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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.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.annotation.Annotation;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Map;

/**
 * Indexes a directory of classes.
 * 
 * The assumption is that the classes are tested at some stage before packaging.
 * That will be the time when we can index the annotations, even if Eclipse
 * decided not to run any annotation processor.
 * 
 * 
 * @author Johannes Schindelin
 */
public class DirectoryIndexer extends AbstractIndexWriter {

	public void index(final File directory) throws IOException {
		try {
			@SuppressWarnings("deprecation")
			final URLClassLoader loader =
				new URLClassLoader(new URL[] { directory.toURL() }, Thread
					.currentThread().getContextClassLoader());
			discoverAnnotations(directory, "", loader);
		}
		catch (MalformedURLException e) {
			throw new IOException(e);
		}
		write(directory);
	}

	protected void discoverAnnotations(final File directory,
		final String classNamePrefix, final ClassLoader loader) throws IOException
	{
		final File[] list = directory.listFiles();
		if (list == null) {
			return;
		}
		for (final File file : list) {
			if (file.isDirectory()) {
				discoverAnnotations(file, classNamePrefix + file.getName() + ".",
					loader);
			}
			else if (file.isFile()) {
				final String fileName = file.getName();
				if (!fileName.endsWith(".class")) {
					continue;
				}
				final String className =
					classNamePrefix + fileName.substring(0, fileName.length() - 6);
				try {
					for (final Map.Entry> entry : ByteCodeAnalyzer
						.getAnnotations(file).entrySet())
					{
						final String annotationName = entry.getKey();
						try {
							if (!isIndexable(loader.loadClass(annotationName))) {
								continue;
							}
						}
						catch (ClassNotFoundException e) {
							// fall back to the class loader that laoded the directory indexer
							if (!isIndexable(Class.forName(annotationName))) {
								continue;
							}
						}
						add(entry.getValue(), annotationName, className);
					}
				}
				catch (Throwable e) {
					System.err.println("Warning: could not load class '" + className +
						"'; skipping");
					e.printStackTrace();
				}
			}
		}
	}

	protected synchronized  void add(final A annotation,
		final String className)
	{
		if (!isIndexable(annotation)) {
			return;
		}
		add(adapt(annotation), annotation.annotationType().getName(), className);
	}

	private static  boolean isIndexable(final A annotation)
	{
		return isIndexable(annotation.annotationType());
	}

	private static boolean isIndexable(final Class annotationClass) {
		return annotationClass.getAnnotation(Indexable.class) != null;
	}

	protected synchronized void write(final File directory) throws IOException {
		final StreamFactory factory = new StreamFactory() {

			@Override
			public InputStream openInput(String annotationName) throws IOException {
				final File file =
					new File(directory, Index.INDEX_PREFIX + annotationName);
				if (file.exists()) {
					return new FileInputStream(file);
				}
				return null;
			}

			@Override
			public OutputStream openOutput(String annotationName) throws IOException {
				final File file =
					new File(directory, Index.INDEX_PREFIX + annotationName);
				final File dir = file.getParentFile();
				if (dir != null && !dir.isDirectory() && !dir.mkdirs()) {
					throw new IOException("Could not make directory " + dir);
				}
				return new FileOutputStream(file) {

					@Override
					public void close() throws IOException {
						super.close();
						if (file.length() == 0) {
							file.delete();
						}
					}
				};
			}

			@Override
			public boolean isClassObsolete(String className) {
				final String classPath = className.replace('.', '/') + ".class";
				return !new File(directory, classPath).exists();
			}
		};

		final File[] possiblyObsoletes =
			new File(directory, Index.INDEX_PREFIX).listFiles();
		if (possiblyObsoletes != null) {
			for (final File candidate : possiblyObsoletes) {
				if (candidate.isFile()) {
					final String annotationName = candidate.getName();
					merge(annotationName, factory);
				}
			}
		}
		write(factory);
	}
}