jdk-24/test/jdk/java/lang/Thread/virtual/TracePinnedThreads.java

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/**
 * @test
 * @bug 8284161 8289284 8322846
 * @summary Basic test of debugging option to trace pinned threads
 * @requires vm.continuations
 * @library /test/lib
 * @run junit/othervm -Djdk.tracePinnedThreads=full TracePinnedThreads
 * @run junit/othervm -Djdk.tracePinnedThreads=short TracePinnedThreads
 */

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;
import java.time.Duration;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.LockSupport;

import jdk.test.lib.thread.VThreadRunner;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class TracePinnedThreads {
    static final Object lock = new Object();

    /**
     * Parks current thread for 1 second.
     */
    private static void park() {
        long nanos = Duration.ofSeconds(1).toNanos();
        LockSupport.parkNanos(nanos);
    }

    /**
     * Invokes the park method through a native frame to park the current
     * thread for 1 second.
     */
    private static native void invokePark();

    /**
     * Test parking inside synchronized block.
     */
    @Test
    void testPinnedCausedBySynchronizedBlock() throws Exception {
        String output = run(() -> {
            synchronized (lock) {
                park();
            }
        });
        assertContains(output, "reason:MONITOR");
        assertContains(output, "<== monitors:1");
    }

    /**
     * Test parking with native frame on stack.
     */
    @Test
    void testPinnedCausedByNativeMethod() throws Exception {
        System.loadLibrary("TracePinnedThreads");
        String output = run(() -> invokePark());
        assertContains(output, "reason:NATIVE");
        assertContains(output, "(Native Method)");
    }

    /**
     * Test parking in class initializer.
     */
    @Test
    void testPinnedCausedByClassInitializer() throws Exception {
        class C {
            static {
                park();
            }
        }
        String output = run(C::new);
        assertContains(output, "reason:NATIVE");
        assertContains(output, "<clinit>");
    }

    /**
     * Test contention writing to System.out when pinned. The test creates four threads
     * that write to System.out when pinned, this is enough to potentially deadlock
     * without the changes in JDK-8322846.
     */
    @Test
    void testContention() throws Exception {
        // use several classes to avoid duplicate stack traces
        class C1 {
            synchronized void print() {
                System.out.println("hello");
            }
        }
        class C2 {
            synchronized void print() {
                System.out.println("hello");
            }
        }
        class C3 {
            synchronized void print() {
                System.out.println("hello");
            }
        }
        class C4 {
            synchronized void print() {
                System.out.println("hello");
            }
        }

        try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
            executor.submit(() -> {
                new C1().print();
            });
            executor.submit(() -> {
                new C2().print();
            });
            executor.submit(() -> {
                new C3().print();
            });
            executor.submit(() -> {
                new C4().print();
            });
        }
    }

    /**
     * Run a task in a virtual thread, returning a String with any output printed
     * to standard output.
     */
    private static String run(Runnable task) throws Exception {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        PrintStream original = System.out;
        System.setOut(new PrintStream(baos, true));
        try {
            VThreadRunner.run(task::run);
        } finally {
            System.setOut(original);
        }
        String output = new String(baos.toByteArray());
        System.out.println(output);
        return output;
    }

    /**
     * Tests that s1 contains s2.
     */
    private static void assertContains(String s1, String s2) {
        assertTrue(s1.contains(s2), s2 + " not found!!!");
    }

    /**
     * Tests that s1 does not contain s2.
     */
    private static void assertDoesNotContain(String s1, String s2) {
        assertFalse(s1.contains(s2), s2 + " found!!");
    }
}