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What is the difference between @ExtendWith and @RegisterExtension in JUnit 6?

JUnit 6 provides two ways to register extensions. The choice between them depends on whether you need programmatic construction (with constructor arguments) or simple declarative annotation use.

@ExtendWith vs @RegisterExtension
Aspect@ExtendWith@RegisterExtension
LocationAnnotation on class or methodField in the test class
Extension constructionJUnit calls default no-arg constructorDeveloper constructs with custom arguments
When to useStateless extensions with no configurationExtensions that need constructor parameters or runtime configuration
Lifecycle controlLimitedFull: static field = class scope; instance field = method scope
Example@ExtendWith(MockitoExtension.class)@RegisterExtension static WireMockExtension wm = WireMockExtension.newInstance().port(8080).build();
// @ExtendWith: declarative, no constructor args
@ExtendWith(MockitoExtension.class)
class MockitoTest {
    @Mock
    UserRepository repo;

    @Test
    void test() { ... }
}

// @RegisterExtension: programmatic, with constructor args
class WireMockTest {

    // Static field -> class-scoped lifecycle (like @BeforeAll/@AfterAll)
    @RegisterExtension
    static WireMockExtension wireMock = WireMockExtension.newInstance()
        .options(wireMockConfig().port(8080))
        .build();

    // Instance field -> method-scoped lifecycle (fresh per test)
    @RegisterExtension
    DatabaseExtension db = new DatabaseExtension("jdbc:h2:mem:test");

    @Test
    void callsExternalService() {
        wireMock.stubFor(get("/api/users").willReturn(okJson("[]")));
        // ...
    }
}

// Multiple extensions can be combined:
@ExtendWith({MockitoExtension.class, SpringExtension.class})
class SpringMockitoTest { ... }

When should you prefer @RegisterExtension over @ExtendWith?
What is the lifecycle difference between a static @RegisterExtension field and an instance (non-static) @RegisterExtension field?

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