ThreadService: improve thread safety

We now use double-checked locking for initialization. Otherwise,
conceivably, two threads could trigger simultaneous creation of
competing ExecutorServices.

And we also synchronize the disposal, so that the ExecutorService
cannot possibly be created after dispose() is called.

Author: ctrueden

src/main/java/org/scijava/thread/DefaultThreadService.java

@@ -141,9 +141,12 @@ public ThreadContext getThreadContext(final Thread thread) {
 	// -- Disposable methods --
 
 	@Override
-	public void dispose() {
+	public synchronized void dispose() {
 		disposed = true;
-		if (executor != null) executor.shutdown();
+		if (executor != null) {
+			executor.shutdown();
+			executor = null;
+		}
 	}
 
 	// -- ThreadFactory methods --
@@ -157,12 +160,15 @@ public Thread newThread(final Runnable r) {
 	// -- Helper methods --
 
 	private ExecutorService executor() {
-		if (executor == null) {
-			executor = Executors.newCachedThreadPool(this);
-		}
+		if (executor == null) initExecutor();
 		return executor;
 	}
 
+	private synchronized void initExecutor() {
+		if (executor != null) return;
+		executor = Executors.newCachedThreadPool(this);
+	}
+
 	private Runnable wrap(final Runnable r) {
 		final Thread parent = Thread.currentThread();
 		return new Runnable() {