PhantomReferentClearing.java

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/**
 * @test
 * @bug 8071507
 * @summary Test that PhantomReferences are cleared when notified.
 * @run main/othervm PhantomReferentClearing
 */

import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.util.ArrayList;
import java.util.List;

public class PhantomReferentClearing {

    private static final long ENQUEUE_TIMEOUT = 1000;   // 1 sec, in millis

    // P1 & P2 are PhantomReference objects
    // O1 & O2 are objects
    //
    // -> is a strong reference
    // => is a referent reference
    //
    //   root -> P1
    //   root -> P2
    //   root -> O1
    //   root -> O2
    //   O1 -> O2
    //   P1 => O1
    //   P2 => O2
    //
    // (1) Remove root -> O1 and collect.  P1 notified, P2 !notified.
    // (2) Remove root -> O2 and collect.
    //
    // If phantom references are cleared when notified, as proposed by
    // 8071507, then P2 should be notified, and the test passes.
    //
    // Otherwise, P2 does not get notified because it remains reachable
    // from O1, which is being retained by P1.  This fails the test.

    private static final ReferenceQueue<Object> Q1 = new ReferenceQueue<>();
    private static final ReferenceQueue<Object> Q2 = new ReferenceQueue<>();

    private static volatile Object O2 = new Object();
    private static volatile List<Object> O1 = new ArrayList<>();
    static {
        O1.add(O2);
    }

    private static final PhantomReference<Object> P1 = new PhantomReference<>(O1, Q1);
    private static final PhantomReference<Object> P2 = new PhantomReference<>(O2, Q2);

    public static void main(String[] args) throws InterruptedException {

        // Collect, and verify neither P1 or P2 notified.
        System.gc();
        if (Q1.remove(ENQUEUE_TIMEOUT) != null) {
            throw new RuntimeException("P1 already notified");
        } else if (Q2.poll() != null) {
            throw new RuntimeException("P2 already notified");
        }

        // Delete root -> O1, collect, verify P1 notified, P2 not notified.
        O1 = null;
        System.gc();
        if (Q1.remove(ENQUEUE_TIMEOUT) == null) {
            throw new RuntimeException("P1 not notified by O1 deletion");
        } else if (Q2.remove(ENQUEUE_TIMEOUT) != null) {
            throw new RuntimeException("P2 notified by O1 deletion.");
        }

        // Delete root -> O2, collect. P2 should be notified.
        O2 = null;
        System.gc();
        if (Q2.remove(ENQUEUE_TIMEOUT) == null) {
            throw new RuntimeException("P2 not notified by O2 deletion");
        }
    }
}