Don't use ObjectIndex when retrieving singletons

There was an inconsistency in that getInstances was using a private
list of singleton instances, but getInstance(Class) was trying to use
the ObjectIndex.

Unfortunately, the use of the ObjectIndex is currently bugged due to
its loading ALL instances when any single instance type is requested.

Thus we update the getInstance(Class) implementation to use a private
hash of available instances.

A proper ObjectIndex implementation will need to wait for 3.0.0.

Author: hinerm

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

@@ -33,7 +33,9 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 import org.scijava.log.LogService;
 import org.scijava.object.LazyObjects;
@@ -61,6 +63,8 @@ public abstract class AbstractSingletonService<PT extends SingletonPlugin>
 	/** List of singleton plugin instances. */
 	private List<PT> instances;
 
+	private Map<Class<? extends PT>, PT> instanceMap;
+
 	// -- SingletonService methods --
 
 	@Override
@@ -69,10 +73,11 @@ public List<PT> getInstances() {
 		return instances;
 	}
 
+	@SuppressWarnings("unchecked")
 	@Override
 	public <P extends PT> P getInstance(final Class<P> pluginClass) {
-		final List<P> objects = objectService.getObjects(pluginClass);
-		return objects == null || objects.isEmpty() ? null : objects.get(0);
+		if (instanceMap == null) initInstances();
+		return (P) instanceMap.get(pluginClass);
 	}
 
 	// -- Service methods --
@@ -98,6 +103,14 @@ private synchronized void initInstances() {
 			Collections.unmodifiableList(filterInstances(getPluginService()
 				.createInstancesOfType(getPluginType())));
 
+		instanceMap = new HashMap<Class<? extends PT>, PT>();
+
+		for (PT plugin : instances) {
+			@SuppressWarnings("unchecked")
+			Class<? extends PT> ptClass = (Class<? extends PT>) plugin.getClass();
+			instanceMap.put(ptClass, plugin);
+		}
+
 		log.info("Found " + instances.size() + " " +
 			getPluginType().getSimpleName() + " plugins.");
 	}