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1: /* ThreadLocal -- a variable with a unique value per thread 2: Copyright (C) 2000, 2002, 2003, 2004, 2005, 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: /** 41: * ThreadLocal objects have a different state associated with every 42: * Thread that accesses them. Every access to the ThreadLocal object 43: * (through the <code>get()</code> and <code>set()</code> methods) 44: * only affects the state of the object as seen by the currently 45: * executing Thread. 46: * 47: * <p>The first time a ThreadLocal object is accessed on a particular 48: * Thread, the state for that Thread's copy of the local variable is set by 49: * executing the method <code>initialValue()</code>. 50: * </p> 51: * 52: * <p>An example how you can use this: 53: * </p> 54: * 55: * <pre> 56: * class Connection 57: * { 58: * private static ThreadLocal owner = new ThreadLocal() 59: * { 60: * public Object initialValue() 61: * { 62: * return("nobody"); 63: * } 64: * }; 65: * ... 66: * } 67: * </pre> 68: * 69: * <p>Now all instances of connection can see who the owner of the currently 70: * executing Thread is by calling <code>owner.get()</code>. By default any 71: * Thread would be associated with 'nobody'. But the Connection object could 72: * offer a method that changes the owner associated with the Thread on 73: * which the method was called by calling <code>owner.put("somebody")</code>. 74: * (Such an owner changing method should then be guarded by security checks.) 75: * </p> 76: * 77: * <p>When a Thread is garbage collected all references to values of 78: * the ThreadLocal objects associated with that Thread are removed. 79: * </p> 80: * 81: * @author Mark Wielaard (mark@klomp.org) 82: * @author Eric Blake (ebb9@email.byu.edu) 83: * @since 1.2 84: * @status updated to 1.5 85: */ 86: public class ThreadLocal<T> 87: { 88: /** 89: * Placeholder to distinguish between uninitialized and null set by the 90: * user. Do not expose this to the public. Package visible for use by 91: * InheritableThreadLocal 92: */ 93: static final Object sentinel = new Object(); 94: 95: /** 96: * The base for the computation of the next hash for a thread local. 97: */ 98: private static int nextHashBase = 1; 99: 100: /** 101: * Allocate a new hash. 102: */ 103: private synchronized int computeNextHash() 104: { 105: return nextHashBase++ * 6709; 106: } 107: 108: /** 109: * Hash code computed for ThreadLocalMap 110: */ 111: final int fastHash; 112: 113: /** 114: * Creates a ThreadLocal object without associating any value to it yet. 115: */ 116: public ThreadLocal() 117: { 118: fastHash = computeNextHash(); 119: } 120: 121: /** 122: * Called once per thread on the first invocation of get(), if set() was 123: * not already called. The default implementation returns <code>null</code>. 124: * Often, this method is overridden to create the appropriate initial object 125: * for the current thread's view of the ThreadLocal. 126: * 127: * @return the initial value of the variable in this thread 128: */ 129: protected T initialValue() 130: { 131: return null; 132: } 133: 134: /** 135: * Gets the value associated with the ThreadLocal object for the currently 136: * executing Thread. If this is the first time the current thread has called 137: * get(), and it has not already called set(), the value is obtained by 138: * <code>initialValue()</code>. 139: * 140: * @return the value of the variable in this thread 141: */ 142: public T get() 143: { 144: ThreadLocalMap map = Thread.getThreadLocals(); 145: // Note that we don't have to synchronize, as only this thread will 146: // ever modify the map. 147: T value = (T) map.get(this); 148: if (value == sentinel) 149: { 150: value = initialValue(); 151: map.set(this, value); 152: } 153: return value; 154: } 155: 156: /** 157: * Sets the value associated with the ThreadLocal object for the currently 158: * executing Thread. This overrides any existing value associated with the 159: * current Thread and prevents <code>initialValue()</code> from being 160: * called if this is the first access to this ThreadLocal in this Thread. 161: * 162: * @param value the value to set this thread's view of the variable to 163: */ 164: public void set(T value) 165: { 166: ThreadLocalMap map = Thread.getThreadLocals(); 167: // Note that we don't have to synchronize, as only this thread will 168: // ever modify the map. 169: map.set(this, value); 170: } 171: 172: /** 173: * Removes the value associated with the ThreadLocal object for the 174: * currently executing Thread. 175: * @since 1.5 176: */ 177: public void remove() 178: { 179: ThreadLocalMap map = Thread.getThreadLocals(); 180: map.remove(this); 181: } 182: }
GNU Classpath (0.98) |