JUnit5 Programmatic Extension Registration with @RegisterExtension

1. Overview

JUnit 5 provides multiple methods for registering extensions. For an overview of some of these methods, refer to our Guide to JUnit 5 Extensions.

In this quick tutorial, we’ll focus on programmatic registration of JUnit 5 extensions, using the @RegisterExtension annotation.

2. @RegisterExtension

We can apply this annotation to fields in test classes. One advantage of this method is that we can access the extension as an object in the test content directly. 

JUnit will call the extension methods at appropriate stages.

For example, if an extension implements BeforeEachCallback, JUnit will call its corresponding interface methods before executing a test method.

3. Using @RegisterExtension with Static Fields

When used with static fields, JUnit will apply the methods of this extension after the class-level @ExtendWith based extensions have been applied.

Also, JUnit will invoke both class-level and method-level callbacks of the extension.

For example, the following extension features both a beforeAll and a beforeEach implementation:

public class LoggingExtension implements 
  BeforeAllCallback, BeforeEachCallback {

    // logger, constructor etc

    @Override
    public void beforeAll(ExtensionContext extensionContext) 
      throws Exception {
        logger.info("Type {} In beforeAll : {}", 
          type, extensionContext.getDisplayName());
    }

    @Override
    public void beforeEach(ExtensionContext extensionContext) throws Exception {
        logger.info("Type {} In beforeEach : {}",
          type, extensionContext.getDisplayName());
    }

    public String getType() {
        return type;
    }
}

Let’s apply this extension to a static field of a test:

public class RegisterExtensionTest {

    @RegisterExtension
    static LoggingExtension staticExtension = new LoggingExtension("static version");

    @Test
    public void demoTest() {
        // assertions
    }
}

The output shows messages from both the beforeAll and beforeEach methods:

Type static version In beforeAll : RegisterExtensionTest
Type static version In beforeEach : demoTest()

4. Using @RegisterExtension with Instance Fields

If we use RegisterExtension with non-static fields, JUnit will only apply the extension after processing all TestInstancePostProcessor callbacks.

In this case, JUnit will ignore class level callbacks like beforeAll.

In the above example, let’s remove the static modifier from LoggingExtension:

@RegisterExtension
LoggingExtension instanceLevelExtension = new LoggingExtension("instance version");

Now JUnit will only invoke the beforeEach method, as seen in the output:

Type instance version In beforeEach : demoTest()

5. Conclusion

In this article, we did an overview of programmatically registering JUnit 5 extensions with @RegisterExtension.

We also covered the difference between applying the extension to static fields vs. instance fields.

As usual, code examples can be found at our Github repository.