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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.Collections; 41: import java.util.Map; 42: import java.util.WeakHashMap; 43: 44: 45: /** 46: * ThreadLocal objects have a different state associated with every 47: * Thread that accesses them. Every access to the ThreadLocal object 48: * (through the <code>get()</code> and <code>set()</code> methods) 49: * only affects the state of the object as seen by the currently 50: * executing Thread. 51: * 52: * <p>The first time a ThreadLocal object is accessed on a particular 53: * Thread, the state for that Thread's copy of the local variable is set by 54: * executing the method <code>initialValue()</code>. 55: * </p> 56: * 57: * <p>An example how you can use this: 58: * </p> 59: * 60: * <pre> 61: * class Connection 62: * { 63: * private static ThreadLocal owner = new ThreadLocal() 64: * { 65: * public Object initialValue() 66: * { 67: * return("nobody"); 68: * } 69: * }; 70: * ... 71: * } 72: * </pre> 73: * 74: * <p>Now all instances of connection can see who the owner of the currently 75: * executing Thread is by calling <code>owner.get()</code>. By default any 76: * Thread would be associated with 'nobody'. But the Connection object could 77: * offer a method that changes the owner associated with the Thread on 78: * which the method was called by calling <code>owner.put("somebody")</code>. 79: * (Such an owner changing method should then be guarded by security checks.) 80: * </p> 81: * 82: * <p>When a Thread is garbage collected all references to values of 83: * the ThreadLocal objects associated with that Thread are removed. 84: * </p> 85: * 86: * @author Mark Wielaard (mark@klomp.org) 87: * @author Eric Blake (ebb9@email.byu.edu) 88: * @since 1.2 89: * @status updated to 1.4 90: */ 91: public class ThreadLocal 92: { 93: /** 94: * Placeholder to distinguish between uninitialized and null set by the 95: * user. Do not expose this to the public. Package visible for use by 96: * InheritableThreadLocal 97: */ 98: static final Object NULL = new Object(); 99: 100: /** 101: * The stored value. Package visible for use by InheritableThreadLocal. */ 102: Object value; 103: 104: /** 105: * Maps Threads to values. Uses a WeakHashMap so if a Thread is garbage 106: * collected the reference to the Value will disappear. A null value means 107: * uninitialized, while NULL means a user-specified null. Only the 108: * <code>set(Thread, Object)</code> and <code>get(Thread)</code> methods 109: * access it. Package visible for use by InheritableThreadLocal. 110: */ 111: final Map valueMap = Collections.synchronizedMap(new WeakHashMap()); 112: 113: /** 114: * Creates a ThreadLocal object without associating any value to it yet. 115: */ 116: public ThreadLocal() 117: { 118: } 119: 120: /** 121: * Called once per thread on the first invocation of get(), if set() was 122: * not already called. The default implementation returns <code>null</code>. 123: * Often, this method is overridden to create the appropriate initial object 124: * for the current thread's view of the ThreadLocal. 125: * 126: * @return the initial value of the variable in this thread 127: */ 128: protected Object initialValue() 129: { 130: return null; 131: } 132: 133: /** 134: * Gets the value associated with the ThreadLocal object for the currently 135: * executing Thread. If this is the first time the current thread has called 136: * get(), and it has not already called set(), the value is obtained by 137: * <code>initialValue()</code>. 138: * 139: * @return the value of the variable in this thread 140: */ 141: public Object get() 142: { 143: Thread currentThread = Thread.currentThread(); 144: // Note that we don't have to synchronize, as only this thread will 145: // ever modify the returned value and valueMap is a synchronizedMap. 146: Object value = valueMap.get(currentThread); 147: if (value == null) 148: { 149: value = initialValue(); 150: valueMap.put(currentThread, value == null ? NULL : value); 151: } 152: return value == NULL ? null : value; 153: } 154: 155: /** 156: * Sets the value associated with the ThreadLocal object for the currently 157: * executing Thread. This overrides any existing value associated with the 158: * current Thread and prevents <code>initialValue()</code> from being 159: * called if this is the first access to this ThreadLocal in this Thread. 160: * 161: * @param value the value to set this thread's view of the variable to 162: */ 163: public void set(Object value) 164: { 165: // Note that we don't have to synchronize, as only this thread will 166: // ever modify the returned value and valueMap is a synchronizedMap. 167: valueMap.put(Thread.currentThread(), value == null ? NULL : value); 168: } 169: }