core/src/main/java/org/apache/calcite/util/DelegatingInvocationHandler.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.calcite.util;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * A class derived from <code>DelegatingInvocationHandler</code> handles a
 * method call by looking for a method in itself with identical parameters. If
 * no such method is found, it forwards the call to a fallback object, which
 * must implement all of the interfaces which this proxy implements.
 *
 * <p>It is useful in creating a wrapper class around an interface which may
 * change over time.</p>
 *
 * <p>Example:</p>
 *
 * <blockquote>
 * <pre>import java.sql.Connection;
 * Connection connection = ...;
 * Connection tracingConnection = (Connection) Proxy.newProxyInstance(
 *     null,
 *     new Class[] {Connection.class},
 *     new DelegatingInvocationHandler() {
 *         protected Object getTarget() {
 *             return connection;
 *         }
 *         Statement createStatement() {
 *             System.out.println("statement created");
 *             return connection.createStatement();
 *         }
 *     });</pre>
 * </blockquote>
 */
public abstract class DelegatingInvocationHandler implements InvocationHandler {
  //~ Methods ----------------------------------------------------------------

  public Object invoke(
      Object proxy,
      Method method,
      Object[] args) throws Throwable {
    Class clazz = getClass();
    Method matchingMethod;
    try {
      matchingMethod =
          clazz.getMethod(
              method.getName(),
              method.getParameterTypes());
    } catch (NoSuchMethodException | SecurityException e) {
      matchingMethod = null;
    }
    try {
      if (matchingMethod != null) {
        // Invoke the method in the derived class.
        return matchingMethod.invoke(this, args);
      } else {
        // Invoke the method on the proxy.
        return method.invoke(
            getTarget(),
            args);
      }
    } catch (InvocationTargetException e) {
      throw e.getTargetException();
    }
  }

  /**
   * Returns the object to forward method calls to, should the derived class
   * not implement the method. Generally, this object will be a member of the
   * derived class, supplied as a parameter to its constructor.
   */
  protected abstract Object getTarget();
}