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 * Creates a ThreadLocal object without associating any value to it yet. 100 */ 101 public ThreadLocal() 102 { 103 constructNative(); 104 } 105 106 /** 107 * Called once per thread on the first invocation of get(), if set() was 108 * not already called. The default implementation returns <code>null</code>. 109 * Often, this method is overridden to create the appropriate initial object 110 * for the current thread's view of the ThreadLocal. 111 * 112 * @return the initial value of the variable in this thread 113 */ 114 protected T initialValue() 115 { 116 return null; 117 } 118 119 /** 120 * Gets the value associated with the ThreadLocal object for the currently 121 * executing Thread. If this is the first time the current thread has called 122 * get(), and it has not already called set(), the value is obtained by 123 * <code>initialValue()</code>. 124 * 125 * @return the value of the variable in this thread 126 */ 127 public native T get(); 128 129 private final Object internalGet() 130 { 131 Map<ThreadLocal<T>,T> map = (Map<ThreadLocal<T>,T>) Thread.getThreadLocals(); 132 // Note that we don't have to synchronize, as only this thread will 133 // ever modify the map. 134 T value = map.get(this); 135 if (value == null) 136 { 137 value = initialValue(); 138 map.put(this, (T) (value == null ? sentinel : value)); 139 } 140 return value == (T) sentinel ? null : value; 141 } 142 143 /** 144 * Sets the value associated with the ThreadLocal object for the currently 145 * executing Thread. This overrides any existing value associated with the 146 * current Thread and prevents <code>initialValue()</code> from being 147 * called if this is the first access to this ThreadLocal in this Thread. 148 * 149 * @param value the value to set this thread's view of the variable to 150 */ 151 public native void set(T value); 152 153 private final void internalSet(Object value) 154 { 155 Map map = Thread.getThreadLocals(); 156 // Note that we don't have to synchronize, as only this thread will 157 // ever modify the map. 158 map.put(this, value == null ? sentinel : value); 159 } 160 161 /** 162 * Removes the value associated with the ThreadLocal object for the 163 * currently executing Thread. 164 * @since 1.5 165 */ 166 public native void remove(); 167 168 private final void internalRemove() 169 { 170 Map map = Thread.getThreadLocals(); 171 map.remove(this); 172 } 173 174 protected native void finalize () throws Throwable; 175 176 private native void constructNative(); 177 178 private gnu.gcj.RawData TLSPointer; 179 }