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+/*
+
+Copyright (c) Xerox Corporation 1998-2002.  All rights reserved.
+
+Use and copying of this software and preparation of derivative works based
+upon this software are permitted.  Any distribution of this software or
+derivative works must comply with all applicable United States export control
+laws.
+
+This software is made available AS IS, and Xerox Corporation makes no warranty
+about the software, its performance or its conformity to any specification.
+
+|<---            this code is formatted to fit into 80 columns             --->|
+|<---            this code is formatted to fit into 80 columns             --->|
+|<---            this code is formatted to fit into 80 columns             --->|
+
+
+RegistrySynchronization.java
+Part of the Spacewar system.
+
+*/
+
+package spacewar;
+
+import coordination.Coordinator;
+
+/**
+ * This aspect ensures synchronized access to methods of the Game in the
+ * presence of several threads.
+ *
+ * It uses the Coordinator class, from the AspectJ coordination library.
+ * (This case is right on the borderline of being too simple to use the
+ * coordination library, but we use it anyways to keep the similarity
+ * with the RegistrySynchronizer.)
+ *
+ * It uses a per-Game coordination scheme, so there is one instance of
+ * this class for each instance of the Game class.  When this class is
+ * constructed, it registers appropriate mutexes and selfexes using
+ * the behavior inherited from Coordinator.
+ *
+ * The coordination constraints for the Game are simple.  We just need to
+ * make sure that newShip and handleCollisions are mutually exclusive.  That
+ * ensures that they we can't destroy a ship that has just been replaced.
+ */
+aspect GameSynchronization extends Coordinator perthis(this(Game)) {
+
+    protected pointcut synchronizationPoint():
+	call(void Game.handleCollisions(..)) || call(Ship Game.newShip(..));
+
+    public GameSynchronization() {
+        addMutex(new String[] {"handleCollisions", "newShip"});
+    }
+
+}