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1: /* InheritableThreadLocal -- a ThreadLocal which inherits values across threads 2: Copyright (C) 2000, 2001, 2002, 2003, 2005 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.ArrayList; 41: import java.util.Collections; 42: import java.util.Iterator; 43: import java.util.List; 44: import java.util.Map; 45: import java.util.WeakHashMap; 46: 47: /** 48: * A ThreadLocal whose value is inherited by child Threads. The value of the 49: * InheritableThreadLocal associated with the (parent) Thread is copied to 50: * the new (child) Thread at the moment of creation. 51: * 52: * <p>It is possible to make the value associated with the child Thread a 53: * function of the value that is associated with the parent Thread by 54: * overriding the <code>childValue()</code> method. The utility of this class 55: * is in transferring items like User ID or Transaction ID across threads 56: * automatically. 57: * 58: * @author Mark Wielaard (mark@klomp.org) 59: * @author Eric Blake (ebb9@email.byu.edu) 60: * @see ThreadLocal 61: * @since 1.2 62: * @status updated to 1.4 63: */ 64: public class InheritableThreadLocal extends ThreadLocal 65: { 66: /** 67: * Maps Threads to a List of InheritableThreadLocals (the heritage of that 68: * Thread). Uses a WeakHashMap so if the Thread is garbage collected the 69: * List can be collected, too. Maps to a list in case the user overrides 70: * equals. 71: */ 72: private static final Map threadMap 73: = Collections.synchronizedMap(new WeakHashMap()); 74: 75: /** 76: * Creates a new InheritableThreadLocal that has no values associated 77: * with it yet. 78: */ 79: public InheritableThreadLocal() 80: { 81: Thread currentThread = Thread.currentThread(); 82: // Note that we don't have to synchronize, as only this thread will 83: // ever modify the returned heritage and threadMap is a synchronizedMap. 84: List heritage = (List) threadMap.get(currentThread); 85: if (heritage == null) 86: { 87: heritage = new ArrayList(); 88: threadMap.put(currentThread, heritage); 89: } 90: heritage.add(this); 91: } 92: 93: /** 94: * Determines the value associated with a newly created child Thread as a 95: * function of the value associated with the currently executing (parent) 96: * Thread. The default implementation just returns the parentValue. 97: * 98: * @param parentValue the value of this object in the parent thread at 99: * the moment of creation of the child 100: * @return the initial value for the child thread 101: */ 102: protected Object childValue(Object parentValue) 103: { 104: return parentValue; 105: } 106: 107: /** 108: * Generates the childValues of all <code>InheritableThreadLocal</code>s 109: * that are in the heritage of the current Thread for the newly created 110: * childThread. Should be called from the contructor Thread. 111: * 112: * @param childThread the newly created thread, to inherit from this thread 113: * @see Thread#Thread(ThreadGroup, Runnable, String) 114: */ 115: static void newChildThread(Thread childThread) 116: { 117: // The currentThread is the parent of the new thread. 118: Thread parentThread = Thread.currentThread(); 119: // Note that we don't have to synchronize, as only this thread will 120: // ever modify the returned heritage and threadMap is a synchronizedMap. 121: ArrayList heritage = (ArrayList) threadMap.get(parentThread); 122: if (heritage != null) 123: { 124: threadMap.put(childThread, heritage.clone()); 125: // Perform the inheritance. 126: Iterator it = heritage.iterator(); 127: int i = heritage.size(); 128: while (--i >= 0) 129: { 130: InheritableThreadLocal local = (InheritableThreadLocal) it.next(); 131: Object parentValue = local.valueMap.get(parentThread); 132: if (parentValue != null) 133: { 134: Object childValue = local.childValue(parentValue == NULL 135: ? null : parentValue); 136: local.valueMap.put(childThread, (childValue == null 137: ? NULL : parentValue)); 138: } 139: } 140: } 141: } 142: }