Using test-and-test-and-set.

See http://en.wikipedia.org/wiki/Spinlock, the section about
optimizations.
See also http://en.wikipedia.org/wiki/Test_and_Test-and-set.

jetty-util/src/main/java/org/eclipse/jetty/util/thread/SpinLock.java

@@ -20,9 +20,7 @@ package org.eclipse.jetty.util.thread;
 
 import java.util.concurrent.atomic.AtomicReference;
 
-
-/* ------------------------------------------------------------ */
-/** Spin Lock
+/**
  * <p>This is a lock designed to protect VERY short sections of
  * critical code.  Threads attempting to take the lock will spin
  * forever until the lock is available, thus it is important that
@@ -43,12 +41,14 @@ public class SpinLock
 
     public Lock lock()
     {
-        Thread thread = Thread.currentThread();
-        while(true)
+        Thread current = Thread.currentThread();
+        while (true)
         {
-            if (!_lock.compareAndSet(null,thread))
+            // Using test-and-test-and-set for better performance.
+            Thread locker = _lock.get();
+            if (locker != null || !_lock.compareAndSet(null, current))
             {
-                if (_lock.get()==thread)
+                if (locker == current)
                     throw new IllegalStateException("SpinLock is not reentrant");
                 continue;
             }
@@ -58,12 +58,7 @@ public class SpinLock
 
     public boolean isLocked()
     {
-        return _lock.get()!=null;
-    }
-    
-    public boolean isLockedThread()
-    {
-        return _lock.get()==Thread.currentThread();
+        return _lock.get() != null;
     }
 
     public class Lock implements AutoCloseable

jetty-util/src/test/java/org/eclipse/jetty/util/thread/SpinLockTest.java

@@ -18,17 +18,16 @@
 
 package org.eclipse.jetty.util.thread;
 
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertTrue;
-
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 
 import org.junit.Test;
 
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
 public class SpinLockTest
 {
-    
     @Test
     public void testLocked()
     {
@@ -70,7 +69,6 @@ public class SpinLockTest
         assertFalse(lock.isLocked());
     }
 
-
     @Test
     public void testContend() throws Exception
     {
@@ -121,7 +119,7 @@ public class SpinLockTest
         };
         thread1.start();
         // thread1 will be spinning here
-        assertFalse(held1.await(100,TimeUnit.MILLISECONDS));
+        assertFalse(held1.await(100, TimeUnit.MILLISECONDS));
         
         // Let thread0 complete
         hold0.countDown();
@@ -135,8 +133,5 @@ public class SpinLockTest
         thread1.join();
 
         assertFalse(lock.isLocked());
-        
     }
-    
-    
 }