Source for java.lang.InheritableThreadLocal

   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: }