/*
 * 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 ObjectPool. 
 By contract, when an ObjectPool delegates to a PoolableObjectFactory, 

 
 makeObject is called whenever a new instance is needed. 
 
 activateObject is invoked on every instance that has been passivated before it is borrowed from the pool. 
 
 validateObject is invoked on activated instances to make sure they can be borrowed from the pool. validateObject may also be used to test an instance being returned to the pool before it is passivated. It will only be invoked on an activated instance. 
 
 passivateObject is invoked on every instance when it is returned to the pool. 
 
 destroyObject is invoked on every instance when it is being "dropped" from the pool (whether due to the response from validateObject,
  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.
 


 PoolableObjectFactory must be thread-safe. The only promise an ObjectPool makes is that the same instance of an object will not be passed to more than one method of a PoolableObjectFactory
at a time.

Author:
Rodney Waldhoff, Sandy McArthur
Type parameters:
<T> – the type of objects held in this pool
See Also:
ObjectPool
Version:
$Revision: 1222388 $ $Date: 2011-12-22 13:28:27 -0500 (Thu, 22 Dec 2011) $
Since:
Pool 1.0/**
 * 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 activated. They will not be activated before being 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.
Returns:
an instance that can be served by the pool./**
   * 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.

It is important for implementations of this method to be aware
that there is no guarantee about what state obj
will be in and the implementation should be prepared to handle
unexpected errors.


Also, an implementation must take in to consideration that
instances lost to the garbage collector may never be destroyed.

Params:
obj – the instance to be destroyed
Throws:
Exception – should be avoided as it may be swallowed by
   the pool implementation.
See Also:
validateObject
ObjectPool.invalidateObject/**
   * 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 false if obj should be destroyed.
Params:
obj – the instance to be validated
Returns:
false if obj is not valid and should
        be dropped from the pool, true otherwise./**
   * 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.
Params:
obj – the instance to be activated
Throws:
Exception – if there is a problem activating obj,
   this exception may be swallowed by the pool.
See Also:
destroyObject/**
   * 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.
Params:
obj – the instance to be passivated
Throws:
Exception – if there is a problem passivating obj,
   this exception may be swallowed by the pool.
See Also:
destroyObject/**
   * 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;
}