143 lines
5.5 KiB
Java
143 lines
5.5 KiB
Java
/* InheritableThreadLocal -- a ThreadLocal which inherits values across threads
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Copyright (C) 2000, 2001, 2002, 2003, 2005 Free Software Foundation, Inc.
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This file is part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; see the file COPYING. If not, write to the
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Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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02110-1301 USA.
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Linking this library statically or dynamically with other modules is
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making a combined work based on this library. Thus, the terms and
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conditions of the GNU General Public License cover the whole
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combination.
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As a special exception, the copyright holders of this library give you
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permission to link this library with independent modules to produce an
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executable, regardless of the license terms of these independent
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modules, and to copy and distribute the resulting executable under
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terms of your choice, provided that you also meet, for each linked
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independent module, the terms and conditions of the license of that
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module. An independent module is a module which is not derived from
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or based on this library. If you modify this library, you may extend
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this exception to your version of the library, but you are not
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obligated to do so. If you do not wish to do so, delete this
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exception statement from your version. */
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package java.lang;
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import java.util.ArrayList;
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import java.util.Collections;
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import java.util.Iterator;
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import java.util.List;
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import java.util.Map;
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import java.util.WeakHashMap;
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/**
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* A ThreadLocal whose value is inherited by child Threads. The value of the
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* InheritableThreadLocal associated with the (parent) Thread is copied to
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* the new (child) Thread at the moment of creation.
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*
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* <p>It is possible to make the value associated with the child Thread a
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* function of the value that is associated with the parent Thread by
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* overriding the <code>childValue()</code> method. The utility of this class
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* is in transferring items like User ID or Transaction ID across threads
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* automatically.
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*
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* @author Mark Wielaard (mark@klomp.org)
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* @author Eric Blake (ebb9@email.byu.edu)
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* @see ThreadLocal
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* @since 1.2
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* @status updated to 1.4
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*/
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public class InheritableThreadLocal extends ThreadLocal
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{
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/**
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* Maps Threads to a List of InheritableThreadLocals (the heritage of that
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* Thread). Uses a WeakHashMap so if the Thread is garbage collected the
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* List can be collected, too. Maps to a list in case the user overrides
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* equals.
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*/
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private static final Map threadMap
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= Collections.synchronizedMap(new WeakHashMap());
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/**
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* Creates a new InheritableThreadLocal that has no values associated
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* with it yet.
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*/
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public InheritableThreadLocal()
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{
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Thread currentThread = Thread.currentThread();
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// Note that we don't have to synchronize, as only this thread will
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// ever modify the returned heritage and threadMap is a synchronizedMap.
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List heritage = (List) threadMap.get(currentThread);
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if (heritage == null)
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{
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heritage = new ArrayList();
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threadMap.put(currentThread, heritage);
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}
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heritage.add(this);
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}
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/**
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* Determines the value associated with a newly created child Thread as a
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* function of the value associated with the currently executing (parent)
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* Thread. The default implementation just returns the parentValue.
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*
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* @param parentValue the value of this object in the parent thread at
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* the moment of creation of the child
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* @return the initial value for the child thread
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*/
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protected Object childValue(Object parentValue)
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{
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return parentValue;
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}
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/**
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* Generates the childValues of all <code>InheritableThreadLocal</code>s
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* that are in the heritage of the current Thread for the newly created
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* childThread. Should be called from the contructor Thread.
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*
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* @param childThread the newly created thread, to inherit from this thread
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* @see Thread#Thread(ThreadGroup, Runnable, String)
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*/
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static void newChildThread(Thread childThread)
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{
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// The currentThread is the parent of the new thread.
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Thread parentThread = Thread.currentThread();
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// Note that we don't have to synchronize, as only this thread will
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// ever modify the returned heritage and threadMap is a synchronizedMap.
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ArrayList heritage = (ArrayList) threadMap.get(parentThread);
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if (heritage != null)
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{
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threadMap.put(childThread, heritage.clone());
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// Perform the inheritance.
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Iterator it = heritage.iterator();
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int i = heritage.size();
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while (--i >= 0)
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{
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InheritableThreadLocal local = (InheritableThreadLocal) it.next();
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Object parentValue = local.valueMap.get(parentThread);
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if (parentValue != null)
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{
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Object childValue = local.childValue(parentValue == NULL
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? null : parentValue);
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local.valueMap.put(childThread, (childValue == null
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? NULL : parentValue));
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}
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}
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}
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}
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}
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