What is the CancellationToken API introduced in JUnit 6?

One of the headline new features in JUnit 6 is the CancellationToken API -- a cooperative cancellation mechanism that allows launchers (Gradle, Maven, IntelliJ, GitHub Actions) to signal a running test suite to stop cleanly, without killing the JVM process.

The problem it solves: in JUnit 5, there was no clean way to abort a long-running test suite mid-run. Build tools had to forcibly kill the JVM, which skipped teardown methods, left database connections open, and could leave the CI environment in an inconsistent state.

// JUnit 6: CancellationToken -- test and extension authors can check
// for cancellation between steps and abort cleanly.

// Extensions can check cancellation:
class CleanupExtension implements BeforeEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        // Check if the test run has been cancelled before starting setup
        if (context.getCancellationToken().isCancellationRequested()) {
            return;  // skip setup gracefully
        }
        // ... normal setup ...
    }
}

// --fail-fast mode in ConsoleLauncher:
// The first test failure triggers a CancellationToken that propagates
// through the test suite, stopping remaining tests cleanly.
java -jar junit-platform-console-standalone-6.1.1.jar \
    --scan-class-path \
    --fail-fast
// Equivalent in Maven Surefire:
// <configuration>
//   <runOrder>random</runOrder>
//   <forkCount>1</forkCount>
// </configuration>
// (use --fail-at-end or failIfNoTests per tool docs)

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