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Common reflection code used in support of annotation processing
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/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * License: GNU Lesser General Public License (LGPL), version 2.1 or later.
 * See the lgpl.txt file in the root directory or .
 */
package org.hibernate.annotations.common.annotationfactory;

import java.lang.annotation.Annotation;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.SortedSet;
import java.util.TreeSet;

/**
 * A concrete implementation of Annotation that pretends it is a
 * "real" source code annotation. It's also an InvocationHandler.
 * 
 * When you create an AnnotationProxy, you must initialize it
 * with an AnnotationDescriptor.
 * The adapter checks that the provided elements are the same elements defined
 * in the annotation interface. However, it does not check that their
 * values are the right type. If you omit an element, the adapter will use the
 * default value for that element from the annotation interface, if it exists.
 * If no default exists, it will throw an exception.
 * 
 * Warning: this class does not implement hashCode() and
 * equals() - it just uses the ones it inherits from Object.
 * This means that an AnnotationProxy does not follow the
 * recommendations of the Annotation javadoc about these two
 * methods. That's why you should never mix AnnotationProxies
 * with "real" annotations. For example, don't put them into the same
 * Collection.
 *
 * @author Paolo Perrotta
 * @author Davide Marchignoli
 * @see java.lang.annotation.Annotation
 */
public final class AnnotationProxy implements Annotation, InvocationHandler {

	private final Class annotationType;
	//FIXME it's probably better to use String as a key rather than Method
	//      to speed up and avoid any fancy permsize/GC issue
	//      I'd better check the litterature on the subject
	private final Map values;

	public AnnotationProxy(AnnotationDescriptor descriptor) {
		this.annotationType = descriptor.type();
		this.values = getAnnotationValues( annotationType, descriptor );
	}

	private static Map getAnnotationValues(
			Class annotationType,
			AnnotationDescriptor descriptor) {
		Map result = new HashMap();
		int processedValuesFromDescriptor = 0;
		for ( Method m : annotationType.getDeclaredMethods() ) {
			if ( descriptor.containsElement( m.getName() ) ) {
				result.put( m, descriptor.valueOf( m.getName() ) );
				processedValuesFromDescriptor++;
			}
			else if ( m.getDefaultValue() != null ) {
				result.put( m, m.getDefaultValue() );
			}
			else {
				throw new IllegalArgumentException( "No value provided for " + m.getName() );
			}
		}
		if ( processedValuesFromDescriptor != descriptor.numberOfElements() ) {
			throw new RuntimeException( "Trying to instanciate " + annotationType + " with unknown elements" );
		}
		return toSmallMap( result );
	}

	static  Map toSmallMap(Map map) {
		switch ( map.size() ) {
			case 0:
				return Collections.emptyMap();
			case 1:
				Map.Entry entry = map.entrySet().iterator().next();
				return Collections.singletonMap( entry.getKey(), entry.getValue() );
			default:
				return map;
		}
	}

	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
		if ( values.containsKey( method ) ) {
			return values.get( method );
		}
		return method.invoke( this, args );
	}

	public Class annotationType() {
		return annotationType;
	}

	public String toString() {
		StringBuilder result = new StringBuilder();
		result.append( '@' ).append( annotationType().getName() ).append( '(' );
		for ( Method m : getRegisteredMethodsInAlphabeticalOrder() ) {
			result.append( m.getName() ).append( '=' ).append( values.get( m ) ).append( ", " );
		}
		// remove last separator:
		if ( values.size() > 0 ) {
			result.delete( result.length() - 2, result.length() );
			result.append( ")" );
		}
		else {
			result.delete( result.length() - 1, result.length() );
		}

		return result.toString();
	}

	private SortedSet getRegisteredMethodsInAlphabeticalOrder() {
		SortedSet result = new TreeSet(
				new Comparator() {
					public int compare(Method o1, Method o2) {
						return o1.getName().compareTo( o2.getName() );
					}
				}
		);
		result.addAll( values.keySet() );
		return result;
	}
}