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    /**
041     * A ThreadLocal whose value is inherited by child Threads. The value of the
042     * InheritableThreadLocal associated with the (parent) Thread is copied to
043     * the new (child) Thread at the moment of creation.
044     *
045     * <p>It is possible to make the value associated with the child Thread a
046     * function of the value that is associated with the parent Thread by
047     * overriding the <code>childValue()</code> method. The utility of this class
048     * is in transferring items like User ID or Transaction ID across threads
049     * automatically.
050     *
051     * @author Mark Wielaard (mark@klomp.org)
052     * @author Eric Blake (ebb9@email.byu.edu)
053     * @author Tom Tromey (tromey@redhat.com)
054     * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
055     * @see ThreadLocal
056     * @since 1.2
057     * @status updated to 1.4
058     */
059    public class InheritableThreadLocal<T> extends ThreadLocal<T>
060    {
061    
062      /**
063       * Creates a new InheritableThreadLocal that has no values associated
064       * with it yet.
065       */
066      public InheritableThreadLocal()
067      {
068      }
069    
070      /**
071       * Determines the value associated with a newly created child Thread as a
072       * function of the value associated with the currently executing (parent)
073       * Thread. The default implementation just returns the parentValue.
074       *
075       * @param parentValue the value of this object in the parent thread at
076       *        the moment of creation of the child
077       * @return the initial value for the child thread
078       */
079      protected T childValue(T parentValue)
080      {
081        return parentValue;
082      }
083    
084      /**
085       * Generates the childValues of all <code>InheritableThreadLocal</code>s
086       * that are in the heritage of the current Thread for the newly created
087       * childThread. Should be called from the constructor Thread.
088       *
089       * @param childThread the newly created thread, to inherit from this thread
090       * @see Thread#Thread(ThreadGroup, Runnable, String)
091       */
092      static void newChildThread(Thread childThread)
093      {
094        // The currentThread is the parent of the new thread.
095        Thread parentThread = Thread.currentThread();
096        childThread.locals.inherit(parentThread.locals);
097      }
098    }