What is the @RegisterExtension annotation and how does it differ from @ExtendWith?

// @ExtendWith: declarative, no configuration
@ExtendWith(MockitoExtension.class)
class SimpleTest { ... }

// @RegisterExtension: programmatic, configurable
class ConfigurableTest {

    // Static field: extension scope covers ALL tests (like @BeforeAll)
    @RegisterExtension
    static WireMockExtension server =
        WireMockExtension.newInstance()
            .options(WireMockConfiguration.options().port(8089))
            .build();

    // Instance field: extension scope covers EACH test (like @BeforeEach)
    @RegisterExtension
    TempDirectory tempDir = new TempDirectory();

    @Test
    void callsExternalApi() {
        server.stubFor(get("/orders")
            .willReturn(aResponse().withStatus(200)));
        // test code
    }

    @Test
    void writesToTempFile() throws IOException {
        Path file = tempDir.getRoot().resolve("output.txt");
        Files.writeString(file, "hello");
        assertEquals("hello", Files.readString(file));
    }
}

// Key rule: static @RegisterExtension field
// -> extension callbacks run around the class (BeforeAll/AfterAll)
// Instance @RegisterExtension field
// -> callbacks run around each test (BeforeEach/AfterEach)