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1: /* ThreadLocal -- a variable with a unique value per thread 2: Copyright (C) 2000, 2002, 2003 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: import java.util.WeakHashMap; 42: 43: 44: /** 45: * ThreadLocal objects have a different state associated with every 46: * Thread that accesses them. Every access to the ThreadLocal object 47: * (through the <code>get()</code> and <code>set()</code> methods) 48: * only affects the state of the object as seen by the currently 49: * executing Thread. 50: * 51: * <p>The first time a ThreadLocal object is accessed on a particular 52: * Thread, the state for that Thread's copy of the local variable is set by 53: * executing the method <code>initialValue()</code>. 54: * </p> 55: * 56: * <p>An example how you can use this: 57: * </p> 58: * 59: * <pre> 60: * class Connection 61: * { 62: * private static ThreadLocal owner = new ThreadLocal() 63: * { 64: * public Object initialValue() 65: * { 66: * return("nobody"); 67: * } 68: * }; 69: * ... 70: * } 71: * </pre> 72: * 73: * <p>Now all instances of connection can see who the owner of the currently 74: * executing Thread is by calling <code>owner.get()</code>. By default any 75: * Thread would be associated with 'nobody'. But the Connection object could 76: * offer a method that changes the owner associated with the Thread on 77: * which the method was called by calling <code>owner.put("somebody")</code>. 78: * (Such an owner changing method should then be guarded by security checks.) 79: * </p> 80: * 81: * <p>When a Thread is garbage collected all references to values of 82: * the ThreadLocal objects associated with that Thread are removed. 83: * </p> 84: * 85: * @author Mark Wielaard (mark@klomp.org) 86: * @author Eric Blake (ebb9@email.byu.edu) 87: * @since 1.2 88: * @status updated to 1.4 89: */ 90: public class ThreadLocal 91: { 92: /** 93: * Placeholder to distinguish between uninitialized and null set by the 94: * user. Do not expose this to the public. Package visible for use by 95: * InheritableThreadLocal 96: */ 97: static final Object NULL = new Object(); 98: 99: /** 100: * Serves as a key for the Thread.locals WeakHashMap. 101: * We can't use "this", because a subclass may override equals/hashCode 102: * and we need to use object identity for the map. 103: */ 104: final Key key = new Key(); 105: 106: class Key 107: { 108: ThreadLocal get() 109: { 110: return ThreadLocal.this; 111: } 112: } 113: 114: /** 115: * Creates a ThreadLocal object without associating any value to it yet. 116: */ 117: public ThreadLocal() 118: { 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 Object 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 Object get() 143: { 144: Map map = Thread.getThreadLocals(); 145: // Note that we don't have to synchronize, as only this thread will 146: // ever modify the map. 147: Object value = map.get(key); 148: if (value == null) 149: { 150: value = initialValue(); 151: map.put(key, value == null ? NULL : value); 152: } 153: return value == NULL ? null : 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(Object value) 165: { 166: Map map = Thread.getThreadLocals(); 167: // Note that we don't have to synchronize, as only this thread will 168: // ever modify the map. 169: map.put(key, value == null ? NULL : value); 170: } 171: }
GNU Classpath (0.18) |