A Guide to JUnit 5 Extensions

In this article, we will explore JUnit 5 extensions, a powerful way to customize and extend the functionality of the JUnit 5 testing framework. These extensions allow you to include custom actions at any point in the test lifecycle, such as setup, teardown, and test execution.

JUnit 5 Extensions

JUnit 5 introduces a flexible extension model that can modify test execution behavior, inject dependencies, and manage shared resources. Extensions can be triggered at various stages of the testing lifecycle, including before and after all tests or individual tests and when specific events occur (e.g., exceptions, parameter resolution).

The JUnit 5 architecture consists of three components:

• JUnit Platform: A foundation for running and discovering tests.
• JUnit Jupiter: The programming and extension model in JUnit 5.
• JUnit Vintage: Allows backward compatibility for running JUnit 3 and 4 tests on the JUnit 5 platform.

JUnit 5 requires Java 8 or later to run, but it can still be used to test code written in older Java versions.

JUnit 5 = JUnit Jupiter + JUnit Vintage + JUnit Platform

JUnit Jupiter and Vintage

JUnit Jupiter is the programming and extension model that introduces new features and flexibility for writing tests and creating custom extensions. It uses a TestEngine to execute tests on the JUnit 5 platform.

JUnit Vintage allows older tests (written with JUnit 3 and 4) to run on JUnit 5. It requires JUnit 4.12 or later to be available in the classpath, ensuring seamless migration to JUnit 5 while maintaining the ability to run legacy tests.

Common Use Cases for JUnit 5 Extensions

JUnit 5 extensions can be used for:

• Customizing test behavior.
• Injecting dependencies during test execution.
• Modifying the flow of test execution.
• Setting up or tearing down shared resources.

Below are some key extensions provided by JUnit 5:

1. BeforeAllCallback

The BeforeAllCallback extension allows executing code before all tests in a test class are run. It is commonly used for global setup tasks, such as initializing database connections or setting up external resources.

@ExtendWith(GlobalSetupExtension.class)
class MyTest {

    @Test
    void testExample() {
        System.out.println("Running test example.");
    }
}

Output:

Global setup before all tests.
Running test example.

2. AfterAllCallback

The AfterAllCallback extension is executed after all the test methods in a test class have finished. This is useful for global cleanup tasks, such as closing database connections or releasing other resources.

@ExtendWith(GlobalTeardownExtension.class)
class MyTest {

    @Test
    void testExample() {
        System.out.println("Running test example.");
    }
}

Output:

Running test example.
Global teardown after all tests.

3. BeforeEachCallback

BeforeEachCallback is executed before each test method in a test class. This extension is useful for preparing resources or resetting states before each test, such as resetting mocks or reinitializing test data.

@ExtendWith(SetupBeforeEachExtension.class)
class MyTest {

    @Test
    void testMethod1() {
        System.out.println("Running test method 1.");
    }

    @Test
    void testMethod2() {
        System.out.println("Running test method 2.");
    }
}

Output:

Setting up before each test.
Running test method 1.

Setting up before each test.
Running test method 2.

4. AfterEachCallback

The AfterEachCallback extension runs after each test method finishes. It is typically used for cleanup tasks such as closing file streams, resetting states, or clearing temporary data after each test execution.

  @ExtendWith(TeardownAfterEachExtension.class)
class MyTest {

    @Test
    void testMethod1() {
        System.out.println("Running test method 1.");
    }

    @Test
    void testMethod2() {
        System.out.println("Running test method 2.");
    }
}

Output:

Running test method 1.
Cleaning up after each test.

Running test method 2.
Cleaning up after each test.

Conclusion

JUnit 5’s extension model provides a flexible way to customize and extend the test execution process. Extensions like BeforeAllCallback, BeforeEachCallback, AfterEachCallback, and more, help with setting up resources, injecting dependencies, and cleaning up after tests. With these capabilities, you can create robust, maintainable, and efficient test suites tailored to your specific needs.