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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 both 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
 * 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
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * #L%
 */

package org.scijava.annotations;

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.net.URL;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

/**
 * Makes the annotation indexes accessible.
 * 
 * You would call it like this:

 * 
 * for (IndexItem item : Index.load(MyAnnotation.class)) {

 *    // do something with item.annotation() and/or item.className()

 * }

 * 
 * 
 * 
 * @author Johannes Schindelin
 */
public class Index implements Iterable> {

	/**
	 * Loads the index of all classes annotated with the specified annotation.
	 * 
	 * The specified annotation needs to be annotated with {@link Indexable} for
	 * the annotation indexing to work properly, of course.
	 * 
	 * 
	 * @param annotation the annotation type
	 * @return the index
	 */
	public static  Index load(final Class annotation)
	{
		return load(annotation, Thread.currentThread().getContextClassLoader());
	}

	/**
	 * Loads the index of all classes annotated with the specified annotation.
	 * 
	 * 
	 * 
	 * @param annotation the annotation type
	 * @param loader the class loader to use when loading {@link Class}-type
	 *          annotation fields
	 * @return the index
	 */
	public static  Index load(final Class annotation,
		final ClassLoader loader)
	{
		EclipseHelper.updateAnnotationIndex(loader);
		return new Index(annotation, loader);
	}

	static final String INDEX_PREFIX = "META-INF/json/";
	private static final String LEGACY_INDEX_PREFIX = "META-INF/annotations/";

	private final Class annotation;
	private final ClassLoader loader;

	private Index(final Class annotation, final ClassLoader loader) {
		this.annotation = annotation;
		this.loader = loader;
	}

	private class IndexItemIterator implements Iterator> {

		private Enumeration urls;
		private IndexReader indexReader;
		private IndexItem next;
		private Set seen;

		private Map legacyURLs;

		public IndexItemIterator(final Class annotation) {
			seen = new HashSet();
			try {
				legacyURLs = new LinkedHashMap();
				final Enumeration legacy =
					loader.getResources(LEGACY_INDEX_PREFIX + annotation.getName());
				final int legacySuffixLength =
					LEGACY_INDEX_PREFIX.length() + annotation.getName().length();
				while (legacy.hasMoreElements()) {
					final URL url = legacy.nextElement();
					final String string = url.toString();
					final String key =
						string.substring(0, string.length() - legacySuffixLength) +
							INDEX_PREFIX + annotation.getName();
					legacyURLs.put(key, url);
				}
				if (legacyURLs.isEmpty()) {
					legacyURLs = null;
				}

				urls = loader.getResources(INDEX_PREFIX + annotation.getName());
				readNext();
			}
			catch (final IOException e) {
				e.printStackTrace();
			}
		}

		private void readNext() throws IOException {
			for (;;) {
				if (indexReader == null) {
					try {
						indexReader = getNextReader();
					} catch (IOException e) {
						e.printStackTrace();
						continue;
					}
					if (indexReader == null) {
						urls = null;
						next = null;
						return;
					}
				}
				@SuppressWarnings("unchecked")
				final Map map =
					(Map) indexReader.next();
				if (map != null) {
					final String className = (String) map.get("class");
					@SuppressWarnings("unchecked")
					final Map values =
						(Map) map.get("values");
					next = new IndexItem(annotation, loader, className, values);
					return;
				}
				indexReader.close();
				indexReader = null;
			}
		}

		private IndexReader getNextReader() throws IOException {
			if (urls == null) {
				return null;
			}
			while (urls.hasMoreElements()) {
				final URL url = urls.nextElement();
				if (seen.contains(url)) continue;
				if (legacyURLs != null) {
					legacyURLs.remove(url.toString());
				}
				seen.add(url);
				return new IndexReader(url.openStream());
			}
			if (legacyURLs != null && !legacyURLs.isEmpty()) {
				final Entry entry =
					legacyURLs.entrySet().iterator().next();
				legacyURLs.remove(entry.getKey());
				return IndexReader.getLegacyReader(entry.getValue().openStream());
			}
			return null;
		}

		@Override
		public boolean hasNext() {
			return next != null;
		}

		@Override
		public IndexItem next() {
			final IndexItem result = next;
			try {
				readNext();
			}
			catch (final IOException e) {
				e.printStackTrace();
				next = null;
			}
			return result;
		}

		@Override
		public void remove() {
			throw new UnsupportedOperationException();
		}
	}

	@Override
	public Iterator> iterator() {
		return new IndexItemIterator(annotation);
	}
}