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SciJava Common is a shared library for SciJava software. It provides a plugin framework, with an extensible mechanism for service discovery, backed by its own annotation processor, so that plugins can be loaded dynamically. It is used by downstream projects in the SciJava ecosystem, such as ImageJ and SCIFIO.

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/*
 * #%L
 * SciJava Common shared library for SciJava software.
 * %%
 * Copyright (C) 2009 - 2016 Board of Regents of the University of
 * Wisconsin-Madison, Broad Institute of MIT and Harvard, and Max Planck
 * Institute of Molecular Cell Biology and Genetics.
 * %%
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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package org.scijava.plugin;

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.ObjectService;

/**
 * Abstract base class for {@link SingletonService}s.
 * 
 * @author Curtis Rueden
 * @param  Plugin type of the {@link SingletonPlugin}s being managed.
 */
public abstract class AbstractSingletonService
	extends AbstractPTService implements SingletonService
{

	@Parameter
	private LogService log;

	@Parameter
	private ObjectService objectService;

	// TODO: Listen for PluginsAddedEvent and PluginsRemovedEvent
	// and update the list of singletons accordingly.

	/** List of singleton plugin instances. */
	private List instances;

	private Map, PT> instanceMap;

	// -- SingletonService methods --

	@Override
	public ObjectService objectService() {
		return objectService;
	}

	@Override
	public List getInstances() {
		if (instances == null) initInstances();
		return instances;
	}

	@SuppressWarnings("unchecked")
	@Override
	public  P getInstance(final Class pluginClass) {
		if (instanceMap == null) initInstances();
		return (P) instanceMap.get(pluginClass);
	}

	// -- Helper methods --

	private synchronized void initInstances() {
		if (instances != null) return;

		final List list = Collections.unmodifiableList(filterInstances(
			pluginService().createInstancesOfType(getPluginType())));

		final HashMap, PT> map =
			new HashMap<>();

		for (final PT plugin : list) {
			@SuppressWarnings("unchecked")
			final Class ptClass =
				(Class) plugin.getClass();
			map.put(ptClass, plugin);
		}

		log.debug("Found " + list.size() + " " + getPluginType().getSimpleName() +
			" plugins.");

		instanceMap = map;
		instances = list;
	}

}