libjava/java/lang/ThreadLocal.java

   1 /* ThreadLocal -- a variable with a unique value per thread
   2    Copyright (C) 2000, 2002, 2003 Free Software Foundation, Inc.
   3
   4 This file is part of GNU Classpath.
   5
   6 GNU Classpath is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 GNU Classpath is distributed in the hope that it will be useful, but
  12 WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GNU Classpath; see the file COPYING.  If not, write to the
  18 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  19 02110-1301 USA.
  20
  21 Linking this library statically or dynamically with other modules is
  22 making a combined work based on this library.  Thus, the terms and
  23 conditions of the GNU General Public License cover the whole
  24 combination.
  25
  26 As a special exception, the copyright holders of this library give you
  27 permission to link this library with independent modules to produce an
  28 executable, regardless of the license terms of these independent
  29 modules, and to copy and distribute the resulting executable under
  30 terms of your choice, provided that you also meet, for each linked
  31 independent module, the terms and conditions of the license of that
  32 module.  An independent module is a module which is not derived from
  33 or based on this library.  If you modify this library, you may extend
  34 this exception to your version of the library, but you are not
  35 obligated to do so.  If you do not wish to do so, delete this
  36 exception statement from your version. */
  37
  38 package java.lang;
  39
  40 import java.util.Collections;
  41 import java.util.Map;
  42 import java.util.WeakHashMap;
  43
  44
  45 /**
  46  * ThreadLocal objects have a different state associated with every
  47  * Thread that accesses them. Every access to the ThreadLocal object
  48  * (through the <code>get()</code> and <code>set()</code> methods)
  49  * only affects the state of the object as seen by the currently
  50  * executing Thread.
  51  *
  52  * <p>The first time a ThreadLocal object is accessed on a particular
  53  * Thread, the state for that Thread's copy of the local variable is set by
  54  * executing the method <code>initialValue()</code>.
  55  * </p>
  56  *
  57  * <p>An example how you can use this:
  58  * </p>
  59  *
  60  * <pre>
  61  * class Connection
  62  * {
  63  *   private static ThreadLocal owner = new ThreadLocal()
  64  *     {
  65  *       public Object initialValue()
  66  *       {
  67  *         return("nobody");
  68  *       }
  69  *     };
  70  * ...
  71  * }
  72  * </pre>
  73  *
  74  * <p>Now all instances of connection can see who the owner of the currently
  75  * executing Thread is by calling <code>owner.get()</code>. By default any
  76  * Thread would be associated with 'nobody'. But the Connection object could
  77  * offer a method that changes the owner associated with the Thread on
  78  * which the method was called by calling <code>owner.put("somebody")</code>.
  79  * (Such an owner changing method should then be guarded by security checks.)
  80  * </p>
  81  *
  82  * <p>When a Thread is garbage collected all references to values of
  83  * the ThreadLocal objects associated with that Thread are removed.
  84  * </p>
  85  *
  86  * @author Mark Wielaard (mark@klomp.org)
  87  * @author Eric Blake (ebb9@email.byu.edu)
  88  * @since 1.2
  89  * @status updated to 1.4
  90  */
  91 public class ThreadLocal
  92 {
  93   /**
  94    * Placeholder to distinguish between uninitialized and null set by the
  95    * user. Do not expose this to the public. Package visible for use by
  96    * InheritableThreadLocal
  97    */
  98   static final Object NULL = new Object();
  99
 100   /**
 101    * The stored value. Package visible for use by InheritableThreadLocal. */
 102   Object value;
 103
 104   /**
 105    * Maps Threads to values. Uses a WeakHashMap so if a Thread is garbage
 106    * collected the reference to the Value will disappear. A null value means
 107    * uninitialized, while NULL means a user-specified null. Only the
 108    * <code>set(Thread, Object)</code> and <code>get(Thread)</code> methods
 109    * access it. Package visible for use by InheritableThreadLocal.
 110    */
 111   final Map valueMap = Collections.synchronizedMap(new WeakHashMap());
 112
 113   /**
 114    * Creates a ThreadLocal object without associating any value to it yet.
 115    */
 116   public ThreadLocal()
 117   {
 118   }
 119
 120   /**
 121    * Called once per thread on the first invocation of get(), if set() was
 122    * not already called. The default implementation returns <code>null</code>.
 123    * Often, this method is overridden to create the appropriate initial object
 124    * for the current thread's view of the ThreadLocal.
 125    *
 126    * @return the initial value of the variable in this thread
 127    */
 128   protected Object initialValue()
 129   {
 130     return null;
 131   }
 132
 133   /**
 134    * Gets the value associated with the ThreadLocal object for the currently
 135    * executing Thread. If this is the first time the current thread has called
 136    * get(), and it has not already called set(), the value is obtained by
 137    * <code>initialValue()</code>.
 138    *
 139    * @return the value of the variable in this thread
 140    */
 141   public Object get()
 142   {
 143     Thread currentThread = Thread.currentThread();
 144     // Note that we don't have to synchronize, as only this thread will
 145     // ever modify the returned value and valueMap is a synchronizedMap.
 146     Object value = valueMap.get(currentThread);
 147     if (value == null)
 148       {
 149         value = initialValue();
 150         valueMap.put(currentThread, value == null ? NULL : value);
 151       }
 152     return value == NULL ? null : value;
 153   }
 154
 155   /**
 156    * Sets the value associated with the ThreadLocal object for the currently
 157    * executing Thread. This overrides any existing value associated with the
 158    * current Thread and prevents <code>initialValue()</code> from being
 159    * called if this is the first access to this ThreadLocal in this Thread.
 160    *
 161    * @param value the value to set this thread's view of the variable to
 162    */
 163   public void set(Object value)
 164   {
 165     // Note that we don't have to synchronize, as only this thread will
 166     // ever modify the returned value and valueMap is a synchronizedMap.
 167     valueMap.put(Thread.currentThread(), value == null ? NULL : value);
 168   }
 169 }