Make sure that event handlers are not GC'ed prematurely

dscho · dscho · commit a58313b2e0fb · 2013-07-30T15:05:05.000-05:00
Unfortunately, our ProxySubscriber instances were garbage collected
rather quickly because there was no reference to them (references to the
event handler object do not prevent ProxySubscribers from being GCed).
So let's make sure that there are references to each ProxySubscriber as
long as there are still references to the corresponding event handler
object.

Signed-off-by: Johannes Schindelin &lt;johannes.schindelin@gmx.de&gt;
diff --git a/src/main/java/org/scijava/event/DefaultEventService.java b/src/main/java/org/scijava/event/DefaultEventService.java
@@ -40,6 +40,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import java.util.WeakHashMap;
 
 import org.bushe.swing.event.annotation.AbstractProxySubscriber;
 import org.bushe.swing.event.annotation.BaseProxySubscriber;
@@ -209,6 +210,37 @@ private Class<? extends SciJavaEvent> getEventClass(final Method m) {
 		return eventClass;
 	}
 
+	// -- Event handlers garbage collection preventer --
+
+	private WeakHashMap<Object, List<ProxySubscriber<?>>> keepEm =
+			new WeakHashMap<Object, List<ProxySubscriber<?>>>();
+
+	/**
+	 * Prevents {@link ProxySubscriber} instances from being garbage collected
+	 * prematurely.
+	 * <p>
+	 * We instantiate a {@link ProxySubscriber} for each method with an
+	 * {@link EventHandler} annotation. These instances are then passed to the
+	 * EventBus. The way the instances are created ensures that the event handlers
+	 * will be held only as weak references. But they are weak references to the
+	 * {@link ProxySubscriber} instances rather than the object containing the
+	 * {@link EventHandler}-annotated methods. Therefore, we have to make sure
+	 * that there is a non-GC'able reference to each {@link ProxySubscriber} as
+	 * long as there is a reference to the containing event handler object.
+	 * </p>
+	 * 
+	 * @param o the object containing {@link EventHandler}-annotated methods
+	 * @param subscriber a {@link ProxySubscriber} for a particular {@link EventHandler}
+	 */
+	private synchronized void keepIt(final Object o, final ProxySubscriber<?> subscriber) {
+		List<ProxySubscriber<?>> list = keepEm.get(o);
+		if (list == null) {
+			list = new ArrayList<ProxySubscriber<?>>();
+			keepEm.put(o, list);
+		}
+		list.add(subscriber);
+	}
+
 	// -- Helper classes --
 
 	/**
@@ -229,6 +261,7 @@ private class ProxySubscriber<E extends SciJavaEvent> extends
 
 		public ProxySubscriber(final Class<E> c, final Object o, final Method m) {
 			super(o, m, ReferenceStrength.WEAK, eventBus, false);
+			keepIt(o, this);
 			this.c = c;
 
 			// allow calling of non-public methods
