What is the @TempDir annotation and how has it changed in JUnit 6?

import org.junit.jupiter.api.io.TempDir;
import org.junit.jupiter.api.io.CleanupMode;

class TempDirDemo {

    // Field injection: directory shared across all tests in class
    @TempDir
    static Path sharedTempDir;

    // Parameter injection: fresh directory per test method
    @Test
    void writeAndReadFile(@TempDir Path tempDir) throws Exception {
        Path file = tempDir.resolve("output.txt");
        Files.writeString(file, "Hello JUnit 6");
        assertEquals("Hello JUnit 6", Files.readString(file));
        // tempDir is automatically deleted after this test
    }

    // JUnit 6: cleanup attribute -- control when directory is deleted
    @Test
    void keepDirOnFailure(
        @TempDir(cleanup = CleanupMode.ON_SUCCESS) Path debugDir
    ) throws Exception {
        Files.writeString(debugDir.resolve("log.txt"), "debug info");
        // If test PASSES: directory is deleted
        // If test FAILS:  directory is KEPT for inspection
    }

    // cleanup modes:
    // ALWAYS (default): always delete
    // ON_SUCCESS:       keep on failure (useful for debugging)
    // NEVER:            never delete (manual cleanup)
}