1 /*
   2  * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package com.sun.istack.internal;
  27 
  28 import java.util.concurrent.ConcurrentLinkedQueue;
  29 import java.lang.ref.WeakReference;
  30 
  31 /**
  32  * Pool of reusable objects that are indistinguishable from each other,
  33  * such as JAXB marshallers.
  34  *
  35  * @author Kohsuke Kawaguchi
  36  */
  37 public interface Pool<T> {
  38 
  39     /**
  40      * Gets a new object from the pool.
  41      *
  42      * <p>
  43      * If no object is available in the pool, this method creates a new one.
  44      */
  45     @NotNull T take();
  46 
  47     /**
  48      * Returns an object back to the pool.
  49      */
  50     void recycle(@NotNull T t);
  51 
  52     /**
  53      * Default implementation that uses {@link ConcurrentLinkedQueue}
  54      * as the data store.
  55      *
  56      * <h2>Note for Implementors</h2>
  57      * <p>
  58      * Don't rely on the fact that this class extends from {@link ConcurrentLinkedQueue}.
  59      */
  60     public abstract class Impl<T> implements Pool<T> {
  61 
  62         private volatile WeakReference<ConcurrentLinkedQueue<T>> queue;
  63 
  64         /**
  65          * Gets a new object from the pool.
  66          *
  67          * <p>
  68          * If no object is available in the pool, this method creates a new one.
  69          *
  70          * @return
  71          *      always non-null.
  72          */
  73         public final @NotNull T take() {
  74             T t = getQueue().poll();
  75             if(t==null) {
  76                 return create();
  77             }
  78             return t;
  79         }
  80 
  81         /**
  82          * Returns an object back to the pool.
  83          */
  84         public final void recycle(T t) {
  85             getQueue().offer(t);
  86         }
  87 
  88         private ConcurrentLinkedQueue<T> getQueue() {
  89             WeakReference<ConcurrentLinkedQueue<T>> q = queue;
  90             if (q != null) {
  91                 ConcurrentLinkedQueue<T> d = q.get();
  92                 if (d != null) {
  93                     return d;
  94                 }
  95             }
  96             // overwrite the queue
  97             ConcurrentLinkedQueue<T> d = new ConcurrentLinkedQueue<T>();
  98             queue = new WeakReference<ConcurrentLinkedQueue<T>>(d);
  99 
 100             return d;
 101         }
 102 
 103         /**
 104          * Creates a new instance of object.
 105          *
 106          * <p>
 107          * This method is used when someone wants to
 108          * {@link #take() take} an object from an empty pool.
 109          *
 110          * <p>
 111          * Also note that multiple threads may call this method
 112          * concurrently.
 113          */
 114         protected abstract @NotNull T create();
 115     }
 116 }