001/* InheritableThreadLocal -- a ThreadLocal which inherits values across threads 002 Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc. 003 004This file is part of GNU Classpath. 005 006GNU Classpath is free software; you can redistribute it and/or modify 007it under the terms of the GNU General Public License as published by 008the Free Software Foundation; either version 2, or (at your option) 009any later version. 010 011GNU Classpath is distributed in the hope that it will be useful, but 012WITHOUT ANY WARRANTY; without even the implied warranty of 013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 014General Public License for more details. 015 016You should have received a copy of the GNU General Public License 017along with GNU Classpath; see the file COPYING. If not, write to the 018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 01902110-1301 USA. 020 021Linking this library statically or dynamically with other modules is 022making a combined work based on this library. Thus, the terms and 023conditions of the GNU General Public License cover the whole 024combination. 025 026As a special exception, the copyright holders of this library give you 027permission to link this library with independent modules to produce an 028executable, regardless of the license terms of these independent 029modules, and to copy and distribute the resulting executable under 030terms of your choice, provided that you also meet, for each linked 031independent module, the terms and conditions of the license of that 032module. An independent module is a module which is not derived from 033or based on this library. If you modify this library, you may extend 034this exception to your version of the library, but you are not 035obligated to do so. If you do not wish to do so, delete this 036exception statement from your version. */ 037 038package 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 */ 059public 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}