001 /* ThreadLocal -- a variable with a unique value per thread
002 Copyright (C) 2000, 2002, 2003, 2006 Free Software Foundation, Inc.
003
004 This file is part of GNU Classpath.
005
006 GNU Classpath is free software; you can redistribute it and/or modify
007 it under the terms of the GNU General Public License as published by
008 the Free Software Foundation; either version 2, or (at your option)
009 any later version.
010
011 GNU Classpath is distributed in the hope that it will be useful, but
012 WITHOUT ANY WARRANTY; without even the implied warranty of
013 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
014 General Public License for more details.
015
016 You should have received a copy of the GNU General Public License
017 along with GNU Classpath; see the file COPYING. If not, write to the
018 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019 02110-1301 USA.
020
021 Linking this library statically or dynamically with other modules is
022 making a combined work based on this library. Thus, the terms and
023 conditions of the GNU General Public License cover the whole
024 combination.
025
026 As a special exception, the copyright holders of this library give you
027 permission to link this library with independent modules to produce an
028 executable, regardless of the license terms of these independent
029 modules, and to copy and distribute the resulting executable under
030 terms of your choice, provided that you also meet, for each linked
031 independent module, the terms and conditions of the license of that
032 module. An independent module is a module which is not derived from
033 or based on this library. If you modify this library, you may extend
034 this exception to your version of the library, but you are not
035 obligated to do so. If you do not wish to do so, delete this
036 exception statement from your version. */
037
038 package java.lang;
039
040 import java.util.Map;
041
042
043 /**
044 * ThreadLocal objects have a different state associated with every
045 * Thread that accesses them. Every access to the ThreadLocal object
046 * (through the <code>get()</code> and <code>set()</code> methods)
047 * only affects the state of the object as seen by the currently
048 * executing Thread.
049 *
050 * <p>The first time a ThreadLocal object is accessed on a particular
051 * Thread, the state for that Thread's copy of the local variable is set by
052 * executing the method <code>initialValue()</code>.
053 * </p>
054 *
055 * <p>An example how you can use this:
056 * </p>
057 *
058 * <pre>
059 * class Connection
060 * {
061 * private static ThreadLocal owner = new ThreadLocal()
062 * {
063 * public Object initialValue()
064 * {
065 * return("nobody");
066 * }
067 * };
068 * ...
069 * }
070 * </pre>
071 *
072 * <p>Now all instances of connection can see who the owner of the currently
073 * executing Thread is by calling <code>owner.get()</code>. By default any
074 * Thread would be associated with 'nobody'. But the Connection object could
075 * offer a method that changes the owner associated with the Thread on
076 * which the method was called by calling <code>owner.put("somebody")</code>.
077 * (Such an owner changing method should then be guarded by security checks.)
078 * </p>
079 *
080 * <p>When a Thread is garbage collected all references to values of
081 * the ThreadLocal objects associated with that Thread are removed.
082 * </p>
083 *
084 * @author Mark Wielaard (mark@klomp.org)
085 * @author Eric Blake (ebb9@email.byu.edu)
086 * @since 1.2
087 * @status updated to 1.5
088 */
089 public class ThreadLocal<T>
090 {
091 /**
092 * Placeholder to distinguish between uninitialized and null set by the
093 * user. Do not expose this to the public. Package visible for use by
094 * InheritableThreadLocal
095 */
096 static final Object sentinel = new Object();
097
098 /**
099 * The base for the computation of the next hash for a thread local.
100 */
101 private static int nextHashBase = 1;
102
103 /**
104 * Allocate a new hash.
105 */
106 private synchronized int computeNextHash()
107 {
108 return nextHashBase++ * 6709;
109 }
110
111 /**
112 * Hash code computed for ThreadLocalMap
113 */
114 final int fastHash;
115
116 /**
117 * Creates a ThreadLocal object without associating any value to it yet.
118 */
119 public ThreadLocal()
120 {
121 constructNative();
122 fastHash = computeNextHash();
123 }
124
125 /**
126 * Called once per thread on the first invocation of get(), if set() was
127 * not already called. The default implementation returns <code>null</code>.
128 * Often, this method is overridden to create the appropriate initial object
129 * for the current thread's view of the ThreadLocal.
130 *
131 * @return the initial value of the variable in this thread
132 */
133 protected T initialValue()
134 {
135 return null;
136 }
137
138 /**
139 * Gets the value associated with the ThreadLocal object for the currently
140 * executing Thread. If this is the first time the current thread has called
141 * get(), and it has not already called set(), the value is obtained by
142 * <code>initialValue()</code>.
143 *
144 * @return the value of the variable in this thread
145 */
146 public native T get();
147
148 private final Object internalGet()
149 {
150 ThreadLocalMap map = Thread.getThreadLocals();
151 // Note that we don't have to synchronize, as only this thread will
152 // ever modify the map.
153 T value = (T) map.get(this);
154 if (value == sentinel)
155 {
156 value = initialValue();
157 map.set(this, value);
158 }
159 return value;
160 }
161
162 /**
163 * Sets the value associated with the ThreadLocal object for the currently
164 * executing Thread. This overrides any existing value associated with the
165 * current Thread and prevents <code>initialValue()</code> from being
166 * called if this is the first access to this ThreadLocal in this Thread.
167 *
168 * @param value the value to set this thread's view of the variable to
169 */
170 public native void set(T value);
171
172 private final void internalSet(Object value)
173 {
174 ThreadLocalMap map = Thread.getThreadLocals();
175 // Note that we don't have to synchronize, as only this thread will
176 // ever modify the map.
177 map.set(this, value);
178 }
179
180 /**
181 * Removes the value associated with the ThreadLocal object for the
182 * currently executing Thread.
183 * @since 1.5
184 */
185 public native void remove();
186
187 private final void internalRemove()
188 {
189 ThreadLocalMap map = Thread.getThreadLocals();
190 map.remove(this);
191 }
192
193 protected native void finalize () throws Throwable;
194
195 private native void constructNative();
196
197 private gnu.gcj.RawData TLSPointer;
198 }