Source for java.lang.ThreadLocal

   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: }