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.Iterator;
  41: import java.util.WeakHashMap;
  42: 
  43: /**
  44:  * A ThreadLocal whose value is inherited by child Threads. The value of the
  45:  * InheritableThreadLocal associated with the (parent) Thread is copied to
  46:  * the new (child) Thread at the moment of creation.
  47:  *
  48:  * <p>It is possible to make the value associated with the child Thread a
  49:  * function of the value that is associated with the parent Thread by
  50:  * overriding the <code>childValue()</code> method. The utility of this class
  51:  * is in transferring items like User ID or Transaction ID across threads
  52:  * automatically.
  53:  *
  54:  * @author Mark Wielaard (mark@klomp.org)
  55:  * @author Eric Blake (ebb9@email.byu.edu)
  56:  * @see ThreadLocal
  57:  * @since 1.2
  58:  * @status updated to 1.4
  59:  */
  60: public class InheritableThreadLocal extends ThreadLocal
  61: {
  62:   /**
  63:    * Creates a new InheritableThreadLocal that has no values associated
  64:    * with it yet.
  65:    */
  66:   public InheritableThreadLocal()
  67:   {
  68:   }
  69: 
  70:   /**
  71:    * Determines the value associated with a newly created child Thread as a
  72:    * function of the value associated with the currently executing (parent)
  73:    * Thread. The default implementation just returns the parentValue.
  74:    *
  75:    * @param parentValue the value of this object in the parent thread at
  76:    *        the moment of creation of the child
  77:    * @return the initial value for the child thread
  78:    */
  79:   protected Object childValue(Object parentValue)
  80:   {
  81:     return parentValue;
  82:   }
  83: 
  84:   /**
  85:    * Generates the childValues of all <code>InheritableThreadLocal</code>s
  86:    * that are in the heritage of the current Thread for the newly created
  87:    * childThread. Should be called from the contructor Thread.
  88:    *
  89:    * @param childThread the newly created thread, to inherit from this thread
  90:    * @see Thread#Thread(ThreadGroup, Runnable, String)
  91:    */
  92:   static void newChildThread(Thread childThread)
  93:   {
  94:     // The currentThread is the parent of the new thread.
  95:     Thread parentThread = Thread.currentThread();
  96:     if (parentThread.locals != null)
  97:       {
  98:         Iterator keys = parentThread.locals.keySet().iterator();
  99:         while (keys.hasNext())
 100:           {
 101:             Key key = (Key)keys.next();
 102:             if (key.get() instanceof InheritableThreadLocal)
 103:               {
 104:                 InheritableThreadLocal local = (InheritableThreadLocal)key.get();
 105:                 Object parentValue = parentThread.locals.get(key);
 106:                 Object childValue = local.childValue(parentValue == NULL
 107:                                                      ? null : parentValue);
 108:                 if (childThread.locals == null)
 109:                     childThread.locals = new WeakHashMap();
 110:                 childThread.locals.put(key, (childValue == null
 111:                                              ? NULL : childValue));
 112:               }
 113:           }
 114:       }
 115:   }
 116: }