ThreadService: add API for backing ExecutorService

For some uses cases (particularly for certain calls to ImgLib2
functionality), it is helpful to have access to the ExecutorService
being used internally by the ThreadService. And while we're at it,
we may as well make the ExecutorService itself configurable.

I have a vague intuition that ExecutorService is probably not applicable
to all threading models of all possible sorts of Java runtimes, so I
include a caveat in the javadoc that the accessor is allowed to return
null, and the mutator is allowed to throw UnsupportedOperationException,
if the ThreadService implementation does not use an ExecutorService.

Author: ctrueden

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

@@ -83,6 +83,16 @@ public Future<?> run(final Runnable code) {
 		return executor().submit(wrap(code));
 	}
 
+	@Override
+	public ExecutorService getExecutorService() {
+		return executor();
+	}
+
+	@Override
+	public void setExecutorService(final ExecutorService executor) {
+		this.executor = executor;
+	}
+
 	@Override
 	public boolean isDispatchThread() {
 		return EventQueue.isDispatchThread();

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

@@ -33,6 +33,7 @@
 
 import java.lang.reflect.InvocationTargetException;
 import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Future;
 import java.util.concurrent.ThreadFactory;
 
@@ -91,6 +92,24 @@ public enum ThreadContext {
 	 */
 	Future<?> run(Runnable code);
 
+	/**
+	 * Gets the {@link ExecutorService} object used when {@link #run} is called.
+	 * 
+	 * @return the {@link ExecutorService}, or null if an {@link ExecutorService}
+	 *         is not used in this {@link ThreadService} implementation.
+	 */
+	ExecutorService getExecutorService();
+
+	/**
+	 * Sets the {@link ExecutorService} object used when {@link #run} is called.
+	 * 
+	 * @param executor The {@link ExecutorService} for this {@link ThreadService}
+	 *          to use internally for {@link #run} calls.
+	 * @throws UnsupportedOperationException if this {@link ThreadService} does
+	 *           not use an {@link ExecutorService} to handle {@link #run} calls.
+	 */
+	void setExecutorService(ExecutorService executor);
+
 	/**
 	 * Gets whether the current thread is a dispatch thread for use with
 	 * {@link #invoke} and {@link #queue}.