ThreadLocal.java

/* ThreadLocal -- a variable with a unique value per thread
   Copyright (C) 2000, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.

GNU Classpath is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

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along with GNU Classpath; see the file COPYING.  If not, write to the
Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
modules, and to copy and distribute the resulting executable under
terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
module.  An independent module is a module which is not derived from
or based on this library.  If you modify this library, you may extend
this exception to your version of the library, but you are not
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exception statement from your version. */

package java.lang;

/**
 * ThreadLocal objects have a different state associated with every
 * Thread that accesses them. Every access to the ThreadLocal object
 * (through the <code>get()</code> and <code>set()</code> methods)
 * only affects the state of the object as seen by the currently
 * executing Thread.
 *
 * <p>The first time a ThreadLocal object is accessed on a particular
 * Thread, the state for that Thread's copy of the local variable is set by
 * executing the method <code>initialValue()</code>.
 * </p>
 *
 * <p>An example how you can use this:
 * </p>
 *
 * <pre>
 * class Connection
 * {
 *   private static ThreadLocal owner = new ThreadLocal()
 *     {
 *       public Object initialValue()
 *       {
 *         return("nobody");
 *       }
 *     };
 * ...
 * }
 * </pre>
 *
 * <p>Now all instances of connection can see who the owner of the currently
 * executing Thread is by calling <code>owner.get()</code>. By default any
 * Thread would be associated with 'nobody'. But the Connection object could
 * offer a method that changes the owner associated with the Thread on
 * which the method was called by calling <code>owner.put("somebody")</code>.
 * (Such an owner changing method should then be guarded by security checks.)
 * </p>
 *
 * <p>When a Thread is garbage collected all references to values of
 * the ThreadLocal objects associated with that Thread are removed.
 * </p>
 *
 * @author Mark Wielaard (mark@klomp.org)
 * @author Eric Blake (ebb9@email.byu.edu)
 * @since 1.2
 * @status updated to 1.5
 */
public class ThreadLocal<T>
{
  /**
   * Placeholder to distinguish between uninitialized and null set by the
   * user. Do not expose this to the public. Package visible for use by
   * InheritableThreadLocal
   */
  static final Object sentinel = new Object();

  /**
   * The base for the computation of the next hash for a thread local.
   */
  private static int nextHashBase = 1;

  /**
   * Allocate a new hash.
   */
  private synchronized int computeNextHash()
  {
    return nextHashBase++ * 6709;
  }

  /**
   * Hash code computed for ThreadLocalMap
   */
  final int fastHash;

  /**
   * Creates a ThreadLocal object without associating any value to it yet.
   */
  public ThreadLocal()
  {
    fastHash = computeNextHash();
  }

  /**
   * Called once per thread on the first invocation of get(), if set() was
   * not already called. The default implementation returns <code>null</code>.
   * Often, this method is overridden to create the appropriate initial object
   * for the current thread's view of the ThreadLocal.
   *
   * @return the initial value of the variable in this thread
   */
  protected T initialValue()
  {
    return null;
  }

  /**
   * Gets the value associated with the ThreadLocal object for the currently
   * executing Thread. If this is the first time the current thread has called
   * get(), and it has not already called set(), the value is obtained by
   * <code>initialValue()</code>.
   *
   * @return the value of the variable in this thread
   */
  public T get()
  {
    ThreadLocalMap map = Thread.getThreadLocals();
    // Note that we don't have to synchronize, as only this thread will
    // ever modify the map.
    T value = (T) map.get(this);
    if (value == sentinel)
      {
        value = initialValue();
        map.set(this, value);
      }
    return value;
  }

  /**
   * Sets the value associated with the ThreadLocal object for the currently
   * executing Thread. This overrides any existing value associated with the
   * current Thread and prevents <code>initialValue()</code> from being
   * called if this is the first access to this ThreadLocal in this Thread.
   *
   * @param value the value to set this thread's view of the variable to
   */
  public void set(T value)
  {
    ThreadLocalMap map = Thread.getThreadLocals();
    // Note that we don't have to synchronize, as only this thread will
    // ever modify the map.
    map.set(this, value);
  }

  /**
   * Removes the value associated with the ThreadLocal object for the
   * currently executing Thread.
   * @since 1.5
   */
  public void remove()
  {
    ThreadLocalMap map = Thread.getThreadLocals();
    map.remove(this);
  }
}