001    /* ThreadLocal -- a variable with a unique value per thread
002       Copyright (C) 2000, 2002, 2003, 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    import java.util.Map;
041    
042    
043    /**
044     * ThreadLocal objects have a different state associated with every
045     * Thread that accesses them. Every access to the ThreadLocal object
046     * (through the <code>get()</code> and <code>set()</code> methods)
047     * only affects the state of the object as seen by the currently
048     * executing Thread.
049     *
050     * <p>The first time a ThreadLocal object is accessed on a particular
051     * Thread, the state for that Thread's copy of the local variable is set by
052     * executing the method <code>initialValue()</code>.
053     * </p>
054     *
055     * <p>An example how you can use this:
056     * </p>
057     *
058     * <pre>
059     * class Connection
060     * {
061     *   private static ThreadLocal owner = new ThreadLocal()
062     *     {
063     *       public Object initialValue()
064     *       {
065     *         return("nobody");
066     *       }
067     *     };
068     * ...
069     * }
070     * </pre>
071     *
072     * <p>Now all instances of connection can see who the owner of the currently
073     * executing Thread is by calling <code>owner.get()</code>. By default any
074     * Thread would be associated with 'nobody'. But the Connection object could
075     * offer a method that changes the owner associated with the Thread on
076     * which the method was called by calling <code>owner.put("somebody")</code>.
077     * (Such an owner changing method should then be guarded by security checks.)
078     * </p>
079     *
080     * <p>When a Thread is garbage collected all references to values of
081     * the ThreadLocal objects associated with that Thread are removed.
082     * </p>
083     *
084     * @author Mark Wielaard (mark@klomp.org)
085     * @author Eric Blake (ebb9@email.byu.edu)
086     * @since 1.2
087     * @status updated to 1.5
088     */
089    public class ThreadLocal<T>
090    {
091      /**
092       * Placeholder to distinguish between uninitialized and null set by the
093       * user. Do not expose this to the public. Package visible for use by
094       * InheritableThreadLocal
095       */
096      static final Object sentinel = new Object();
097    
098      /**
099       * Creates a ThreadLocal object without associating any value to it yet.
100       */
101      public ThreadLocal()
102      {
103        constructNative();
104      }
105    
106      /**
107       * Called once per thread on the first invocation of get(), if set() was
108       * not already called. The default implementation returns <code>null</code>.
109       * Often, this method is overridden to create the appropriate initial object
110       * for the current thread's view of the ThreadLocal.
111       *
112       * @return the initial value of the variable in this thread
113       */
114      protected T initialValue()
115      {
116        return null;
117      }
118    
119      /**
120       * Gets the value associated with the ThreadLocal object for the currently
121       * executing Thread. If this is the first time the current thread has called
122       * get(), and it has not already called set(), the value is obtained by
123       * <code>initialValue()</code>.
124       *
125       * @return the value of the variable in this thread
126       */
127      public native T get();
128    
129      private final Object internalGet()
130      {
131        Map<ThreadLocal<T>,T> map = (Map<ThreadLocal<T>,T>) Thread.getThreadLocals();
132        // Note that we don't have to synchronize, as only this thread will
133        // ever modify the map.
134        T value = map.get(this);
135        if (value == null)
136          {
137            value = initialValue();
138            map.put(this, (T) (value == null ? sentinel : value));
139          }
140        return value == (T) sentinel ? null : value;
141      }
142    
143      /**
144       * Sets the value associated with the ThreadLocal object for the currently
145       * executing Thread. This overrides any existing value associated with the
146       * current Thread and prevents <code>initialValue()</code> from being
147       * called if this is the first access to this ThreadLocal in this Thread.
148       *
149       * @param value the value to set this thread's view of the variable to
150       */
151      public native void set(T value);
152    
153      private final void internalSet(Object value)
154      {
155        Map map = Thread.getThreadLocals();
156        // Note that we don't have to synchronize, as only this thread will
157        // ever modify the map.
158        map.put(this, value == null ? sentinel : value);
159      }
160    
161      /**
162       * Removes the value associated with the ThreadLocal object for the
163       * currently executing Thread.
164       * @since 1.5
165       */
166      public native void remove();
167    
168      private final void internalRemove()
169      {
170        Map map = Thread.getThreadLocals();
171        map.remove(this);
172      }
173    
174      protected native void finalize () throws Throwable;
175    
176      private native void constructNative();
177    
178      private gnu.gcj.RawData TLSPointer;
179    }