Initial revision

From-SVN: r102074

libjava/classpath/java/lang/ThreadLocal.java

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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;
+import java.util.WeakHashMap;
+
+
+/**
+ * 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);
+  }
+}