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 }