--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/EDU/oswego/cs/dl/util/concurrent/LayeredSync.java	Sat Mar 21 11:00:06 2009 +0000
@@ -0,0 +1,90 @@
+/*
+  File: LayeredSync.java
+
+  Originally written by Doug Lea and released into the public domain.
+  This may be used for any purposes whatsoever without acknowledgment.
+  Thanks for the assistance and support of Sun Microsystems Labs,
+  and everyone contributing, testing, and using this code.
+
+  History:
+  Date       Who                What
+  1Aug1998  dl               Create public version
+*/
+
+package EDU.oswego.cs.dl.util.concurrent;
+
+/**
+ * A class that can be used to compose Syncs.
+ * A LayeredSync object manages two other Sync objects,
+ * <em>outer</em> and <em>inner</em>. The acquire operation
+ * invokes <em>outer</em>.acquire() followed by <em>inner</em>.acquire(),
+ * but backing out of outer (via release) upon an exception in inner.
+ * The other methods work similarly.
+ * <p>
+ * LayeredSyncs can be used to compose arbitrary chains
+ * by arranging that either of the managed Syncs be another
+ * LayeredSync.
+ *
+ * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
+**/
+
+
+public class LayeredSync implements Sync {
+
+  protected final Sync outer_;
+  protected final Sync inner_;
+
+  /** 
+   * Create a LayeredSync managing the given outer and inner Sync
+   * objects
+   **/
+
+  public LayeredSync(Sync outer, Sync inner) {
+    outer_ = outer;
+    inner_ = inner;
+  }
+
+  public void acquire() throws InterruptedException {
+    outer_.acquire();
+    try {
+      inner_.acquire();
+    }
+    catch (InterruptedException ex) {
+      outer_.release();
+      throw ex;
+    }
+  }
+
+  public boolean attempt(long msecs) throws InterruptedException {
+
+    long start = (msecs <= 0)? 0 : System.currentTimeMillis();
+    long waitTime = msecs;
+
+    if (outer_.attempt(waitTime)) {
+      try {
+        if (msecs > 0)
+          waitTime = msecs - (System.currentTimeMillis() - start);
+        if (inner_.attempt(waitTime))
+          return true;
+        else {
+          outer_.release();
+          return false;
+        }
+      }
+      catch (InterruptedException ex) {
+        outer_.release();
+        throw ex;
+      }
+    }
+    else
+      return false;
+  }
+
+  public void release() {
+    inner_.release();
+    outer_.release();
+  }
+
+}
+
+