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