[COLLECTIONS-575] Synchronized queue wrapper in QueueUtils.

src/changes/changes.xml

@@ -75,6 +75,9 @@
     <action issue="COLLECTIONS-668" dev="ggregory" type="add" due-to="Gary Gregory">
       Add CollectionUtils containsAny method for primitive array: org.apache.commons.collections4.CollectionUtils.containsAny(Collection&lt;?>, T...).
     </action>
+    <action issue="COLLECTIONS-575" dev="ggregory" type="add" due-to="Guram Savinov, Grzegorz Rożniecki, Bruno P. Kinoshita, Gary Gregory">
+      Synchronized queue wrapper in QueueUtils.
+    </action>
   </release>
   <release version="4.1" date="2015-11-28" description="This is a security and minor release.">
     <action issue="COLLECTIONS-508" dev="tn" type="add">

src/main/java/org/apache/commons/collections4/QueueUtils.java

@@ -20,6 +20,7 @@ import java.util.LinkedList;
 import java.util.Queue;
 
 import org.apache.commons.collections4.queue.PredicatedQueue;
+import org.apache.commons.collections4.queue.SynchronizedQueue;
 import org.apache.commons.collections4.queue.TransformedQueue;
 import org.apache.commons.collections4.queue.UnmodifiableQueue;
 
@@ -43,6 +44,37 @@ public class QueueUtils {
 
     //-----------------------------------------------------------------------
 
+    /**
+     * Returns a synchronized (thread-safe) queue backed by the given queue.
+     * In order to guarantee serial access, it is critical that all access to the
+     * backing queue is accomplished through the returned queue.
+     * <p>
+     * It is imperative that the user manually synchronize on the returned queue
+     * when iterating over it:
+     *
+     * <pre>
+     * Queue queue = QueueUtils.synchronizedQueue(new CircularFifoQueue());
+     * ...
+     * synchronized(queue) {
+     *     Iterator i = queue.iterator(); // Must be in synchronized block
+     *     while (i.hasNext())
+     *         foo(i.next());
+     *     }
+     * }
+     * </pre>
+     *
+     * Failure to follow this advice may result in non-deterministic behavior.
+     *
+     * @param <E> the element type
+     * @param queue the queue to synchronize, must not be null
+     * @return a synchronized queue backed by that queue
+     * @throws NullPointerException if the queue is null
+     * @since 4.2
+     */
+    public static <E> Queue<E> synchronizedQueue(final Queue<E> queue) {
+        return SynchronizedQueue.synchronizedQueue(queue);
+    }
+
     /**
      * Returns an unmodifiable queue backed by the given queue.
      *

src/main/java/org/apache/commons/collections4/queue/SynchronizedQueue.java

@@ -0,0 +1,143 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.collections4.queue;
+
+import java.util.Queue;
+
+import org.apache.commons.collections4.collection.SynchronizedCollection;
+
+/**
+ * Decorates another {@link Queue} to synchronize its behaviour for a multi-threaded environment.
+ * <p>
+ * Methods are synchronized, then forwarded to the decorated queue. Iterators must be separately synchronized around the
+ * loop.
+ *
+ * @param <E> the type of the elements in the collection
+ * @since 4.2
+ */
+public class SynchronizedQueue<E> extends SynchronizedCollection<E> implements Queue<E> {
+
+    /** Serialization version */
+    private static final long serialVersionUID = 1L;
+
+    /**
+     * Factory method to create a synchronized queue.
+     *
+     * @param <E>
+     *            the type of the elements in the queue
+     * @param queue
+     *            the queue to decorate, must not be null
+     * @return a new synchronized Queue
+     * @throws NullPointerException
+     *             if queue is null
+     */
+    public static <E> SynchronizedQueue<E> synchronizedQueue(final Queue<E> queue) {
+        return new SynchronizedQueue<E>(queue);
+    }
+
+    // -----------------------------------------------------------------------
+    /**
+     * Constructor that wraps (not copies).
+     *
+     * @param queue
+     *            the queue to decorate, must not be null
+     * @throws NullPointerException
+     *             if queue is null
+     */
+    protected SynchronizedQueue(final Queue<E> queue) {
+        super(queue);
+    }
+
+    /**
+     * Constructor that wraps (not copies).
+     *
+     * @param queue
+     *            the queue to decorate, must not be null
+     * @param lock
+     *            the lock to use, must not be null
+     * @throws NullPointerException
+     *             if queue or lock is null
+     */
+    protected SynchronizedQueue(final Queue<E> queue, final Object lock) {
+        super(queue, lock);
+    }
+
+    /**
+     * Gets the queue being decorated.
+     *
+     * @return the decorated queue
+     */
+    @Override
+    protected Queue<E> decorated() {
+        return (Queue<E>) super.decorated();
+    }
+
+    @Override
+    public E element() {
+        synchronized (lock) {
+            return decorated().element();
+        }
+    }
+
+    @Override
+    public boolean equals(final Object object) {
+        if (object == this) {
+            return true;
+        }
+        synchronized (lock) {
+            return decorated().equals(object);
+        }
+    }
+
+    // -----------------------------------------------------------------------
+
+    @Override
+    public int hashCode() {
+        synchronized (lock) {
+            return decorated().hashCode();
+        }
+    }
+
+    @Override
+    public boolean offer(final E e) {
+        synchronized (lock) {
+            return decorated().offer(e);
+        }
+    }
+
+    @Override
+    public E peek() {
+        synchronized (lock) {
+            return decorated().peek();
+        }
+    }
+
+    @Override
+    public E poll() {
+        synchronized (lock) {
+            return decorated().poll();
+        }
+    }
+
+    @Override
+    public E remove() {
+        synchronized (lock) {
+            return decorated().remove();
+        }
+    }
+
+}

src/test/java/org/apache/commons/collections4/QueueUtilsTest.java

@@ -16,13 +16,16 @@
  */
 package org.apache.commons.collections4;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
 
 import java.util.LinkedList;
 import java.util.Queue;
 
 import org.apache.commons.collections4.functors.TruePredicate;
 import org.apache.commons.collections4.queue.PredicatedQueue;
+import org.apache.commons.collections4.queue.SynchronizedQueue;
 import org.apache.commons.collections4.queue.TransformedQueue;
 import org.apache.commons.collections4.queue.UnmodifiableQueue;
 import org.junit.Test;
@@ -38,6 +41,18 @@ public class QueueUtilsTest {
 
     // ----------------------------------------------------------------------
 
+    @Test
+    public void testSynchronizedQueue() {
+        Queue<Object> queue = QueueUtils.synchronizedQueue(new LinkedList<Object>());
+        assertTrue("Returned object should be a SynchronizedQueue.", queue instanceof SynchronizedQueue);
+        try {
+            QueueUtils.synchronizedQueue(null);
+            fail("Expecting NullPointerException for null queue.");
+        } catch (final NullPointerException ex) {
+            // expected
+        }
+    }    
+    
     @Test
     public void testUnmodifiableQueue() {
         Queue<Object> queue = QueueUtils.unmodifiableQueue(new LinkedList<>());

src/test/java/org/apache/commons/collections4/queue/SynchronizedQueueTest.java

@@ -0,0 +1,63 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.collections4.queue;
+
+import java.util.LinkedList;
+import java.util.Queue;
+
+import org.apache.commons.collections4.BulkTest;
+import org.junit.Ignore;
+
+import junit.framework.Test;
+
+/**
+ * Extension of {@link AbstractQueueTest} for exercising the
+ * {@link SynchronizedQueue} implementation.
+ *
+ * @since 4.2
+ */
+public class SynchronizedQueueTest<T> extends AbstractQueueTest<T> {
+
+    public static Test suite() {
+        return BulkTest.makeSuite(SynchronizedQueueTest.class);
+    }
+
+    public SynchronizedQueueTest(final String testName) {
+        super(testName);
+    }
+
+    //-----------------------------------------------------------------------
+
+    @Override
+    public String getCompatibilityVersion() {
+        return "4.2";
+    }
+
+    @Override
+    public Queue<T> makeObject() {
+        return SynchronizedQueue.synchronizedQueue(new LinkedList<T>());
+    }
+
+    @Ignore("Run once")
+    public void testCreate() throws Exception {
+        Queue<T> queue = makeObject();
+        writeExternalFormToDisk((java.io.Serializable) queue, "src/test/resources/data/test/SynchronizedQueue.emptyCollection.version4.2.obj");
+        queue = makeFullCollection();
+        writeExternalFormToDisk((java.io.Serializable) queue, "src/test/resources/data/test/SynchronizedQueue.fullCollection.version4.2.obj");
+    }
+
+}