1 /*
   2  * Copyright (c) 2016, 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;
  27 
  28 import java.lang.ref.Reference;
  29 import java.lang.ref.SoftReference;
  30 import java.lang.ref.WeakReference;
  31 
  32 /**
  33  * This abstract ReentrantContextProvider helper class manages the creation,
  34  * storage, and retrieval of concrete ReentrantContext instances which can be
  35  * subclassed to hold cached contextual data.
  36  *
  37  * It supports reentrancy as every call to acquire() provides a new unique context
  38  * instance that must later be returned for reuse by a call to release(ctx)
  39  * (typically in a try/finally block).
  40  *
  41  * It has a couple of abstract implementations which store references in a queue
  42  * and/or thread-local storage.
  43  * The Providers can be configured to hold ReentrantContext instances in memory
  44  * using hard, soft or weak references.
  45  *
  46  * The acquire() and release() methods are used to retrieve and return the contexts.
  47  *
  48  * The {@code newContext()} method remains abstract in all implementations and
  49  * must be provided by the module to create a new subclass of ReentrantContext
  50  * with the appropriate contextual data in it.
  51  *
  52  * Sample Usage:
  53  * - create a subclass ReentrantContextImpl to hold the thread state:
  54  *
  55  * static final class ReentrantContextImpl extends ReentrantContext {
  56  *     // specific cached data
  57  * }
  58  *
  59  * - create the appropriate ReentrantContextProvider:
  60  *
  61  * private static final ReentrantContextProvider<ReentrantContextImpl> contextProvider =
  62  *     new ReentrantContextProviderTL<ReentrantContextImpl>(ReentrantContextProvider.REF_WEAK)
  63  *     {
  64  *         @Override
  65  *         protected ReentrantContextImpl newContext() {
  66  *             return new ReentrantContextImpl();
  67  *         }
  68  *     };
  69  * ...
  70  * void someMethod() {
  71  *     ReentrantContextImpl ctx = contextProvider.acquire();
  72  *     try {
  73  *         // use the context
  74  *     } finally {
  75  *         contextProvider.release(ctx);
  76  *     }
  77  * }
  78  *
  79  * @param <K> ReentrantContext subclass
  80  *
  81  * @see ReentrantContext
  82  */
  83 public abstract class ReentrantContextProvider<K extends ReentrantContext>
  84 {
  85     // thread-local storage: inactive
  86     static final byte USAGE_TL_INACTIVE = 0;
  87     // thread-local storage: in use
  88     static final byte USAGE_TL_IN_USE = 1;
  89     // CLQ storage
  90     static final byte USAGE_CLQ = 2;
  91 
  92     // hard reference
  93     public static final int REF_HARD = 0;
  94     // soft reference
  95     public static final int REF_SOFT = 1;
  96     // weak reference
  97     public static final int REF_WEAK = 2;
  98 
  99     /* members */
 100     // internal reference type
 101     private final int refType;
 102 
 103     /**
 104      * Create a new ReentrantContext provider using the given reference type
 105      * among hard, soft or weak
 106      *
 107      * @param refType reference type
 108      */
 109     protected ReentrantContextProvider(final int refType) {
 110         this.refType = refType;
 111     }
 112 
 113     /**
 114      * Create a new ReentrantContext instance
 115      *
 116      * @return new ReentrantContext instance
 117      */
 118     protected abstract K newContext();
 119 
 120     /**
 121      * Give a ReentrantContext instance for the current thread
 122      *
 123      * @return ReentrantContext instance
 124      */
 125     public abstract K acquire();
 126 
 127     /**
 128      * Restore the given ReentrantContext instance for reuse
 129      *
 130      * @param ctx ReentrantContext instance
 131      */
 132     public abstract void release(K ctx);
 133 
 134     @SuppressWarnings("unchecked")
 135     protected final Reference<K> getOrCreateReference(final K ctx) {
 136         if (ctx.reference == null) {
 137             // Create the reference:
 138             switch (refType) {
 139                 case REF_HARD:
 140                     ctx.reference = new HardReference<K>(ctx);
 141                     break;
 142                 case REF_SOFT:
 143                     ctx.reference = new SoftReference<K>(ctx);
 144                     break;
 145                 default:
 146                 case REF_WEAK:
 147                     ctx.reference = new WeakReference<K>(ctx);
 148                     break;
 149             }
 150         }
 151         return (Reference<K>) ctx.reference;
 152     }
 153 
 154     /* Missing HardReference implementation */
 155     static final class HardReference<V> extends WeakReference<V> {
 156         // kept strong reference:
 157         private final V strongRef;
 158 
 159         HardReference(final V referent) {
 160             // no referent needed for the parent WeakReference:
 161             super(null);
 162             this.strongRef = referent;
 163         }
 164 
 165         @Override
 166         public V get() {
 167             return strongRef;
 168         }
 169     }
 170 }
--- EOF ---