Lean on object service for singleton retrieval

Instead of maintaining our own list of singletons, we should be using
the ObjectService.

Author: hinerm

src/main/java/org/scijava/plugin/AbstractSingletonService.java

@@ -31,7 +31,6 @@
 
 package org.scijava.plugin;
 
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -58,19 +57,16 @@ public abstract class AbstractSingletonService<PT extends SingletonPlugin>
 	private ObjectService objectService;
 
 	// TODO: Listen for PluginsAddedEvent and PluginsRemovedEvent
-	// and update the list of singletons accordingly.
-
-	/** List of singleton plugin instances. */
-	private List<PT> instances;
+	// and update the map of singletons accordingly.
 
 	private Map<Class<? extends PT>, PT> instanceMap;
 
 	// -- SingletonService methods --
 
 	@Override
 	public List<PT> getInstances() {
-		if (instances == null) initInstances();
-		return instances;
+		final List<PT> plugins = objectService.getObjects(getPluginType());
+		return Collections.unmodifiableList(plugins);
 	}
 
 	@SuppressWarnings("unchecked")
@@ -89,7 +85,7 @@ public void initialize() {
 
 			@Override
 			public List<PT> get() {
-				return new ArrayList<PT>(getInstances());
+				return createInstances();
 			}
 
 			@Override
@@ -115,27 +111,30 @@ protected List<PT> filterInstances(final List<PT> list) {
 	// -- Helper methods --
 
 	private synchronized void initInstances() {
-		if (instances != null) return;
-
-		final List<PT> list =
-			Collections.unmodifiableList(filterInstances(getPluginService()
-				.createInstancesOfType(getPluginType())));
+		if (instanceMap != null) return;
 
 		final HashMap<Class<? extends PT>, PT> map =
 			new HashMap<Class<? extends PT>, PT>();
 
+		final List<PT> list = getInstances();
 		for (final PT plugin : list) {
 			@SuppressWarnings("unchecked")
 			final Class<? extends PT> ptClass =
 				(Class<? extends PT>) plugin.getClass();
 			map.put(ptClass, plugin);
 		}
 
-		log.debug("Found " + list.size() + " " + getPluginType().getSimpleName() +
-			" plugins.");
-
 		instanceMap = map;
-		instances = list;
+	}
+
+	private List<PT> createInstances() {
+		final List<PT> instances =
+			filterInstances(getPluginService().createInstancesOfType(getPluginType()));
+
+		log.info("Found " + instances.size() + " " +
+			getPluginType().getSimpleName() + " plugins.");
+
+		return instances;
 	}
 
 }