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 }