Initial revision

From-SVN: r102074

libjava/classpath/java/lang/ref/ReferenceQueue.java

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+/* java.lang.ref.ReferenceQueue
+   Copyright (C) 1999 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.ref;
+
+/**
+ * This is the queue, where references can enqueue themselve on.  Each
+ * reference may be registered to a queue at initialization time and
+ * will be appended to the queue, when the enqueue method is called.
+ *
+ * The enqueue method may be automatically called by the garbage
+ * collector if it detects, that the object is only reachable through
+ * the Reference objects.
+ *
+ * @author Jochen Hoenicke
+ * @see Reference#enqueue()
+ */
+public class ReferenceQueue
+{
+  /**
+   * This is a linked list of references.  If this is null, the list is
+   * empty.  Otherwise this points to the first reference on the queue.
+   * The first reference will point to the next reference via the 
+   * <code>nextOnQueue</code> field.  The last reference will point to
+   * itself (not to null, since <code>nextOnQueue</code> is used to 
+   * determine if a reference is enqueued).
+   */
+  private Reference first;
+
+  /**
+   * Creates a new empty reference queue.
+   */
+  public ReferenceQueue()
+  {
+  }
+
+  /**
+   * Checks if there is a reference on the queue, returning it
+   * immediately.  The reference will be dequeued.
+   *
+   * @return a reference on the queue, if there is one,
+   * <code>null</code> otherwise.  
+   */
+  public synchronized Reference poll()
+  { 
+    return dequeue();
+  }
+
+  /**
+   * This is called by reference to enqueue itself on this queue.  
+   * @param ref the reference that should be enqueued.
+   */
+  synchronized void enqueue(Reference ref)
+  {
+    /* last reference will point to itself */
+    ref.nextOnQueue = first == null ? ref : first;
+    first = ref;
+    /* this wakes only one remove thread. */
+    notify();
+  }
+
+  /**
+   * Remove a reference from the queue, if there is one.
+   * @return the first element of the queue, or null if there isn't any.
+   */
+  private Reference dequeue()
+  {
+    if (first == null)
+      return null;
+
+    Reference result = first;
+    first = (first == first.nextOnQueue) ? null : first.nextOnQueue;
+    result.nextOnQueue = null;
+    return result;
+  }
+
+  /**
+   * Removes a reference from the queue, blocking for <code>timeout</code>
+   * until a reference is enqueued.
+   * @param timeout the timeout period in milliseconds, <code>0</code> means
+   * wait forever.
+   * @return the reference removed from the queue, or 
+   * <code>null</code> if timeout period expired.  
+   * @exception InterruptedException if the wait was interrupted.
+   */
+  public synchronized Reference remove(long timeout)
+    throws InterruptedException
+  {
+    if (first == null)
+      {
+	wait(timeout);
+      }
+
+    return dequeue();
+  }
+    
+
+  /**
+   * Removes a reference from the queue, blocking until a reference is
+   * enqueued.
+   *
+   * @return the reference removed from the queue.  
+   * @exception InterruptedException if the wait was interrupted.
+   */
+  public Reference remove()
+    throws InterruptedException
+  {
+    return remove(0L);
+  }
+}