--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/agent/src/share/classes/sun/jvm/hotspot/utilities/MessageQueueBackend.java	Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,104 @@
+/*
+ * Copyright 2000 Sun Microsystems, Inc.  All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code 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
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ *
+ */
+
+package sun.jvm.hotspot.utilities;
+
+/** The backend for the message queue abstraction. This class is
+    instantiated first and queried to provide the two "sides" of the
+    message queue. */
+
+import java.util.LinkedList;
+
+public class MessageQueueBackend {
+  // The two queues
+  private MessageQueueImpl leftRightQueue;
+  private MessageQueueImpl rightLeftQueue;
+
+  public MessageQueueBackend() {
+    LinkedList leftRightPipe = new LinkedList();
+    LinkedList rightLeftPipe = new LinkedList();
+    leftRightQueue = new MessageQueueImpl(rightLeftPipe, leftRightPipe);
+    rightLeftQueue = new MessageQueueImpl(leftRightPipe, rightLeftPipe);
+  }
+
+  /** Get one of the two symmetric sides of this message queue. */
+  public MessageQueue getFirstQueue() {
+    return leftRightQueue;
+  }
+
+  /** Get second of the two symmetric sides of this message queue. */
+  public MessageQueue getSecondQueue() {
+    return rightLeftQueue;
+  }
+
+  private class MessageQueueImpl implements MessageQueue {
+    private LinkedList readList;
+    private LinkedList writeList;
+
+    public MessageQueueImpl(LinkedList listToReadFrom, LinkedList listToWriteTo) {
+      readList = listToReadFrom;
+      writeList = listToWriteTo;
+    }
+
+    public Object readMessage() {
+      synchronized(readList) {
+        while (readList.isEmpty()) {
+          try {
+            readList.wait();
+          }
+          catch (InterruptedException e) {
+          }
+        }
+        return readList.removeFirst();
+      }
+    }
+
+    public Object readMessageWithTimeout(long millis) {
+      synchronized(readList) {
+        if (readList.isEmpty()) {
+          if (millis == 0) {
+            return null;
+          }
+          try {
+            readList.wait(millis);
+          }
+          catch (InterruptedException e) {
+          }
+        }
+        // If list is still empty after wait, return null
+        if (readList.isEmpty()) {
+          return null;
+        }
+        return readList.removeFirst();
+      }
+    }
+
+    public void writeMessage(Object obj) {
+      synchronized(writeList) {
+        writeList.addLast(obj);
+        writeList.notify();
+      }
+    }
+  }
+}