Software Engineering Glossary

JUnit Extension

Also known as: JUnit 5 Extension Jupiter Extension @ExtendWith @RegisterExtension

Definition

A JUnit Extension is the Jupiter replacement for a JUnit 4 Rule. You implement callbacks such as BeforeEachCallback or AfterAllCallback, then register the extension with @ExtendWith on a class or with a @RegisterExtension field. Extensions wrap test execution the same way Rules did: shared setup, cleanup, parameter injection, and extra conditions, without copying setUp into every class. Current JUnit 6 docs still use this model on the JUnit Platform.

Key Takeaways

New tests on JUnit 5 or 6 should use Extensions, not org.junit.Rule.
@ExtendWith is declarative. @RegisterExtension is the instance-field style closest to @Rule.
@TempDir, assertThrows, @Timeout, and TestInfo replace the old built-in rules.
Spring Boot and Mockito register their own extensions instead of JUnit 4 rules.

How It Works

Jupiter looks for registered extensions and calls their callbacks around each test or class.
ExtensionContext gives you the test class, method, and a store for per-test state.
You can implement several callback interfaces on one class if you need before and after.
JUnit Vintage can still run old Rules on the platform, but it is a migration path, not the long-term API.

Where It Is Used

@ExtendWith(SpringExtension.class) or @SpringBootTest wires the Spring test context.
@ExtendWith(MockitoExtension.class) initializes @Mock fields.
A DatabaseResetExtension that implements BeforeEachCallback is the direct port of a DatabaseResetRule.

JUnit Extension

Definition

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Key Takeaways

How It Works

Where It Is Used

Related glossary terms

JUnit Rule

Java Annotation

Java Virtual Machine