Source for java.lang.InheritableThreadLocal

   1: /* InheritableThreadLocal -- a ThreadLocal which inherits values across threads
   2:    Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005, 2006 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 gnu.java.util.WeakIdentityHashMap;
  41: 
  42: import java.util.Iterator;
  43: 
  44: /**
  45:  * A ThreadLocal whose value is inherited by child Threads. The value of the
  46:  * InheritableThreadLocal associated with the (parent) Thread is copied to
  47:  * the new (child) Thread at the moment of creation.
  48:  *
  49:  * <p>It is possible to make the value associated with the child Thread a
  50:  * function of the value that is associated with the parent Thread by
  51:  * overriding the <code>childValue()</code> method. The utility of this class
  52:  * is in transferring items like User ID or Transaction ID across threads
  53:  * automatically.
  54:  *
  55:  * @author Mark Wielaard (mark@klomp.org)
  56:  * @author Eric Blake (ebb9@email.byu.edu)
  57:  * @author Tom Tromey (tromey@redhat.com)
  58:  * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
  59:  * @see ThreadLocal
  60:  * @since 1.2
  61:  * @status updated to 1.4
  62:  */
  63: public class InheritableThreadLocal<T> extends ThreadLocal<T>
  64: {
  65: 
  66:   /**
  67:    * Creates a new InheritableThreadLocal that has no values associated
  68:    * with it yet.
  69:    */
  70:   public InheritableThreadLocal()
  71:   {
  72:   }
  73: 
  74:   /**
  75:    * Determines the value associated with a newly created child Thread as a
  76:    * function of the value associated with the currently executing (parent)
  77:    * Thread. The default implementation just returns the parentValue.
  78:    *
  79:    * @param parentValue the value of this object in the parent thread at
  80:    *        the moment of creation of the child
  81:    * @return the initial value for the child thread
  82:    */
  83:   protected T childValue(T parentValue)
  84:   {
  85:     return parentValue;
  86:   }
  87: 
  88:   /**
  89:    * Generates the childValues of all <code>InheritableThreadLocal</code>s
  90:    * that are in the heritage of the current Thread for the newly created
  91:    * childThread. Should be called from the constructor Thread.
  92:    *
  93:    * @param childThread the newly created thread, to inherit from this thread
  94:    * @see Thread#Thread(ThreadGroup, Runnable, String)
  95:    */
  96:   static void newChildThread(Thread childThread)
  97:   {
  98:     // The currentThread is the parent of the new thread.
  99:     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: }