libjava/java/lang/InheritableThreadLocal.java

   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 }