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jlechner |
/* ThreadLocal -- a variable with a unique value per thread
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Copyright (C) 2000, 2002, 2003 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.Collections;
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import java.util.Map;
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import java.util.WeakHashMap;
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/**
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* ThreadLocal objects have a different state associated with every
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* Thread that accesses them. Every access to the ThreadLocal object
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* (through the <code>get()</code> and <code>set()</code> methods)
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* only affects the state of the object as seen by the currently
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* executing Thread.
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*
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* <p>The first time a ThreadLocal object is accessed on a particular
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* Thread, the state for that Thread's copy of the local variable is set by
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* executing the method <code>initialValue()</code>.
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* </p>
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*
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* <p>An example how you can use this:
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* </p>
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*
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* <pre>
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* class Connection
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* {
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* private static ThreadLocal owner = new ThreadLocal()
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* {
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* public Object initialValue()
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* {
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* return("nobody");
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* }
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* };
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* ...
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* }
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* </pre>
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*
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* <p>Now all instances of connection can see who the owner of the currently
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* executing Thread is by calling <code>owner.get()</code>. By default any
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* Thread would be associated with 'nobody'. But the Connection object could
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* offer a method that changes the owner associated with the Thread on
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* which the method was called by calling <code>owner.put("somebody")</code>.
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* (Such an owner changing method should then be guarded by security checks.)
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* </p>
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*
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* <p>When a Thread is garbage collected all references to values of
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* the ThreadLocal objects associated with that Thread are removed.
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* </p>
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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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* @since 1.2
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* @status updated to 1.4
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*/
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public class ThreadLocal
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{
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/**
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* Placeholder to distinguish between uninitialized and null set by the
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* user. Do not expose this to the public. Package visible for use by
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* InheritableThreadLocal
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*/
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static final Object NULL = new Object();
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/**
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* The stored value. Package visible for use by InheritableThreadLocal. */
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Object value;
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/**
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* Maps Threads to values. Uses a WeakHashMap so if a Thread is garbage
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* collected the reference to the Value will disappear. A null value means
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* uninitialized, while NULL means a user-specified null. Only the
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* <code>set(Thread, Object)</code> and <code>get(Thread)</code> methods
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* access it. Package visible for use by InheritableThreadLocal.
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*/
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final Map valueMap = Collections.synchronizedMap(new WeakHashMap());
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/**
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* Creates a ThreadLocal object without associating any value to it yet.
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*/
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public ThreadLocal()
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{
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}
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/**
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* Called once per thread on the first invocation of get(), if set() was
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* not already called. The default implementation returns <code>null</code>.
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* Often, this method is overridden to create the appropriate initial object
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* for the current thread's view of the ThreadLocal.
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*
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* @return the initial value of the variable in this thread
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*/
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protected Object initialValue()
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{
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return null;
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}
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/**
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* Gets the value associated with the ThreadLocal object for the currently
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* executing Thread. If this is the first time the current thread has called
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* get(), and it has not already called set(), the value is obtained by
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* <code>initialValue()</code>.
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*
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* @return the value of the variable in this thread
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*/
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public Object get()
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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 value and valueMap is a synchronizedMap.
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Object value = valueMap.get(currentThread);
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if (value == null)
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{
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value = initialValue();
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valueMap.put(currentThread, value == null ? NULL : value);
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}
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return value == NULL ? null : value;
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}
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/**
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* Sets the value associated with the ThreadLocal object for the currently
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* executing Thread. This overrides any existing value associated with the
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* current Thread and prevents <code>initialValue()</code> from being
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* called if this is the first access to this ThreadLocal in this Thread.
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*
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* @param value the value to set this thread's view of the variable to
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*/
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public void set(Object value)
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{
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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 value and valueMap is a synchronizedMap.
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valueMap.put(Thread.currentThread(), value == null ? NULL : value);
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}
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}
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