Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.
Get started with mocking and improve your application tests using our Mockito guide:
Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.
Get started with understanding multi-threaded applications with our Java Concurrency guide:
Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:
Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.
But these can also be overused and fall into some common pitfalls.
To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:
Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:
Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.
I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.
You can explore the course here:
Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.
Get started with Spring Data JPA through the guided reference course:
Refactor Java code safely — and automatically — with OpenRewrite.
Refactoring big codebases by hand is slow, risky, and easy to put off. That’s where OpenRewrite comes in. The open-source framework for large-scale, automated code transformations helps teams modernize safely and consistently.
Each month, the creators and maintainers of OpenRewrite at Moderne run live, hands-on training sessions — one for newcomers and one for experienced users. You’ll see how recipes work, how to apply them across projects, and how to modernize code with confidence.
Join the next session, bring your questions, and learn how to automate the kind of work that usually eats your sprint time.
Regression testing is an important step in the release process, to ensure that new code doesn't break the existing functionality. As the codebase evolves, we want to run these tests frequently to help catch any issues early on.
The best way to ensure these tests run frequently on an automated basis is, of course, to include them in the CI/CD pipeline. This way, the regression tests will execute automatically whenever we commit code to the repository.
In this tutorial, we'll see how to create regression tests using Selenium, and then include them in our pipeline using GitHub Actions:, to be run on the LambdaTest cloud grid:
Regression testing is an important step in the release process, to ensure that new code doesn't break the existing functionality. As the codebase evolves, we want to run these tests frequently to help catch any issues early on.
The best way to ensure these tests run frequently on an automated basis is, of course, to include them in the CI/CD pipeline. This way, the regression tests will execute automatically whenever we commit code to the repository.
In this tutorial, we'll see how to create regression tests using Selenium, and then include them in our pipeline using GitHub Actions:, to be run on the LambdaTest cloud grid:
1. Overview
In this article, we’ll understand why the java.lang.NoClassDefFoundError occurs in JUnit and how to fix it. This issue is mainly related to IDE’s configurations. Therefore, we’ll be focusing on the most popular IDEs: Visual Studio Code, Eclipse, and IntelliJ to reproduce and resolve this error.
2. What Is java.lang.NoClassDefFoundError?
When the Java Runtime runs a Java program, it does not load all the classes and dependencies at once. Instead, it calls upon the Java Classloader to load classes in memory as-and-when-required. While loading a class, if the Classloader cannot find the class’s definition, it throws the NoClassDefFoundError.
There are a couple of reasons for which Java cannot find the class’s definition, which are:
- Missing a few dependent jars which is the most common reason.
- All jars are added as dependencies but in the wrong path.
- Version mismatches in the dependencies.
3. VS Code
For writing Junit4 test cases, we require the Junit4 jar. However, the Junit4 has an internal dependency on the hamcrest-core jar.
If we miss adding the hamcrest-core jar as a dependency in our classpath, Java throws the NoClassDefFoundError. The classpath is as follows:
One other scenario is when we added both the jars, but the versions don’t match. For example, if we have added JUnit jar version 4.13.2 and the hamcrest-core jar version 2.2, the NoClassDefFoundError is thrown:
In both cases, the same stack trace is printed:
java.lang.NoClassDefFoundError: org/hamcrest/SelfDescribing
at java.base/java.lang.ClassLoader.defineClass1(Native Method)
at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1010)
at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:855)
at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:753)
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:676) ...To resolve the error in both the scenarios (missing dependencies and version mismatch), we need to add the correct dependencies. The correct dependencies in the case of Junit4 are junit-4.13.2.jar and hamcrest-core-1.3.jar. Adding these two jars in the dependencies (Referenced Libraries) resolves the error. The instructions to add and remove external jars in VS Code are present here. Our referenced library section should be set up as:
4. Eclipse
In Eclipse IDE that supports Java 9 and above, we have a classpath and a module path. To resolve module dependency, we use the module path. However, adding external jars in the module path does not make them available for the class loader. Hence the class loader considers them as missing dependencies and throws the NoClassDefFoundError.
Hence if our dependency looks like the below image, running a Junit test case results in a NoClassDefFoundError:
The stack trace generated on running the JUnit test is:
java.lang.NoClassDefFoundError: org/junit/runner/manipulation/Filter
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:377)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.loadTestLoaderClass(RemoteTestRunner.java:381)In Eclipse, we need to add the jars under the classpath and not in the module path. So, to add external jars correctly, follow the path:
right-click on the Project -> Build Path -> Configure Build Path
In the window that opens, remove the jars from under the module path and add them under the classpath. This resolves the NoClassDefFoundError. The correct classpath for running JUnit should be similar to:
5. IntelliJ
Running JUnit 5 test cases requires both the Jupiter engine and Jupiter API. The Jupiter engine is internally dependent on the Jupiter API, and hence most of the time, it is sufficient to add just the Jupiter engine dependency in the pom.xml. However, Adding only the Jupiter API dependency in our pom.xml and missing the Jupiter engine dependency results in the NoClassDefFoundError.
The incorrect setup in the pom.xml would be like this:
<dependencies>
<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter-api</artifactId>
<version>5.11.0-M2</version>
<scope>test</scope>
</dependency>
</dependencies>Running a simple test case with this setup results in the following stack trace:
Exception in thread "main" java.lang.NoClassDefFoundError: org/junit/platform/engine/TestDescriptor
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Class.java:375)
at com.intellij.rt.junit.JUnitStarter.getAgentClass(JUnitStarter.java:230)
....In IntelliJ, to correct the dependencies, we need to correct the pom.xml. The corrected pom.xml looks like this:
<dependencies>
<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter-api</artifactId>
<version>5.11.0-M2</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter-engine</artifactId>
<version>5.11.0-M2</version>
<scope>test</scope>
</dependency>
</dependencies>Alternatively, we can add junit-jupiter-engine since adding it automatically adds the junit-jupiter-api jar to the classpath and resolves the error.
6. Summary
In this article, we saw different reasons for the java.lang.NoClassDefFoundError to occur in JUnit. We also saw how we resolve the error in different IDEs. The entire code for this tutorial is available over on GitHub.
