core/src/main/java/org/apache/commons/proxy2/invoker/DuckTypingInvoker.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.commons.proxy2.invoker;

import java.lang.reflect.Method;

import org.apache.commons.proxy2.Invoker;
import org.apache.commons.proxy2.ObjectProvider;

/**
 * An invoker which supports <a href="http://en.wikipedia.org/wiki/Duck_typing">&quot;duck typing&quot;</a>, meaning
 * that it finds a matching method on the object returned from the target provider and invokes it. This class is useful
 * for adapting an existing class to an interface it does not implement.
 * <p>
 * <b>Example:</b>
 * </p>
 * <p>
 * 
 * <pre>
 * public class LegacyDuck // Does not implement interface!
 * {
 *   public void quack()
 *   {
 *     // Quacking logic...
 *   }
 * }
 * <p/>
 * public interface Duck
 * {
 *   public void quack();
 * }
 * <p/>
 * ObjectProvider targetProvider = new ConstantProvider(new LegacyDuck()); // Always returns a "legacy" duck
 * DuckTypingInvoker invoker = new DuckTypingInvoker(targetProvider);
 * Duck duck = ( Duck )proxyFactory.createInvokerProxy( invoker, new Class[] { Duck.class } );
 * </pre>
 * 
 * </p>
 */
public class DuckTypingInvoker implements Invoker
{
    /** Serialization version */
    private static final long serialVersionUID = 1L;

    //******************************************************************************************************************
    // Fields
    //******************************************************************************************************************

    private final ObjectProvider<?> targetProvider;

    //******************************************************************************************************************
    // Constructors
    //******************************************************************************************************************

    /**
     * Create a new DuckTypingInvoker instance.
     * 
     * @param targetProvider
     */
    public DuckTypingInvoker(final ObjectProvider<?> targetProvider)
    {
        this.targetProvider = targetProvider;
    }

    //******************************************************************************************************************
    // Invoker Implementation
    //******************************************************************************************************************

    /**
     * {@inheritDoc}
     */
    @Override
    public Object invoke(final Object proxy, final Method method, final Object[] arguments) throws Throwable
    {
        final Object target = targetProvider.getObject();
        final Class<?> targetClass = target.getClass();
        try
        {
            final Method targetMethod = targetClass.getMethod(method.getName(), method.getParameterTypes());
            if (method.getReturnType().isAssignableFrom(targetMethod.getReturnType()))
            {
                return targetMethod.invoke(target, arguments);
            }
            throw new UnsupportedOperationException("Target type " + targetClass.getName()
                    + " method has incompatible return type.");
        }
        catch (NoSuchMethodException e)
        {
            throw new UnsupportedOperationException("Target type " + targetClass.getName()
                    + " does not have a method matching " + method + ".", e);
        }
    }
}