001/* ThreadLocal -- a variable with a unique value per thread 002 Copyright (C) 2000, 2002, 2003, 2006 Free Software Foundation, Inc. 003 004This file is part of GNU Classpath. 005 006GNU Classpath is free software; you can redistribute it and/or modify 007it under the terms of the GNU General Public License as published by 008the Free Software Foundation; either version 2, or (at your option) 009any later version. 010 011GNU Classpath is distributed in the hope that it will be useful, but 012WITHOUT ANY WARRANTY; without even the implied warranty of 013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 014General Public License for more details. 015 016You should have received a copy of the GNU General Public License 017along with GNU Classpath; see the file COPYING. If not, write to the 018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 01902110-1301 USA. 020 021Linking this library statically or dynamically with other modules is 022making a combined work based on this library. Thus, the terms and 023conditions of the GNU General Public License cover the whole 024combination. 025 026As a special exception, the copyright holders of this library give you 027permission to link this library with independent modules to produce an 028executable, regardless of the license terms of these independent 029modules, and to copy and distribute the resulting executable under 030terms of your choice, provided that you also meet, for each linked 031independent module, the terms and conditions of the license of that 032module. An independent module is a module which is not derived from 033or based on this library. If you modify this library, you may extend 034this exception to your version of the library, but you are not 035obligated to do so. If you do not wish to do so, delete this 036exception statement from your version. */ 037 038package java.lang; 039 040import 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 */ 089public 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}