileLength(); //this may throw an IOException

int recordCount = (int) (length / RECORD_SIZE);

return recordCount;

}

The getRecordCount method here includes a throw clause, which ensures that any IOException is thrown up to the calling method.

Our fourth example shows what happens when you don't catch an exception or throw it.

public static int getRecordCount()

{

long length = getFileLength(); //this may throw an IOException

int recordCount = (int) (length / RECORD_SIZE);

return recordCount;

}

Since your code fails to catch or throw a checked exception, you'll receive a compile error:

MyExceptionTest.java.26: unreported exception java.io.IOException;

must be caught or declared to be thrown

The following is a quick summary of best practices and guidelines that enable you to take advantage of Java's exception handling mechanism, without succumbing to common missteps in the use of exceptions.

Begin Early It is a good idea to incorporate exception handling into your applications from the get go, before you even start programming. That is, you must do this at the design stage, as it gets much harder to do it after implementing your applications.

Don't Ever Ignore an Exception Regardless of whether you're dealing with a checked or an unchecked exception, don't ever ignore the exception. Sometimes, you might see code such as the following:

//Following is an empty catch block

try {

...

} catch (SomeException e) {

}

The empty catch block means that the exception is simply ignored - it's not dealt with. You don't want to do this in your programs. At the minimum, if you must really, really ignore the exception, put a comment explaining why you wanted to ignore the exception. You can use printStackTrace to output a simple error message to tell you there was a problem. If you ignore the exception, because of a condition that you've predicted, it means that the program will run on despite the error. However, you're going to run the risk of a complete program failure down the road when conditions change. If you propagate the exception out, you can ensure that the program fails quickly and captures information that helps you fix the error.

Don't use Exception Handling for every Statement The goal of exception handling is to clarify programs - try not to use a try/catch/finally mechanism for every potential exception generating statement. A simple strategy is to enclose a significant amount of code within the try block and specify multiple catch blocks to account for all possible exceptions. End the whole thing with a single finally block if you need to release any resources held by the program. In any case, avoid placing try/catch/finally around each statement that might throw an exception. Your goal is to use exception handling to remove error processing code from the main program code.

Maintain Failure Atomicity Always strive to ensure that when an object throws an exception, the object continues to remain in a well-defined and usable state. Ideally, even after a method fails due to an exception, the object should be left in the state it was before the method was invoked, When a method can do this, it's said to possess the property of failure atomicity.

You can achieve failure atomicity by using any of the following programming techniques.

Design Immutable Objects The easiest way to achieve failure atomicity is by designing immutable objects. Although when you create an object you can change its contents, occasionally, it's a good idea to create objects whose contents cannot be changed after creation. An object such as this is called an immutable object and its class, an immutable class. A good example is the String class, which is an immutable class. You get automatic failure atomicity when an object is immutable, because the state of an object can't be modified after you create the object.

Check Parameters for Validity before the Operation If you're stuck with mutable objects, the best way to achieve failure atomicity is to perform a check for the validity of parameters before the program commences the modification of an object. The following example shows how you do this:

public Object pop() {

if (size == 0)

throw new EmptyStackException();

Object result = elements[--size];

elements[size] = null; // Eliminate obsolete reference

return result;

}

The initial size check ensures that the method will do two things when it tries to pop an element from an empty stack: it will throw an exception and it will do so while leaving the size field in a consistent state. Otherwise, the attempt to pop the element from an empty stack will leave the size field in a negative state which, of course, is an inconsistent state for the object to be in.

Attempt to Execute the Code before you Modify the Object You can write your programs in a way where the code that is susceptible to a potential exception, is run before any code that modifies an object. A good example would be where you would like to add an element to a TreeMap, wherein the new element must be compared using the TreeMap's ordering. If your code attempts to add an incorrect type element, it will fail when a search for the element in the tree is made, before the tree is modified.

Intercept the Failure Another strategy to achieve failure atomicity is to put in code that intercepts a failure that occurs during an operation, and which rolls back the object's state to what it was before the operation began. This strategy is often adopted when modifying disk based data, that is, data contained in a relational database, for example.

Perform Operations on a Temporary Copy of the Object You can limit state changes made to an object by changing a temporary copy of the object. You replace the contents of the object with the temporary copy once you complete the computational operations. If, for some reason, the computational work (say, a sort) ends up failing, no harm is done to the original object, as you're working with just a copy of the object.

Collect Failure Details If you're troubleshooting an easily reproducible failure, it's no big deal whether you're collecting detailed failure information. When dealing with exceptions that are not easily reproducible, it is critical that your exception's toString method captures sufficient details about the failure - to help diagnose and fix the failure.

When an exception occurs, your details message of an exception should capture the current values of all parameters and values that contributed to the exception. If your error message simply states IndexOutofBoundException, it's only of limited use and may, or may not, help you fix the data problem. If, on the other hand, your detail message for the IndexOutofBoundException contains the lower and upper bounds and the index value, you have something meaningful in your hands with which to proceed towards a fix of the failure condition.

Since the stack trace of the exception always contains the precise file and line number that threw the exception, you don't need a lot of information about the source code in your exception messages. To ensure that your exceptions always contain adequate failure details in their details message, use one of the constructors I explained about earlier in this chapter. It is better to do this than to capture the details within a simple string detail message.

The following example shows how you can use a more helpful constructor than the simple String constructor.

/**

* Construct an IndexOutOfBoundsException.

* @param lowerBound the lowest legal index value.

* @param upperBound the highest legal index value plus one.

* @param index the actual index value.

public IndexOutOfBoundsException(int lowerBound, int upperBound,

int index) {

// Generate a detail message that captures the failure

super("Lower bound: " + lowerBound +

", Upper bound: " + upperBound +

", Index: " + index);

// Save failure information for programmatic access

this.lowerBound = lowerBound;

this.upperBound = upperBound;

this.index = index;

}

Top Tip: Provide accessor methods for all exceptions, especially for checked exceptions.

As mentioned earlier, it's sometimes useful to provide accessor methods to capture details regarding exactly why a failure happened. In the case of the example just shown, you can, for example, provide accessor methods for lowerBound, upperBound and index. It's critical that you provide such accessor methods for all accessor methods.My site list to string in java

Use Standard Java Exceptions As discussed earlier in the book, the Java platform offers a built-in set of unchecked exceptions which will take care of most of your exception mechanism setting needs. Make a point to try and use these preexisting exceptions before you run off coding your own custom exceptions. Other programmers are familiar with these preexisting exceptions and, therefore, they can easily understand your code. Also, since you'll be using just a set of basic exception classes and not a whole bunch of custom exceptions, your programs need less time to load.

Here are brief descriptions of the most common standard Java exceptions.

IllegalArgumentException

You throw this exception when a caller passes in an argument with an inappropriate value. For example, a calling program that passes a negative number in a parameter that denotes how many times you repeat an action.

IllegalStateException

This is the exception thrown when an object is illegally invoked because of its state - say, when a calling program tries to use an object before it has been initialized.

NullPointerException

Used when a parameter value is null when it ought not to be.

IndexOutofBoundsException

An exception thrown when a caller passes an out of range value in a parameter representing an index into a sequence.

ConcurrentModificationException

Thrown when an object designed for single thread usage finds that it was, or is, being concurrently modified.

UnsupportedOperationException

Thrown when an object doesn't support a specific operation such as when a program tries to delete an element from an append-only list.

Although in most cases it's quite apparent as to which exception applies to an error condition, sometimes, you may find that more than one exception might be correctly used for an error condition. There aren't any cut and dry rules for the specification of exceptions - as with some other things in Java, handling exceptions is an art as well as a science!

Document All of a Method's Exceptions Spend the necessary time to fully document all exceptions thrown by each method in an application. Declare all checked exceptions individually and use the Javadoc tag @throws to document the conditions under which each of the exceptions can be thrown.

It is critical that you don't fall into the bad habit of declaring that a method throws some super class of multiple exception cases that the method is set up to throw. That means that you never declare a method that can throw the generic exception classes Exception or Throwable! Using generic exceptions might handle the exception all right, but it keeps you from understanding the precise error that caused the exception to be thrown.

Top Tip: Always attempt to document every single exception that can potentially be thrown by each method in a program.

Take care to document both checked and unchecked exceptions. Since unchecked exceptions normally represent programming errors, documenting them can keep you from avoiding those errors. You can use the same Javadoc tag @Throws to document unchecked exceptions, as is the case for checked exceptions.

If multiple methods in a class can throw the same exception, you may document the exception once in the class's documentation comment. This is much more efficient than documenting the exception separately for each of the methods.

Finally, a useful rule of thumb when handling Java exceptions is to always throw early and catch late!

This finally wraps up our discussion of Java's exception handling mechanisms and how to make the best use of them in your code. Our next topic is the effective use of pre and post conditions in your code within the framework of what's called Java assertions, primarily as a way of testing code before you move it to production.

#list to string in java java list to string

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thinlascl · 3 years ago

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Change spinner selected text color android

Change spinner selected text color android android#

Change spinner selected text color android code#

For that we have used parent.getItemAtPosition(position).toString() method. We can get the value of the selected class name from the ClassSpinner. set divSpinner Visibility to void onNothingSelected(AdapterView parent) GetResources().getStringArray(R.ems_div_class_4))) GetResources().getStringArray(R.ems_div_class_3))) GetResources().getStringArray(R.ems_div_class_2))) GetResources().getStringArray(R.ems_div_class_1))) assigning div item list defined in XML to the div SpinnerĭtAdapter(new ArrayAdapter(MainActivity.this,Īndroid.R.layout.simple_spinner_dropdown_item, String selectedClass = parent.getItemAtPosition(position).toString()

Change spinner selected text color android code#

So add the following code inside classSpinner's onItemSelected() method.ĬtOnItemSelectedListener(new void onItemSelected(AdapterView parent, View view, int position, long id) options to divSpinner from the string-array resource. Inside the onItemSelected() method of classSpinner, you have to get the selected item from the dropdown list and based on that value, you have to assign entries i.e. Spinner classSpinner, void onCreate(Bundle savedInstanceState) Changing Value of second Spinner based on first Spinner value MainActivity.java public class MainActivity extends AppCompatActivity We will start by creating instances of both the spinners and then, assign the setOnItemSelectedListener() on both the spinners. To handle the GUI events, we need to implement the code inside the MainActivity.java file. We will do this through Java code as the user will select from the first dropdown at run time i.e. Doing so will assign the values present in the items_class array to the classSpinner.Īs per the user's selection of the option from the first spinner, our second Spinner will appear along with options based on the first spinner's selection. To add these entries to the spinner all we have to do is add a property in the main activity layout XML file. In our dataset, string-array named items_class will be assigned to the classSpinner to display the class items in the dropdown list. We have also defined our dataset for the Spinners.We have defined the layout XML for the User interface.

Change spinner selected text color android android#

We have understood the design that we are creating in our Android App, which will have 1 TextView and 2 Spinners.

In Android, we must put the data XML files like our strings.xml file in app → res → values → strings.xml.īy now, we are done with the following things: We can define a string array in XML like we have shown below in the strings.xml file. But for this example, we will try the other way i.e. One by declaring an array and defining the items in it. To add items to the Spinner, there are two possible ways to provide it with a set of options. Once you will create that, the error in this file will be resolved.Īs you can see, we have one TextView and two Spinner views inside the xml file along with a few properties specified. NOTE: If you are getting an error in this, that is because you have not yet created the below data XML file. If you want, copy paste the below XML to start with this example in your local machine. So let's start, here we have the layout XML file. When user selects an optin from the second spinner too, then we will create a Toast and display the chosen values on screen. The first Spinner( classSpinner) holds the list of classes(in school) to be selected by the user and based on that choice we will assign the values to the second Spinner( divSpinner). But you can try and change the value of isVisible to VISIBLE to see if it is there or not. Hence, in the image below, it looks like it's not there. The second spinner has the property isVisible equals to GONE, which means it exists in the layout but it will not be visible(or it will be hidden). So we need to customize the layout(shown in below image) where one Spinner is placed below the TextView and the other spinner is below the first spinner.Īs the options inside the second Spinner will depend on what we select in the first Spinner, hence initially the second spinner will be hidden and it will only appear when user has selected one option from the first spinner. The main layout of the application will contain our one TextView and two spinners as its child component views. In this tutorial, we are going to create an application that works with two Spinners, which are inder-dependent. So let's checkout an example and learn how we can implement it in our Android App. You have already studied about what Spinners are and how they work.

Android SDK Manager & required Packages.

#Change spinner selected text color android

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samanthablake02 · 3 months ago

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Supercharge Your Apps: Mastering Kotlin Programming in 2025

Did you know that companies using Kotlin programming report up to a 40% reduction in code compared to Java? That's a monumental gain in efficiency and maintainability. But diving into Kotlin programming can feel like navigating a complex labyrinth if you don’t have the right guide.

This post provides that comprehensive roadmap. It steers clear of the superficial and delves into actionable strategies, vital tools, and key insights that separate adept Kotlin programming from rudimentary endeavors. You'll bypass common pitfalls, grasp best practices, and develop a foundational understanding that will propel your mobile app development.

For any android app development company, mastering Kotlin is essential to stay competitive in the evolving tech landscape.

Demystifying Core Strategies in Kotlin Programming

Let’s dissect some powerful tactics that will enhance your efficacy in Kotlin programming .

Embracing Null Safety: This is arguably one of Kotlin's strongest assets. Its built-in null safety features greatly mitigates the risk of NullPointerExceptions, a prevalent bane in Java.

Nullable Types: Declare a variable nullable using ?. Example: var name: String? = "Kotlin"

Safe Calls: Use the ?. operator to safely access properties or methods on a nullable variable. Example: name?.length (will only access length if name is not null)

Elvis Operator: Use the ?: operator to provide a default value when a variable is null. Example: val length = name?.length ?: -1 (assigns -1 to length if name is null)

Leveraging Coroutines for Asynchronous Programming: Managing asynchronous operations with callbacks can swiftly transform code into an unreadable mess. Coroutines streamline this process, enabling you to write asynchronous code in a sequential style.

Mastering Data Classes: Tired of boilerplate code for simple data holding classes? Kotlin data classes automatically generate methods like equals(), hashCode(), toString(), and copy(), minimizing manual coding.

Evading Common Errors in Your Kotlin Programming Journey

Even experienced developers occasionally fall into common traps. Recognize and circumvent these pitfalls.

Overusing Nullable Types: While null safety is important, relying excessively on nullable types can muddle your code and necessitate redundant null checks. Aim to keep your variables non-nullable where it's logically tenable.

Ignoring Extension Functions: Kotlin's extension functions let you append new methods to existing classes without inheritance. It's easy to overlook this power, leading to repetitive code and diminished modularity.

Not Fully Capitalizing on Data Classes: Data classes auto-generate common methods. Missing out on this auto-generation equates to needless repetition and avoidable chances for errors.

Indispensable Tools for Flourishing in Kotlin Programming

Equipping yourself with the appropriate tools streamlines development, boosts productivity, and elevates code quality.

IntelliJ IDEA: This IDE has outstanding support for Kotlin programming, boasting code completion, debugging capabilities, and refactoring tools.

Android Studio: Built atop IntelliJ IDEA, Android Studio provides tailored tools for Android development with Kotlin.

Kotlin Standard Library: Master this, covering collections, sequences, I/O, and more. It enhances expressiveness and reduces boilerplate.

Expert Insights into Elevating Your Kotlin Programming

Go beyond basic proficiency by following insights from experienced Kotlin practitioners.

Code Reviews are Essential: Routine code reviews uncover subtle errors, guarantee code uniformity, and foster knowledge-sharing within the team.

Staying Updated: The Kotlin language continuously develops. Remain current on new features and recommended practices through official documentation and community forums.

Craft Testable Code: Structure code with testability in mind. Employ dependency injection to segregate components and streamline unit testing. "A major boon of Kotlin programming is its inter-operability with Java. You can gradually migrate large Java codebases and realize incremental benefits. " - John, Senior Software Architect

Consider an anecdote from my personal experience with code reviews. I initially thought my code was impeccable until a colleague identified a glaring potential concurrency issue that would have cost my company greatly in maintenance expenses and down time in a system upgrade scenario. The fresh perspectives gleaned during code reviews has proved invaluable.

Diving Deeper: Extending Kotlin's Functionality with Method Addition

Now, let’s scrutinize adding methods in Kotlin , particularly when expanding existing classes (the essence of extension functions).

How to Add Methods (Extension Functions): A Detailed Guide

This approach does not alter the source code of the original class; instead, it permits you to append a new function that behaves as if it's a member function of that class. Here are the steps involved:

Identify the Class to Extend: Determine which class you want to augment with additional functionality. This could be a class in the Kotlin standard library (like String, List) or a custom class defined in your project.

Create the Extension Function: Declare the extension function using the fun keyword, followed by the class name, a dot (.), and the name of the new function. Here's the generic format: fun ClassName.newFunctionName(parameters: ParameterType): ReturnType { // Function body return someValue }

Access the Receiver Type: Inside the extension function, the class being extended is referred to as the "receiver." You can access its members using the this keyword, though typically you can refer to the properties and methods of the receiver class directly.

Add Functionality: This is where you incorporate your custom logic. Your function can perform any operation on the receiver object or interact with other parts of your application.

Call the Extension Function: Once defined, call the extension function just as you would any member function of the extended class. Here's an example: val myString = "Kotlin Rocks" val wordCount = myString.wordCount() // Calls the extension function println("Word count: $wordCount")

Placement Considerations: Ideally, place extension functions near where they are used or in a dedicated extensions file to keep your code organized and maintainable. Consistency in placement facilitates readability and collaboration. Practical Example: Adding a Word Count Function to String

Let’s create a specific example—an extension function that counts the number of words in a string:fun String.wordCount(): Int { val words = this.trim().split("\\s+".toRegex()) return if (words.first().isEmpty()) 0 else words.size } fun main() { val myString = " This is a Kotlin Example " val count = myString.wordCount() println("Number of words: $count") // Output: Number of words: 5 }

In this example:

We define wordCount() as an extension function on the String class.

The this keyword refers to the string on which the function is called.

We utilize the trim() function to remove leading and trailing whitespace, ensuring accurate word counting.

We employ the split() function to break the string into words using whitespace as the delimiter.

We then calculate the word count by determining the size of the resultant list of words.

By diligently pursuing this guide, you enhance classes seamlessly, thereby amplifying their usefulness and the modularity of your overall architecture. This embodies Kotlin's design goal to allow programmers to stretch its capabilities creatively.

Key Takeaways

Kotlin programming enhances code brevity and reduces errors with features like null safety and data classes.

Prioritize null safety, learn to handle exceptions effectively and apply coroutines for improved performance.

Continually refine your skills through community participation and continuous education in Kotlin programming updates.

Master extension functions and take steps for better programming practices.

Frequently Asked Questions

Can Kotlin programming truly replace Java in Android Development?

Absolutely! Kotlin is now the favored language for Android app development and interoperates effectively with existing Java code. Migrating piece by piece becomes easy, so that's why Kotlin programming is now a preferred option.

Why is Null Safety a much lauded feature of Kotlin?

Kotlin’s built-in null safety alleviates many common NullPointerException that happens when accessing null variables that occurs during runtime in Java. Safe calls and the Elvis operator can help create stronger applications with greater protection from crashing.

How complex is migrating Java code to Kotlin programming?

Thanks to its full interoperability, code can migrate iteratively without re-writing the entire program at once which has encouraged adoption. Each bit is compiled into binary for use by each platform so gradual migration is manageable and can allow quicker deployment.

Can I use my existing Java skills while using Kotlin programming?

Yes! Given that it interoperates seamlessly with Java, prior Java skillsets become an immense value and drastically reduce learning curves when approaching this new way to build mobile apps! You will not have to rebuild all those applications; just move little parts.

What are the specific performance advantages associated with Kotlin programming?

Coroutines enable more effective asynchronous coding management and reduces the need for messy callbacks. Coupled with Kotlin’s compact syntax can lead to enhanced and effective codes. This gives users speedier service and higher level of usefulness!

Recommendation

We highly urge the adoption of Kotlin programming given the high level of improvement the company is using and given its benefits with interoperability as we have previously covered here in the blog. With this easy move towards its acceptance and continued application to build out the company's programming assets; our resources and efforts can be leveraged at scale!

#kotlin programming #mastering kotlin #kotlin 2025 #android development #kotlin for beginners #advanced kotlin

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jarviscodinghub · 4 years ago

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CSC 220 – Lab 8 solved

Objective Complete several small programs in Java based on examples in class. In this lab, you will play with the class design, toString() method, and String. Java programs: • StopWatch: One of the hallmarks of object-oriented programming is the idea of easily modeling real-world objects by creating abstract programming objects. As a simple example, consider StopWatch, which implements the…

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simplexianpo · 5 years ago

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Spring Digging Road 2-> Assembly of SpringBean

# Assembly of SpringBean

##SpringBean allocation plan

Spring container is responsible for creating bean in applications and coordinating the relationships among these objects through dependency injection.

But as a developer, you need to tell Spring which bean to create and how to assemble them together. When describing how bean are assembled, Spring has great flexibility and provides three main assembly mechanisms:

```

1. Implicit bean discovery mechanism-annotation;

2. Explicit configuration in Java -- JavaConfig；

3. Explicit configuration in XML -- XML configuration.

```

###Annotation assembly Bean

Spring realizes automatic assembly from two angles:

```

Component scanning : Spring will automatically discover bean created in the application context

Automatic assembly : Spring automatically satisfies the dependencies between bean.

```

**PS : The combination of component scanning and automatic assembly can minimize explicit configuration.

```

1.Create bean that can be discovered

2.Name the bean scanned by the component

3.Set the basic package for component scanning

4.Automatic assembly by adding annotations to bean

```

The directory structure is as follows

First of all, let everyone look at the contents of pom.xml and springConfig.xml on my side, as shown in the figure:

Several ways to annotate Bean assembly

```

Member variable injection

Constructor injection

Setting mode injection

```

Ok, it's time to code.

The following is member variable injection, which mainly adds @Autowire annotation to A to be passed into Class B.

public class SpringTest {

public static void main(String[] args) {

AbstractApplicationContext context = new ClassPathXmlApplicationContext("/spring/springConfig.xml");

B b = (B) context.getBean("b");

b.print();

}

@Component

class A {

@Override

public String toString() {

return "introduce Class A";

}

@Component

class B {

@Autowired

A a;

void print() {

System.out.println("In the method of Class B" + a);

}

Then there is constructor injection, at this time, the constructor of B is added @Autowired

public class SpringTest {

public static void main(String[] args) {

AbstractApplicationContext context = new ClassPathXmlApplicationContext("/spring/springConfig.xml");

B b = (B) context.getBean("b");

b.print();

}

@Component

class A {

@Override

public String toString() {

return "introduce Class A";

}

@Component

class B {

A a;

@Autowired

B(A a) {

this.a = a;

}

void print() {

System.out.println("In the method of Class B" + a);

}

Finally, set the way to inject. In one method, automatic assembly is performed. The method name can be arbitrary, only @Autowired needs to be added to the method name. here, for the sake of standardization, setA is used as the method name

public class SpringTest {

public static void main(String[] args) {

AbstractApplicationContext context = new ClassPathXmlApplicationContext("/spring/springConfig.xml");

B b = (B) context.getBean("b");

b.print();

}

@Component

class A {

@Override

public String toString() {

return "introduce Class A";

}

@Component

class B {

A a;

@Autowired

void setA(A a) {

this.a = a;

}

void print() {

System.out.println("In the method of Class B" + a);

}

###Java code assembly Bean

Although it is a more recommended way to realize automatic configuration of Spring through component scanning and annotation in many scenarios, sometimes the scheme of automatic configuration does not work, so it is necessary to explicitly configure Spring Bean.

For example, when you want to assemble a component in a third-party library into an application, you can't add @Component and @Autowired annotations to its class, so you can't use the automatic assembly scheme.

In this case, explicit configuration must be adopted. There are two alternatives for explicit configuration: JavaConfig and XML, and JavaConfig is a better one because it is more powerful, type-safe and friendly to refactoring. After all, JavaConfig belongs to Java code.

At the same time, JavaConfig is different from other Java codes, which is conceptually different from business logic and domain codes in applications. Although it uses the same language as other components, JavaConfig is the configuration code.

This means that it should not contain any business logic, and JavaConfig should not intrude into business logic code. Although it is not necessary, JavaConfig is usually put in a separate package, which makes it separate from other application logic, so that there is no confusion about its intention.

Several ways of assembling Bean with Java code:

```

Constructor injection

Setting mode injection

```

The first is constructor injection

@Configuration

public class JavaConfigTest {

@Bean

A newA() {

return new A();

}

@Bean

B newB() {

return new B(newA());

}

public static void main(String[] args) {

AbstractApplicationContext context = new AnnotationConfigApplicationContext(JavaConfigTest.class);

B b = (B) context.getBean("newB");

b.print();

}

class A {

@Override

public String toString() {

return "introduce Class A";

}

class B {

A a;

B(A a) {

this.a = a;

}

void print() {

System.out.println("In the method of Class B" + a);

}

Then setting mode injection

@Configuration

public class JavaConfigTest {

@Bean

A newA() {

return new A();

}

@Bean

B newB() {

B b = new B();

b.setA(newA());

return b;

}

public static void main(String[] args) {

AbstractApplicationContext context = new AnnotationConfigApplicationContext(JavaConfigTest.class);

B b = (B) context.getBean("newB");

b.print();

}

class A {

@Override

public String toString() {

return "introduce Class A";

}

class B {

A a;

void setA(A a) {

this.a = a;

}

void print() {

System.out.println("In the method of Class B" + a);

}

###Assemble Bean through XML

When Spring first appeared, XML was the main way to describe configuration, but XML was no longer the only alternative way to configure Spring. However, in view of the large number of Spring configuration projects based on XML, it is very important to understand how to assemble Sping projects through XML.

Several ways to assemble Bean through XML

```

Constructor injection

Setting mode injection

```

The first is constructor injection:

Project code is:

public class XMLTest {

public static void main(String[] args) {

AbstractApplicationContext context = new ClassPathXmlApplicationContext("/xml/springConfig.xml");

B b = (B) context.getBean("b");

b.print();

}

class A {

@Override

public String toString() {

return "introduce Class A";

}

class B {

A a;

B(A a) {

this.a = a;

}

void print() {

System.out.println("In the method of Class B" + a);

}

springconfig.xml As shown in figure:

Then there is the setting mode injection

Project code is:

public class XMLTest {

public static void main(String[] args) {

AbstractApplicationContext context = new ClassPathXmlApplicationContext("/xml/springConfig.xml");

B b = (B) context.getBean("b");

b.print();

}

class A {

@Override

public String toString() {

return "introduce Class A";

}

class B {

A a;

public void setA(A a) {

this.a = a;

}

void print() {

System.out.println("In the method of Class B" + a);

}

springconfig.xml Figure：

###Spring annotation

@Autowired : Automatically assemble member variables, and @Autowired(required=false) will not report an error if no matching Bean can be found.

@Qualifier : Comments specify the ID of the injected bean

@Component : Define a bean

@Controller : Controller

@Service : Define a service bean

@Repository : Used to take the class identity of data access layer (DAO) as a bean

@Scope : Set the type of bean

###Others:

Singleton: In the whole application, only one instance of bean is created :

```

value=ConfigurableBeanFactory.SCOPE_SINGLETON

```

Prototype : A new bean instance is created every time it is injected or acquired through Spring context

```

value=ConfigurableBeanFactory.SCOPE_PROTOTYPEE

```

Request : In a Web application, a bean instance is created for each request :

```

value=WebApplicationContext.SCOPE_REQUEST

```

Session : In a Web application, a bean instance is created for each session

```

value=WebApplicationContext.SCOPE_SESSION

```

#Java #Spring #Tips

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enterinit · 5 years ago

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Download Java MSI

Download Java MSI. Java logo 512x512 for Microsoft EndPoint Configuration Manager (SCCM/MECM/MEMCM) deployments:

Java 8.0.2510.8

TLS Application-Layer Protocol Negotiation Extension JEP 244 has enhanced the Java Secure Socket Extension (JSSE) to provide support for the TLS Application-Layer Protocol Negotiation Extension (RFC 7301). New methods have been added to the javax.net.ssl classes SSLEngine, SSLSocket, and SSLParameters to allow clients and servers to negotiate an application layer value as part of the TLS handshake. Added Support for PKCS#1 v2.2 Algorithms Including RSASSA-PSS Signature The SunRsaSign and SunJCE providers have been enhanced with support for more algorithms defined in PKCS#1 v2.2, such as RSASSA-PSS signature and OAEP using FIPS 180-4 digest algorithms. New constructors and methods have been added to relevant JCA/JCE classes under the java.security.spec and javax.crypto.spec packages for supporting additional RSASSA-PSS parameters. WebEngine Limits JavaScript Method Calls for Certain Classes JavaScript programs that are run in the context of a web page loaded by WebEngine can communicate with Java objects passed from the application to the JavaScript program. JavaScript programs that reference java.lang.Class objects are now limited to the following methods:getCanonicalName getEnumConstants getFields getMethods getName getPackageName getSimpleName getSuperclass getTypeName getTypeParameters isAssignableFrom isArray isEnum isInstance isInterface isLocalClass isMemberClass isPrimitive isSynthetic toGenericString toString No methods can be called on the following classes:java.lang.ClassLoader java.lang.Module java.lang.Runtime java.lang.System java.lang.invoke.* java.lang.module.* java.lang.reflect.* java.security.* sun.misc.* New Oracle Specific JDK 8 Updates System Property to Fallback to Legacy Base64 Encoding Format Oracle JDK 8u231 upgraded the Apache Santuario libraries to v2.1.3. This upgrade introduced an issue where XML signature using Base64 encoding resulted in appending or to the encoded output. This behavioral change was made in the Apache Santuario codebase to comply with RFC 2045. The Santuario team has adopted a position of keeping their libraries compliant with RFC 2045. Oracle JDK 8u221 using the legacy encoder returns encoded data in a format without or . Therefore, a new Oracle JDK 8 Updates only system property, - com.sun.org.apache.xml.internal.security.lineFeedOnly, is made available to fall back to legacy Base64 encoded format. Users can set this flag in one of two ways: -Dcom.sun.org.apache.xml.internal.security.lineFeedOnly=trueSystem.setProperty("com.sun.org.apache.xml.internal.security.lineFeedOnly", "true") This new system property is disabled by default. It has no effect on default behavior nor when com.sun.org.apache.xml.internal.security.ignoreLineBreaks property is set. Later JDK family versions might only support the recommended property: com.sun.org.apache.xml.internal.security.ignoreLineBreaks x86 msiexec.exe /i "jre1.8.0_251.msi"/qn JU=0 JAVAUPDATE=0 AUTOUPDATECHECK=0 RebootYesNo=No WEB_JAVA=1 x64 msiexec.exe /i "jre1.8.0_25164.msi" /qn JU=0 JAVAUPDATE=0 AUTOUPDATECHECK=0 RebootYesNo=No WEB_JAVA=1 REMOVEOLDERJRES=1 For uninstall use: x86 msiexec /x {26A24AE4-039D-4CA4-87B4-2F32180251F0} /qn /norestart x64 msiexec /x {26A24AE4-039D-4CA4-87B4-2F64180251F0} /qn /norestart Java MSI x86 download Java MSI x64 download

Java MSI 8.0.2410.7

Release Highlights IANA Data 2019c JDK 8u241 contains IANA time zone data version 2019c. New Feature: Allow SASL Mechanisms to Be Restricted A security property named jdk.sasl.disabledMechanisms has been added that can be used to disable SASL mechanisms. Any disabled mechanism will be ignored if it is specified in the mechanisms argument of Sasl.createSaslClient or the mechanism argument of Sasl.createSaslServer. The default value for this security property is empty, which means that no mechanisms are disabled out-of-the-box. New Feature: SunPKCS11 Provider Upgraded with Support for PKCS#11 v2.40 The SunPKCS11 provider has been updated with support for PKCS#11 v2.40. This version adds support for more algorithms such as the AES/GCM/NoPadding cipher, DSA signatures using SHA-2 family of message digests, and RSASSA-PSS signatures when the corresponding PKCS11 mechanisms are supported by the underlying PKCS11 library. Other notes: New Checks on Trust Anchor Certificates New checks have been added to ensure that trust anchors are CA certificates and contain proper extensions. Trust anchors are used to validate certificate chains used in TLS and signed code. Trust anchor certificates must include a Basic Constraints extension with the cA field set to true. Also, if they include a Key Usage extension, the keyCertSign bit must be set. Other notes: Exact Match Required for Trusted TLS Server Certificate A TLS server certificate must be an exact match of a trusted certificate on the client in order for it to be trusted when establishing a TLS connection. Other notes: Added LuxTrust Global Root 2 Certificate LuxTrust root certificate has been added to the cacerts truststore Other notes: Added 4 Amazon Root CA Certificates Amazon root certificate has been added to the cacerts truststore Bug Fixes: Support for OpenType CFF Fonts Previously, Oracle JDK 8 did not include OpenType CFF fonts (.otf fonts) into the standard logical fonts (such as "Dialog" and "SansSerif"). This resulted in missing glyphs when rendering text. In the most extreme cases where only CFF fonts were installed on the system, a Java exception could be thrown. Several Linux distributions were affected by this issue because they rely on CFF fonts to support some languages, which is common for CJK (Chinese, Japanese, and Korean) languages. Oracle JDK 8 now uses these CFF fonts, and this issue has been resolved. Bug Fixes: Better Serial Filter Handling The jdk.serialFilter system property can only be set on the command line. If the filter has not been set on the command line, it can be set can be set with java.io.ObjectInputFilter.Config.setSerialFilter. Setting the jdk.serialFilter with java.lang.System.setProperty has no effect. For Configuration Manager deployments (or another authomated deployments) use: x86 msiexec.exe /i "jre1.8.0_241.msi"/qn JU=0 JAVAUPDATE=0 AUTOUPDATECHECK=0 RebootYesNo=No WEB_JAVA=1 x64 msiexec.exe /i "jre1.8.0_24164.msi" /qn JU=0 JAVAUPDATE=0 AUTOUPDATECHECK=0 RebootYesNo=No WEB_JAVA=1 REMOVEOLDERJRES=1 For uninstall use: x86 msiexec /x {26A24AE4-039D-4CA4-87B4-2F32180241F0} /qn /norestart x64 msiexec /x {26A24AE4-039D-4CA4-87B4-2F64180241F0} /qn /norestart Java MSI x86 download Java MSI x64 download

Java MSI 8.0.2310.11

Release Highlights IANA Data 2019b JDK 8u231 contains IANA time zone data version 2019b. For more information, refer to Timezone Data Versions in the JRE Software. New Feature: New jdk.jceks.iterationCount System Property A new system property has been introduced to control the iteration count value used for the jceks keystore. The default value remains at 200000 but values between 10000 and 5000000 may be specified. The new system property name is jdk.jceks.iterationCount and the value supplied should be an integer in the accepted range. The default value will be used if a parsing error is encountered.New Feature: New Java Flight Recorder (JFR) Security Events Four new JFR events have been added to the security library area. These events are disabled by default and can be enabled via the JFR configuration files or via standard JFR options.Removed Features and Options: Removal of T2K Rasterizer and ICU Layout Engine From JavaFX. The T2K rasterizer and ICU layout engine have been removed from JavaFX.Other notes: GTK3 Is Now the Default on Linux/Unix. Newer versions of Linux, Solaris, and other Unix flavor desktop environments use GTK3, while still supporting GTK2. Previously, the JDK would default to loading the older GTK2 libraries. However, in this release, it defaults to loading GTK3 libraries. Loading is typically triggered by using the Swing GTK Look And Feel. The old behavior can be restored by using the system property: -Djdk.gtk.version=2.2Other notes: Remove Obsolete NIST EC Curves from the Default TLS Algorithms. This change removes obsolete NIST EC curves from the default Named Groups used during TLS negotiation. The curves removed are sect283k1, sect283r1, sect409k1, sect409r1, sect571k1, sect571r1, and secp256k1. To re-enable these curves, use the jdk.tls.namedGroups system property. The property contains a comma-separated list within quotation marks of enabled named groups in preference order.For example: java -Djdk.tls.namedGroups="secp256r1, secp384r1, secp521r1, sect283k1, sect283r1, sect409k1, sect409r1, sect571k1, sect571r1, secp256k1" ... For System Center Configuration Manager deployments (or another authomated deployments) use: x86 msiexec.exe /i "jre1.8.0_231.msi"/qn JU=0 JAVAUPDATE=0 AUTOUPDATECHECK=0 RebootYesNo=No WEB_JAVA=1 x64 msiexec.exe /i "jre1.8.0_23164.msi" /qn JU=0 JAVAUPDATE=0 AUTOUPDATECHECK=0 RebootYesNo=No WEB_JAVA=1 REMOVEOLDERJRES=1 For uninstall use: x86 msiexec /x {26A24AE4-039D-4CA4-87B4-2F32180231F0} /qn /norestart x64 msiexec /x {26A24AE4-039D-4CA4-87B4-2F64180231F0} /qn /norestart Java MSI x86 download Java MSI x64 download Read the full article

#Java #MECM #MEMCM #MicrosoftEndpointConfigurationManager #MSI #SystemCenter #SystemCenterConfigurationManager #Windows #Windows10

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techandguru-blog · 6 years ago

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Did you ever forget to bring what your girlfriend asked you to take from the market? What is your reaction? Oh no honey, I was supposed to do this and that but forget and feel sorry about that. Similarly, while writing programs, programmers sometimes forget to handle scenarios which hamper the program execution. Such problems in programming languages are often called Exceptions. Java Exceptions are the way to identify such problems and Exception handling is the same as managing your girlfriend so that execution goes on smoothly.

Java provides rich implementation to handle java exceptions. Exceptions are event occurred during compiling or execution of a program which hamper the normal execution of the program. If exceptions are not handled properly, they can abort the program execution. An exception can happen because of numerous reasons e.g. file is not found, invalid data encountered, divide by zero condition, network connection lost in the middle of operations, etc.

So, cause of exception can arise from user input, programmer mistake or resource on the system, etc. Based on the various scenario, exceptions can be divided into below categories as follows

CHECKED JAVA EXCEPTIONS

A checked exception is thrown by the compiler at compile time so these are also called compile-time exceptions. These can not be ignored and must be handled by the programmer. e.g. If you try to read a non-existing file, then compiler throws FileNotFoundException at compile time.

import java.io.FileReader; import java.io.File; public class CompileTimeExceptionExample { public static void main(String args[]) { File file = new File("C://myfile.txt"); FileReader fr = new FileReader(file); } }

If you try to compile the above program using the command

“javac CompileTimeExceptionExample.java”, it will throw FileNotFoundException which will read like below

CompileTimeExceptionExample.java:6: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file); ^ 1 error

Note: read(), close(), write() etc operations thrown IOException so compiler notifies to handle IOException along with FileNotFoundException.

UNCHECKED JAVA EXCEPTIONS

These exceptions occur at runtime and are a result of improper use of operation and APIs. These are also called runtime exception. Runtime exceptions are ignored at compile time. e.g exception is thrown when divide by zero is attempted.

Also, you have n elements in an array and tries to access (n+1)th element and ArrayIndexOutOfBoundException is thrown.

ERRORS IN JAVA

These are beyond the control of the programmer and a programmer can hardly do anything with them. Errors are very severe and result in program abort.

Now we are familiar with what are exceptions and how many times of are they. Let’s look into the Exceptions hierarchy.

All exception classes are a subclass of java.lang.Exception class. Exception class is driven from Throwable class. Error class is also a subclass of Throwable. Error is a severe condition in program execution which can not be handled by code. Errors are thrown by JVM like OutOfMemoryException.

Java Exception Classes

Exceptions can be divide mainly in two main categories namely Runtime exception and IO exceptions (compile-time exceptions).

Below table lists the methods exposed by Exception class

Method Description public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor. public Throwable getCause() Returns the cause of the exception as represented by a Throwable object. public String toString() Returns the name of the class concatenated with the result of getMessage(). public void printStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream. public StackTraceElement [] getStackTrace() Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack. public Throwable fillInStackTrace() Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace.

HANDLING EXCEPTION IN JAVA

Exceptions can be handled by using “try{}catch(){}” block. The code snippet likely to produce Exception is put inside try block and is called protected code. And if anything unexpected happens and catch block and handles it according to exception defined in the definition of catch block.

Syntax of using try-catch block is below:

try { // Protected code..likely to produc exceptions } catch (ExceptionName e1) { // ExceptionName represents exception type handled in catch block. // Catch block }

If an exception occurs in the protected code, then control is passed to the catch block with an exception object.

A single try-catch block and handle multiple exceptions in the catch block. One can specify actions for each type of exception which are likely to occur in the protected code placed inside the try block. Syntax of handling the multiple exceptions in the catch block is shown below:

try { // Protected code likely to encounter exception } catch (ExceptionType1 e1) { // Catch block } catch (ExceptionType2 e2) { // Catch block } catch (ExceptionType3 e3) { // Catch block }

A single try block can have any number of catch blocks to handle exceptions. The type of the exception thrown in the try block is matched against the exception handled in catch block from TOP to BOTTOM and if anywhere down the ladder, the exception thrown and exception handled by catch block are matched then control is passed to matching catch block and is handled by that catch block.

If no catch block handles the exception thrown then the exception is thrown to the last method in the execution stack and current method execution stops here.

Since Java 7, single catch block and handle multiple exceptions. This approach simplifies the code. e.g.

catch (IOException|FileNotFoundException ex) { logger.log(ex); throw ex; }

Throws and Throw usages in Java

If the method in execution does not handle the checked exception then it must declare exceptions it is going to throw using “throws” keywords. "Throws" is appended at the end of the method signature and specify the Exceptions it can throw using comma like below

import java.io.*; public class ThrowsExample { public void methodName(double amount) throws Exception1,Exception2 { // Method implementation throw new Exception1(); ….. throw new Exception2(); } // Remainder of class definition }

throws is used to postpone the handling of exception and throw is used to raise the exception explicitly.

Finally{} block: finally block is placed after either try or catch depending on whether the catch is used or not. It also occurs after catch if catch block is used otherwise put after try block. “finally” block always executes irrespective of any exception is thrown in try block or not. This block is normally used to do cleanup and resource release kind of activities.

Syntax of finally block

try { // Protected code } catch (ExceptionType1 e1) { // Catch block } catch (ExceptionType2 e2) { // Catch block } catch (ExceptionType3 e3) { // Catch block }finally { // The finally block always executes. }

Popular use of finally block is found in closing sessions/connection after doing DB operations. It is where your program normally release the resources if it has to.

KEY POINTS TO BE NOTED HERE

- try block has to have at least one out of catch block and finally block.

- no code can appear between try-catch and finally block

- catch or finally can not appear without try block

- finally block is not a compulsion to have

try-with-resource:

Since Java 7, try-with-resources is supported. What it means is it automatically releases the resources used inside try block. Generally, we have to close the connection, streams created in try block using finally block. To avoid such explicit code management, Java 7 provides try with resources wherein it automatically releases the resources as the control moves out of try block. Let’s look at an example:

import java.io.FileReader; import java.io.IOException; public class TryWithResourceExample { public static void main(String args[]) { try(FileReader fr = new FileReader("E://temp.txt")) { char [] a = new char[50]; fr.read(a); // reads the content to the array a for(char c : a) System.out.print(c); // prints the characters one by one } catch (IOException e) { e.printStackTrace(); } } }

try-with-resource is also referred to as automatic resource management. To use automatic resource management, you just need to declare the resource inside parentheses with a try statement like in the above program.

KEY POINTS TO USE try-with-resource MECHANISM:

- To use a class with try-with-resource, the class must implement AutoCloseable interface. Close() method is implicitly called after try-catch

- multiple class can be used with try to automatically manage them

- All instances of the class declared with try are instantiated before try block and are final by default.

USER DEFINED JAVA EXCEPTIONS:

User can create own exception by extending Throwable class in java. To create a self-defined Exception class, one should following key points listed below:

- All exceptions must be a child of Throwable

- to write check exception, one must extend the Exception class.

- To create a Runtime exception class, one must implement the RuntimeException class.

Syntax to define Exception class

class MyException extends Exception { }

Basis the origin of exception, Exceptions in java can be divided into two categories like

JVM Exceptions: these are thrown by JVM. e.g. NullPointerException, ArrayIndexOutOfBoundException, ClassCastException

Programmatic Exception: thrown by a programmer like IllegalArgumentException, IllegalStateException etc

So key take away from the post:

- Error are not something programmer can handle through code but however writing code efficiently can reduce the chances of their occurrences.

- checked exceptions are caught by the compiler at compile time and include IOExceptions/ FileNotFoundExceptions etc

- unchecked exceptions occur at runtime and are something programmers can manage using try-catch block

- finally block always execute regardless of what happened in the try block.

That's it, folks!!

Related items: Java Interview Questions and Java Basics

#java exceptions and exception handling #exception handling best practices #error in java #checked and uncheck exceptions in java

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jarviscodinghub · 5 years ago

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CSC 220 – Lab 8

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voidstarzero · 4 years ago

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DPoP with Spring Boot and Spring Security

Solid is an exciting project that I first heard about back in January. Its goal is to help “re-decentralize” the Web by empowering users to control access to their own data. Users set up “pods” to store their data, which applications can securely interact with using the Solid protocol. Furthermore, Solid documents are stored as linked data, which allows applications to interoperate more easily, hopefully leading to less of the platform lock-in that exists with today’s Web.

I’ve been itching to play with this for months, and finally got some free time over the past few weekends to try building a Solid app. Solid's authentication protocol, Solid OIDC, is built on top of regular OIDC with a mechanism called DPoP, or "Demonstration of Proof of Possession". While Spring Security makes it fairly easy to configure OIDC providers and clients, it doesn't yet have out-of-the-box support for DPoP. This post is a rough guide on adding DPoP to a Spring Boot app using Spring Security 5, which gets a lot of the way towards implementing the Solid OIDC flow. The full working example can be found here.

DPoP vs. Bearer Tokens

What's the point of DPoP? I will admit it's taken me a fair amount of reading and re-reading over the past several weeks to feel like I can grasp what DPoP is about. My understanding thus far: If a regular bearer token is stolen, it can potentially be used by a malicious client to impersonate the client that it was intended for. Adding audience information into the token mitigates some of the danger, but also constrains where the token can be used in a way that might be too restrictive. DPoP is instead an example of a "sender-constrained" token pattern, where the access token contains a reference to an ephemeral public key, and every request where it's used must be additionally accompanied by a request-specific token that's signed by the corresponding private key. This proves that the client using the access token also possesses the private key for the token, which at least allows the token to be used with multiple resource servers with less risk of it being misused.

So, the DPoP auth flow differs from Spring's default OAuth2 flow in two ways: the initial token request contains more information than the usual token request; and, each request made by the app needs to create and sign a JWT that will accompany the request in addition to the access token. Let's take a look at how to implement both of these steps.

Overriding the Token Request

In the authorization code grant flow for requesting access tokens, the authorization process is kicked off by the client sending an initial request to the auth server's authorization endpoint. The auth server then responds with a code, which the client includes in a final request to the auth server's token endpoint to obtain its tokens. Solid OIDC recommends using a more secure variation on this exchange called PKCE ("Proof Key for Code Exchange"), which adds a code verifier into the mix; the client generates a code verifier and sends its hash along with the authorization request, and when it makes its token request, it must also include the original code verifier so that the auth server can confirm that it originated the authorization request.

Spring autoconfigures classes that implement both the authorization code grant flow and the PKCE variation, which we can reuse for the first half of our DPoP flow. What we need to customize is the second half -- the token request itself.

To do this we implement the OAuth2AccessTokenResponseClient interface, parameterized with OAuth2AuthorizationCodeGrantRequest since DPoP uses the authorization code grant flow. (For reference, the default implementation provided by Spring can be found in the DefaultAuthorizationCodeTokenResponseClient class.) In the tokenRequest method of our class, we do the following:

retrieve the code verifier generated during the authorization request

retrieve the code received in response to the authorization request

generate an ephemeral key pair, and save it somewhere the app can access it during the lifetime of the session

construct a JWT with request-specific info, and sign it using our generated private key

make a request to the token endpoint using the above data, and return the result as an OAuth2AccessTokenResponse.

Here's the concrete implementation of all of that. We get the various data that we need from the OAuth2AuthorizationCodeGrantRequest object passed to our method. We then call on RequestContextHolder to get the current session ID and use that to save the session keys we generate to a map in the DPoPUtils bean. We create and sign a JWT which goes into the DPoP header, make the token request, and finally convert the response to an OAuth2AccessTokenResponse.

Using the DPoP Access Token

Now, to make authenticated requests to a Solid pod our app will need access to both an Authentication object (provided automatically by Spring) containing the DPoP access token obtained from the above, as well as DPoPUtils for the key pair needed to use the token.

On each request, the application must generate a fresh JWT and place it in a DPoP header as demonstrated by the authHeaders method below:

private fun authHeaders( authToken: String, sessionId: String, method: String, requestURI: String ): HttpHeaders { val headers = HttpHeaders() headers.add("Authorization", "DPoP $authToken") dpopUtils.sessionKey(sessionId)?.let { key -> headers.add("DPoP", dpopUtils.dpopJWT(method, requestURI, key)) } return headers }

The body of the JWT created by DPoPUtils#dpopJWT contains claims that identify the HTTP method and the target URI of the request:

private fun payload(method: String, targetURI: String) : JWTClaimsSet = JWTClaimsSet.Builder() .jwtID(UUID.randomUUID().toString()) .issueTime(Date.from(Instant.now())) .claim("htm", method) .claim("htu", targetURI) .build()

A GET request, for example, would then look something like this:

val headers = authHeaders( authToken, sessionId, "GET", requestURI ) val httpEntity = HttpEntity(headers) val response = restTemplate.exchange( requestURI, HttpMethod.GET, httpEntity, String::class.java )

A couple of last things to note: First, the session ID passed to the above methods is not retrieved from RequestContextHolder as before, but from the Authentication object provided by Spring:

val sessionId = ((authentication as OAuth2AuthenticationToken) .details as WebAuthenticationDetails).sessionId

And second, we want the ephemeral keys we generate during the token request to be removed from DPoPUtils when the session they were created for is destroyed. To accomplish this, we create an HttpSessionListener and override its sessionDestroyed method:

@Component class KeyRemovalSessionListener( private val dPoPUtils: DPoPUtils ) : HttpSessionListener { override fun sessionDestroyed(se: HttpSessionEvent) { val securityContext = se.session .getAttribute("SPRING_SECURITY_CONTEXT") as SecurityContextImpl val webAuthDetails = securityContext.authentication.details as WebAuthenticationDetails val sessionId = webAuthDetails.sessionId dPoPUtils.removeSessionKey(sessionId) } }

This method will be invoked on user logout as well as on session timeout.

#solidproject #Linked Data #oidc #spring #spring security #kotlin

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