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/* ThreadLocal -- a variable with a unique value per thread

   Copyright (C) 2000, 2002, 2003 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.

You should have received a copy of the GNU General Public License

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

conditions of the GNU General Public License cover the whole

combination.

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

obligated to do so.  If you do not wish to do so, delete this

exception statement from your version. */

package java.lang;

import java.util.Map;

/**

 * 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 1.4

 */

public class ThreadLocal

{

  /**

   * 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 NULL = new Object();

  /**

   * Serves as a key for the Thread.locals WeakHashMap.

   * We can't use "this", because a subclass may override equals/hashCode

   * and we need to use object identity for the map.

   */

  final Key key = new Key();

  class Key

  {

    ThreadLocal get()

    {

      return ThreadLocal.this;

    }

  }

  /**

   * Creates a ThreadLocal object without associating any value to it yet.

   */

  public ThreadLocal()

  {

  }

  /**

   * 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 Object 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 Object get()

  {

    Map map = Thread.getThreadLocals();

    // Note that we don't have to synchronize, as only this thread will

    // ever modify the map.

    Object value = map.get(key);

    if (value == null)

      {

        value = initialValue();

        map.put(key, value == null ? NULL : value);

      }

    return value == NULL ? null : 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(Object value)

  {

    Map map = Thread.getThreadLocals();

    // Note that we don't have to synchronize, as only this thread will

    // ever modify the map.

    map.put(key, value == null ? NULL : value);

  }

}