1 /* ThreadLocal -- a variable with a unique value per thread
2    Copyright (C) 2000, 2002, 2003, 2006 Free Software Foundation, Inc.
3 
4 This file is part of GNU Classpath.
5 
6 GNU Classpath is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2, or (at your option)
9 any later version.
10 
11 GNU Classpath is distributed in the hope that it will be useful, but
12 WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
14 General Public License for more details.
15 
16 You should have received a copy of the GNU General Public License
17 along with GNU Classpath; see the file COPYING.  If not, write to the
18 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
19 02110-1301 USA.
20 
21 Linking this library statically or dynamically with other modules is
22 making a combined work based on this library.  Thus, the terms and
23 conditions of the GNU General Public License cover the whole
24 combination.
25 
26 As a special exception, the copyright holders of this library give you
27 permission to link this library with independent modules to produce an
28 executable, regardless of the license terms of these independent
29 modules, and to copy and distribute the resulting executable under
30 terms of your choice, provided that you also meet, for each linked
31 independent module, the terms and conditions of the license of that
32 module.  An independent module is a module which is not derived from
33 or based on this library.  If you modify this library, you may extend
34 this exception to your version of the library, but you are not
35 obligated to do so.  If you do not wish to do so, delete this
36 exception statement from your version. */
37 
38 package java.lang;
39 
40 import java.util.Map;
41 
42 
43 /**
44  * ThreadLocal objects have a different state associated with every
45  * Thread that accesses them. Every access to the ThreadLocal object
46  * (through the <code>get()</code> and <code>set()</code> methods)
47  * only affects the state of the object as seen by the currently
48  * executing Thread.
49  *
50  * <p>The first time a ThreadLocal object is accessed on a particular
51  * Thread, the state for that Thread's copy of the local variable is set by
52  * executing the method <code>initialValue()</code>.
53  * </p>
54  *
55  * <p>An example how you can use this:
56  * </p>
57  *
58  * <pre>
59  * class Connection
60  * {
61  *   private static ThreadLocal owner = new ThreadLocal()
62  *     {
63  *       public Object initialValue()
64  *       {
65  *         return("nobody");
66  *       }
67  *     };
68  * ...
69  * }
70  * </pre>
71  *
72  * <p>Now all instances of connection can see who the owner of the currently
73  * executing Thread is by calling <code>owner.get()</code>. By default any
74  * Thread would be associated with 'nobody'. But the Connection object could
75  * offer a method that changes the owner associated with the Thread on
76  * which the method was called by calling <code>owner.put("somebody")</code>.
77  * (Such an owner changing method should then be guarded by security checks.)
78  * </p>
79  *
80  * <p>When a Thread is garbage collected all references to values of
81  * the ThreadLocal objects associated with that Thread are removed.
82  * </p>
83  *
84  * @author Mark Wielaard (mark@klomp.org)
85  * @author Eric Blake (ebb9@email.byu.edu)
86  * @since 1.2
87  * @status updated to 1.5
88  */
89 public class ThreadLocal<T>
90 {
91   /**
92    * Placeholder to distinguish between uninitialized and null set by the
93    * user. Do not expose this to the public. Package visible for use by
94    * InheritableThreadLocal
95    */
96   static final Object sentinel = new Object();
97 
98   /**
99    * 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    */
computeNextHash()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    */
ThreadLocal()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    */
initialValue()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    */
get()146   public native T get();
147 
internalGet()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    */
set(T value)170   public native void set(T value);
171 
internalSet(Object value)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    */
remove()185   public native void remove();
186 
internalRemove()187   private final void internalRemove()
188   {
189     ThreadLocalMap map = Thread.getThreadLocals();
190     map.remove(this);
191   }
192 
finalize()193   protected native void finalize () throws Throwable;
194 
constructNative()195   private native void constructNative();
196 
197   private gnu.gcj.RawData TLSPointer;
198 }
199