Java example source code file (ReferenceEnqueuePending.java)

This example Java source code file (ReferenceEnqueuePending.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

end, failed, integer, interruptedexception, numberedweakreference, referenceenqueuepending, referencequeue, runtimeexception, test, thread

The ReferenceEnqueuePending.java Java example source code

/*
 * Copyright (c) 2011, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/* @test
 * @bug 4243978
 * @summary Test if Reference.enqueue() works properly with pending references
 */
import java.lang.ref.*;

public class ReferenceEnqueuePending {
    static class NumberedWeakReference extends WeakReference<Integer> {
        //  Add an integer to identify the weak reference object.
        int number;

        NumberedWeakReference(Integer referent, ReferenceQueue<Integer> q, int i) {
            super(referent, q);
            number = i;
        }
    }

    final static boolean debug = System.getProperty("test.debug") != null;
    final static int iterations = 1000;
    final static int gc_trigger = 99;
    static int[] a = new int[2 * iterations];
    // Keep all weak references alive with the following array.
    static NumberedWeakReference[] b = new NumberedWeakReference[iterations];

    public static void main(String[] argv) throws Exception {
        if (debug) {
            System.out.println("Starting the test.");
        }
        // Raise thread priority to match the referenceHandler
        // priority, so that they can race also on a uniprocessor.
        raisePriority();

        ReferenceQueue<Integer> refQueue = new ReferenceQueue<>();

        // Our objective is to let the mutator enqueue
        // a Reference object that may already be in the
        // pending state because of having been identified
        // as weakly reachable at a previous garbage collection.
        // To this end, we create many Reference objects, each with a
        // a unique integer object as its referant.
        // We let the referents become eligible for collection,
        // while racing with the garbage collector which may
        // have pended some of these Reference objects.
        // Finally we check that all of the Reference objects
        // end up on the their queue. The test was originally
        // submitted to show that such races could break the
        // pending list and/or the reference queue, because of sharing
        // the same link ("next") for maintaining both lists, thus
        // losing some of the Reference objects on either queue.

        Integer obj = new Integer(0);
        NumberedWeakReference weaky = new NumberedWeakReference(obj, refQueue, 0);
        for (int i = 1; i < iterations; i++) {
            // Create a new object, dropping the onlY strong reference to
            // the previous Integer object.
            obj = new Integer(i);
            // Trigger gc each gc_trigger iterations.
            if ((i % gc_trigger) == 0) {
                forceGc(0);
            }
            // Enqueue every other weaky.
            if ((i % 2) == 0) {
                weaky.enqueue();
            }
            // Remember the Reference objects, for testing later.
            b[i - 1] = weaky;
            // Get a new weaky for the Integer object just
            // created, which may be explicitly enqueued in
            // our next trip around the loop.
            weaky = new NumberedWeakReference(obj, refQueue, i);
        }

        // Do a final collection to discover and process all
        // Reference objects created above, allowing enough time
        // for the ReferenceHandler thread to queue the References.
        forceGc(100);
        forceGc(100);

        // Verify that all WeakReference objects ended up queued.
        checkResult(refQueue, obj, iterations-1);
        System.out.println("Test passed.");
    }

    private static void checkResult(ReferenceQueue<Integer> queue,
                                    Integer obj,
                                    int expected) {
        if (debug) {
            System.out.println("Reading the queue");
        }

        // Empty the queue and record numbers into a[];
        NumberedWeakReference weakRead = (NumberedWeakReference) queue.poll();
        int length = 0;
        while (weakRead != null) {
            a[length++] = weakRead.number;
            weakRead = (NumberedWeakReference) queue.poll();
        }
        if (debug) {
            System.out.println("Reference Queue had " + length + " elements");
        }
        // Use the last Reference object of those created above, so as to keep it "alive".
        System.out.println("I must write " + obj + " to prevent compiler optimizations.");


        // verify the queued references: all but the last Reference object
        // should have been in the queue.
        if (debug) {
            System.out.println("Start of final check");
        }

        // Sort the first "length" elements in array "a[]".
        sort(length);

        boolean fail = (length != expected);
        for (int i = 0; i < length; i++) {
            if (a[i] != i) {
                if (debug) {
                    System.out.println("a[" + i + "] is not " + i + " but " + a[i]);
                }
                fail = true;
            }
        }
        if (fail) {
             printMissingElements(length, expected);
             throw new RuntimeException("TEST FAILED: only " + length
                    + " reference objects have been queued out of "
                    + expected);
        }
    }

    private static void printMissingElements(int length, int expected) {
        System.out.println("The following numbers were not found in the reference queue: ");
        int missing = 0;
        int element = 0;
        for (int i = 0; i < length; i++) {
            while ((a[i] != element) & (element < expected)) {
                System.out.print(element + " ");
                if (missing % 20 == 19) {
                    System.out.println(" ");
                }
                missing++;
                element++;
            }
            element++;
        }
        System.out.print("\n");
    }

    private static void forceGc(long millis) throws InterruptedException {
        Runtime.getRuntime().gc();
        Thread.sleep(millis);
    }

    // Bubble sort the first "length" elements in array "a".
    private static void sort(int length) {
        int hold;
        if (debug) {
            System.out.println("Sorting. Length=" + length);
        }
        for (int pass = 1; pass < length; pass++) {    // passes over the array
            for (int i = 0; i < length - pass; i++) {  //  a single pass
                if (a[i] > a[i + 1]) {  // then swap
                    hold = a[i];
                    a[i] = a[i + 1];
                    a[i + 1] = hold;
                }
            }  // End of i loop
        } // End of pass loop
    }

    // Raise thread priority so as to increase the
    // probability of the mutator succeeding in enqueueing
    // an object that is still in the pending state.
    // This is (probably) only required for a uniprocessor.
    static void raisePriority() {
        Thread tr = Thread.currentThread();
        tr.setPriority(Thread.MAX_PRIORITY);
    }
}   // End of class ReferenceEnqueuePending