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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, 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.lang.annotation.Annotation;
import java.lang.reflect.Array;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.List;
import java.util.Map;

/**
 * Allows access to individual annotations.
 * 
 * @author Johannes Schindelin
 */
public class IndexItem {

	private final Class annotation;
	private final ClassLoader loader;
	private final String className;
	private final Map map;

	IndexItem(final Class annotation, final ClassLoader loader,
		final String className, final Map map)
	{
		this.annotation = annotation;
		this.loader = loader;
		this.className = className;
		this.map = map;
	}

	/**
	 * Obtains the annotation values.
	 * 
	 * @return the annotation values
	 */
	public A annotation() {
		return proxy(annotation, loader, className, map);
	}

	/**
	 * Returns the name of the annotated class.
	 * 
	 * @return the name of the annotated class.
	 */
	public String className() {
		return className;
	}

	@SuppressWarnings("unchecked")
	private static  A proxy(final Class annotation,
		final ClassLoader loader, final String className,
		final Map map)
	{
		return (A) Proxy.newProxyInstance(loader, new Class[] { annotation },
			new InvocationHandler() {

				@Override
				public Object invoke(Object proxy, Method method, Object[] args)
					throws Throwable
				{
					if (map.containsKey(method)) {
						return map.get(method);
					}
					final String name = method.getName();
					Object value;
					if (map.containsKey(name)) {
						value = map.get(name);
						final Class expectedType = method.getReturnType();
						if (!expectedType.isAssignableFrom(value.getClass())) {
							value = adapt(value, loader, expectedType, className);
						}
					}
					else if (name.equals("toString") &&
						(args == null || args.length == 0))
					{
						value = "@" + annotation.getName() + map;
					}
					else if (name.equals("annotationType") &&
						(args == null || args.length == 0))
					{
						value = annotation;
					}
					else if (name.equals("hashCode") &&
						(args == null || args.length == 0))
					{
						return annotation.hashCode() ^ map.hashCode();
					}
					else if (name.equals("equals") && args != null && args.length == 1) {
						if (!(args[0] instanceof Annotation) ||
							((Annotation) args[0]).annotationType() != annotation)
						{
							return false;
						}
						for (Method method2 : annotation.getMethods()) {
							if (method2.getDeclaringClass() == annotation) {
								if (!invoke(proxy, method2, new Object[0]).equals(
									method2.invoke(args[0])))
								{
									return false;
								}
							}
						}
						return true;
					}
					else {
						value = method.getDefaultValue();
						if (value == null) {
							throw new IllegalArgumentException("Could not find value for " +
								name);
						}
					}
					map.put(method, value);
					return value;
				}
			});
	}

	private static Object adapt(final Object o, final ClassLoader loader,
		final Class expectedType, final String className)
	{
		if (o == null) {
			return null;
		}
		else if (expectedType.isAssignableFrom(o.getClass())) {
			return o;
		}
		else if (o instanceof Boolean) {
			return (boolean) (Boolean) o;
		}
		else if (o instanceof Long) {
			long l = (Long) o;
			if (expectedType == Byte.TYPE) {
				return (byte) l;
			}
			else if (expectedType == Short.TYPE) {
				return (short) l;
			}
			else if (expectedType == Integer.TYPE) {
				return (int) l;
			}
			else if (expectedType == Long.TYPE) {
				return l;
			}
		}
		else if (o instanceof Double) {
			double d = (Double) o;
			if (expectedType == Float.TYPE) {
				return (float) d;
			}
			return d;
		}
		else if (expectedType == Character.TYPE) {
			String s = (String) o;
			if (s.length() == 1) {
				return s.charAt(0);
			}
		}
		else if (expectedType == Class.class) {
			try {
				return loader.loadClass((String) o);
			}
			catch (Throwable t) {
				throw cce(o, expectedType, className, t);
			}
		}
		else if (expectedType.isArray()) {
			@SuppressWarnings("unchecked")
			List