001 /* InheritableThreadLocal -- a ThreadLocal which inherits values across threads
002 Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
003
004 This file is part of GNU Classpath.
005
006 GNU Classpath is free software; you can redistribute it and/or modify
007 it under the terms of the GNU General Public License as published by
008 the Free Software Foundation; either version 2, or (at your option)
009 any later version.
010
011 GNU Classpath is distributed in the hope that it will be useful, but
012 WITHOUT ANY WARRANTY; without even the implied warranty of
013 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
014 General Public License for more details.
015
016 You should have received a copy of the GNU General Public License
017 along with GNU Classpath; see the file COPYING. If not, write to the
018 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019 02110-1301 USA.
020
021 Linking this library statically or dynamically with other modules is
022 making a combined work based on this library. Thus, the terms and
023 conditions of the GNU General Public License cover the whole
024 combination.
025
026 As a special exception, the copyright holders of this library give you
027 permission to link this library with independent modules to produce an
028 executable, regardless of the license terms of these independent
029 modules, and to copy and distribute the resulting executable under
030 terms of your choice, provided that you also meet, for each linked
031 independent module, the terms and conditions of the license of that
032 module. An independent module is a module which is not derived from
033 or based on this library. If you modify this library, you may extend
034 this exception to your version of the library, but you are not
035 obligated to do so. If you do not wish to do so, delete this
036 exception statement from your version. */
037
038 package java.lang;
039
040 import gnu.java.util.WeakIdentityHashMap;
041
042 import java.util.Iterator;
043
044 /**
045 * A ThreadLocal whose value is inherited by child Threads. The value of the
046 * InheritableThreadLocal associated with the (parent) Thread is copied to
047 * the new (child) Thread at the moment of creation.
048 *
049 * <p>It is possible to make the value associated with the child Thread a
050 * function of the value that is associated with the parent Thread by
051 * overriding the <code>childValue()</code> method. The utility of this class
052 * is in transferring items like User ID or Transaction ID across threads
053 * automatically.
054 *
055 * @author Mark Wielaard (mark@klomp.org)
056 * @author Eric Blake (ebb9@email.byu.edu)
057 * @author Tom Tromey (tromey@redhat.com)
058 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
059 * @see ThreadLocal
060 * @since 1.2
061 * @status updated to 1.4
062 */
063 public class InheritableThreadLocal<T> extends ThreadLocal<T>
064 {
065
066 /**
067 * Creates a new InheritableThreadLocal that has no values associated
068 * with it yet.
069 */
070 public InheritableThreadLocal()
071 {
072 }
073
074 /**
075 * Determines the value associated with a newly created child Thread as a
076 * function of the value associated with the currently executing (parent)
077 * Thread. The default implementation just returns the parentValue.
078 *
079 * @param parentValue the value of this object in the parent thread at
080 * the moment of creation of the child
081 * @return the initial value for the child thread
082 */
083 protected T childValue(T parentValue)
084 {
085 return parentValue;
086 }
087
088 /**
089 * Generates the childValues of all <code>InheritableThreadLocal</code>s
090 * that are in the heritage of the current Thread for the newly created
091 * childThread. Should be called from the constructor Thread.
092 *
093 * @param childThread the newly created thread, to inherit from this thread
094 * @see Thread#Thread(ThreadGroup, Runnable, String)
095 */
096 static void newChildThread(Thread childThread)
097 {
098 // The currentThread is the parent of the new thread.
099 Thread parentThread = Thread.currentThread();
100 if (parentThread.locals != null)
101 {
102 Iterator keys = parentThread.locals.keySet().iterator();
103 while (keys.hasNext())
104 {
105 Object key = keys.next();
106 if (key instanceof InheritableThreadLocal)
107 {
108 InheritableThreadLocal local = (InheritableThreadLocal)key;
109 Object parentValue = parentThread.locals.get(key);
110 Object childValue = local.childValue(parentValue == sentinel
111 ? null : parentValue);
112 if (childThread.locals == null)
113 childThread.locals = new WeakIdentityHashMap();
114 childThread.locals.put(key, (childValue == null
115 ? sentinel : childValue));
116 }
117 }
118 }
119 }
120 }