/*
 * 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.pool;

/**
 * An interface defining life-cycle methods for
 * instances to be served by an {@link ObjectPool}.
 * <p>
 * By contract, when an {@link ObjectPool}
 * delegates to a {@link PoolableObjectFactory},
 * <ol>
 *  <li>
 *   {@link #makeObject makeObject}
 *   is called whenever a new instance is needed.
 *  </li>
 *  <li>
 *   {@link #activateObject activateObject}
 *   is invoked on every instance that has been
 *   {@link #passivateObject passivated} before it is
 *   {@link ObjectPool#borrowObject borrowed} from the pool.
 *  </li>
 *  <li>
 *   {@link #validateObject validateObject}
 *   is invoked on {@link #activateObject activated} instances to make sure
 *   they can be {@link ObjectPool#borrowObject borrowed} from the pool.
 *   <code>validateObject</code> <strong>may</strong> also be used to test an
 *   instance being {@link ObjectPool#returnObject returned} to the pool
 *   before it is {@link #passivateObject passivated}. It will only be invoked
 *   on an activated instance.
 *  </li>
 *  <li>
 *   {@link #passivateObject passivateObject}
 *   is invoked on every instance when it is returned to the pool.
 *  </li>
 *  <li>
 *   {@link #destroyObject destroyObject}
 *   is invoked on every instance when it is being "dropped" from the
 *   pool (whether due to the response from <code>validateObject</code>,
 *   or for reasons specific to the pool implementation.) There is no
 *   guarantee that the instance being destroyed will
 *   be considered active, passive or in a generally consistent state.
 *  </li>
 * </ol>
 * </p>
 * <p>
 * {@link PoolableObjectFactory} must be thread-safe. The only promise
 * an {@link ObjectPool} makes is that the same instance of an object will not
 * be passed to more than one method of a <code>PoolableObjectFactory</code>
 * at a time.
 * </p>
 *
 * @param <T> the type of objects held in this pool
 * 
 * @see ObjectPool
 *
 * @author Rodney Waldhoff
 * @author Sandy McArthur
 * @version $Revision: 1222388 $ $Date: 2011-12-22 13:28:27 -0500 (Thu, 22 Dec 2011) $
 * @since Pool 1.0
 */
public interface PoolableObjectFactory<T> {
  /**
   * Creates an instance that can be served by the pool.
   * Instances returned from this method should be in the
   * same state as if they had been
   * {@link #activateObject activated}. They will not be
   * activated before being served by the pool.
   *
   * @return an instance that can be served by the pool.
   * @throws Exception if there is a problem creating a new instance,
   *    this will be propagated to the code requesting an object.
   */
  T makeObject() throws Exception;

  /**
   * Destroys an instance no longer needed by the pool.
   * <p>
   * It is important for implementations of this method to be aware
   * that there is no guarantee about what state <code>obj</code>
   * will be in and the implementation should be prepared to handle
   * unexpected errors.
   * </p>
   * <p>
   * Also, an implementation must take in to consideration that
   * instances lost to the garbage collector may never be destroyed.
   * </p>
   *
   * @param obj the instance to be destroyed
   * @throws Exception should be avoided as it may be swallowed by
   *    the pool implementation.
   * @see #validateObject
   * @see ObjectPool#invalidateObject
   */
  void destroyObject(T obj) throws Exception;

  /**
   * Ensures that the instance is safe to be returned by the pool.
   * Returns <code>false</code> if <code>obj</code> should be destroyed.
   *
   * @param obj the instance to be validated
   * @return <code>false</code> if <code>obj</code> is not valid and should
   *         be dropped from the pool, <code>true</code> otherwise.
   */
  boolean validateObject(T obj);

  /**
   * Reinitialize an instance to be returned by the pool.
   *
   * @param obj the instance to be activated
   * @throws Exception if there is a problem activating <code>obj</code>,
   *    this exception may be swallowed by the pool.
   * @see #destroyObject
   */
  void activateObject(T obj) throws Exception;

  /**
   * Uninitialize an instance to be returned to the idle object pool.
   *
   * @param obj the instance to be passivated
   * @throws Exception if there is a problem passivating <code>obj</code>,
   *    this exception may be swallowed by the pool.
   * @see #destroyObject
   */
  void passivateObject(T obj) throws Exception;
}
