What is the TestExecutionListener SPI in JUnit 6?

// Implement a custom TestExecutionListener:
public class JsonReportListener implements TestExecutionListener {

    private final List<TestResult> results = new ArrayList<>();

    @Override
    public void testStarted(TestIdentifier id) {
        System.out.println("Starting: " + id.getDisplayName());
    }

    @Override
    public void executionFinished(
            TestIdentifier id,
            TestExecutionResult result) {
        results.add(new TestResult(
            id.getDisplayName(),
            result.getStatus()
        ));
    }

    @Override
    public void testPlanExecutionFinished(TestPlan plan) {
        // Write JSON report after all tests complete
        writeJsonReport(results);
    }
}

// Register via ServiceLoader (META-INF/services):
// File: src/test/resources/META-INF/services/
//   org.junit.platform.launcher.TestExecutionListener
// Content:
//   com.example.JsonReportListener

// Or register programmatically via LauncherDiscoveryRequest:
LauncherDiscoveryRequest request = LauncherDiscoveryRequestBuilder
    .request()
    .selectors(selectPackage("com.example"))
    .build();

Launcher launcher = LauncherFactory.create();
launcher.discover(request);
launcher.execute(request, new JsonReportListener());