core/api/src/main/java/org/apache/myfaces/extensions/cdi/core/api/tools/annotate/DefaultAnnotation.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.myfaces.extensions.cdi.core.api.tools.annotate;

import java.lang.annotation.Annotation;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * <p>A small helper class to create an Annotation instance of the given annotation class
 * via {@link java.lang.reflect.Proxy}. The annotation literal gets filled with the default values.</p>
 * <p/>
 * <p>usage:</p>
 * <pre>
 * Class<? extends annotation> annotationClass = ...;
 * Annotation a = DefaultAnnotation.of(annotationClass)
 * </pre>
 *
 * @author Mark Struberg
 * @author Gerhard Petracek
 */
public class DefaultAnnotation implements InvocationHandler
{
    private Class<? extends Annotation> annotationClass;

    /**
     * Required to use the result of the factory instead of a default implementation
     * of {@link javax.enterprise.util.AnnotationLiteral}.
     *
     * @param annotationClass class of the target annotation
     */
    private DefaultAnnotation(Class<? extends Annotation> annotationClass)
    {
        this.annotationClass = annotationClass;
    }

    static volatile Map<Class<? extends Annotation>, Annotation> annotationCache
            = new ConcurrentHashMap<Class<? extends Annotation>, Annotation>();

    public static Annotation of(Class<? extends Annotation> annotationClass)
    {
        Annotation annotation = annotationCache.get(annotationClass);

        if (annotation == null)
        {
            // switch into paranoia mode
            synchronized (annotationCache)
            {
                annotation = annotationCache.get(annotationClass);
                if (annotation == null)
                {
                    annotation = (Annotation) Proxy.newProxyInstance(
                            annotationClass.getClassLoader(),
                            new Class[]{annotationClass},
                            new DefaultAnnotation(annotationClass));
                    
                    annotationCache.put(annotationClass, annotation);
                }
            }
        }

        return annotation;
    }

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable
    {
        if ("hashCode".equals(method.getName()))
        {
            return hashCode();
        }
        else if ("equals".equals(method.getName()))
        {
            return equals(args[0]);
        }
        else if ("annotationType".equals(method.getName()))
        {
            return this.annotationClass;
        }
        else if ("toString".equals(method.getName()))
        {
            return "Proxy for " + this.annotationClass.getName() + " (" + getClass().getName() + ")";
        }

        return method.getDefaultValue();
    }
}