core/api/src/main/java/org/apache/polygene/api/composite/DecoratorMixin.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.polygene.api.composite;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import org.apache.polygene.api.injection.scope.Uses;

/**
 * Generic decorator mixin that allows a Composite to wrap
 * any other Composite as long as they share an interface.
 * <p>
 * Can be used to effectively implement
 * singleton mixins, since the decorated object can be shared between
 * many instances.
 * </p>
 */
public class DecoratorMixin
    implements InvocationHandler
{
    private static final String NL = System.getProperty( "line.separator" );

    private Object delegate;

    public DecoratorMixin( @Uses Object delegate )
    {
        if( delegate instanceof Class )
        {
            Thread.dumpStack();
        }
        this.delegate = delegate;
    }

    @Override
    public Object invoke( Object object, Method method, Object[] args )
        throws Throwable
    {
        if( delegate instanceof InvocationHandler )
        {
            InvocationHandler handler = (InvocationHandler) delegate;
            return handler.invoke( object, method, args );
        }
        else
        {
            try
            {
                return method.invoke( delegate, args );
            }
            catch( InvocationTargetException e )
            {
                throw e.getCause();
            }
            catch( IllegalArgumentException e )
            {
                String message = constructMessage( method, args );
                throw new IllegalArgumentException( message, e );
            }
        }
    }

    private String constructMessage( Method method, Object[] args )
    {
        StringBuilder builder = new StringBuilder();
        builder.append( NL ).append( "method: " );
        builder.append( method.getDeclaringClass().getName() );
        builder.append( "." );
        builder.append( method.getName() );
        builder.append( NL ).append( "delegate: " );
        builder.append( delegate );
        builder.append( NL ).append( "delegateType: " );
        builder.append( delegate == null ? "n/a" : delegate.getClass().getName() );
        builder.append( NL ).append( "arguments:" ).append( NL );
        for( Object arg : args )
        {
            builder.append( "    " );
            Class argClass = arg.getClass();
            if( Proxy.isProxyClass( argClass ) )
            {
                builder.append( Proxy.getInvocationHandler( arg ).getClass().getName() );
            }
            else
            {
                builder.append( argClass.getName() );
            }
            builder.append( NL );
        }
        return builder.toString();
    }
}