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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 - 2017 Board of Regents of the University of
 * Wisconsin-Madison, Broad Institute of MIT and Harvard, Max Planck
 * Institute of Molecular Cell Biology and Genetics, University of
 * Konstanz, and KNIME GmbH.
 * %%
 * 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
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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package org.scijava.plugin;

import java.util.ArrayList;
import java.util.List;

import org.scijava.object.LazyObjects;
import org.scijava.object.ObjectService;

/**
 * A service for managing {@link SingletonPlugin}s of a particular type. The
 * {@code SingletonService} creates and maintain a list of singleton instances.
 * 
 * Note that like {@link PTService}, {@link TypedService} and
 * {@link WrapperService}, {@code SingletonService} is not a service interface
 * defining API for a specific concrete service implementation, but rather a
 * more general layer in a type hierarchy intended to ease creation of services
 * that fit its pattern.
 * 
 * 
 * @author Curtis Rueden
 * @param  Plugin type of the plugins being managed.
 * @see PTService
 */
public interface SingletonService extends
	PTService
{

	/** Gets the {@link ObjectService} upon which this service depends. */
	default ObjectService objectService() {
		return context().getService(ObjectService.class);
	}

	/**
	 * Gets the list of plugin instances. There will be one singleton instance for
	 * each available plugin.
	 */
	List getInstances();

	/** Gets the singleton plugin instance of the given class. */
	 P getInstance(Class
 pluginClass);

	/**
	 * Filters the given list of instances by this service's inclusion criteria.
	 * 
	 * @param list the initial list of instances
	 * @return the filtered list of instances
	 */
	default List filterInstances(final List list) {
		return list;
	}

	// -- PTService methods --

	@Override
	default 
 P create(final Class
 pluginClass) {
		throw new UnsupportedOperationException(
			"Cannot create singleton plugin instance. "
				+ "Use getInstance(Class) instead.");
	}

	// -- Service methods --

	@Override
	default void initialize() {
		// add singleton instances to the object index... IN THE FUTURE!
		objectService().getIndex().addLater(new LazyObjects