001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.pool2;
018
019/**
020 * An interface defining life-cycle methods for instances to be served by an
021 * {@link ObjectPool}.
022 * <p>
023 * By contract, when an {@link ObjectPool} delegates to a
024 * {@link PooledObjectFactory},
025 * </p>
026 * <ol>
027 *  <li>
028 *   {@link #makeObject} is called whenever a new instance is needed.
029 *  </li>
030 *  <li>
031 *   {@link #activateObject} is invoked on every instance that has been
032 *   {@link #passivateObject passivated} before it is
033 *   {@link ObjectPool#borrowObject borrowed} from the pool.
034 *  </li>
035 *  <li>
036 *   {@link #validateObject} may be invoked on {@link #activateObject activated}
037 *   instances to make sure they can be {@link ObjectPool#borrowObject borrowed}
038 *   from the pool. {@link #validateObject} may also be used to
039 *   test an instance being {@link ObjectPool#returnObject returned} to the pool
040 *   before it is {@link #passivateObject passivated}. It will only be invoked
041 *   on an activated instance.
042 *  </li>
043 *  <li>
044 *   {@link #passivateObject} is invoked on every instance when it is returned
045 *   to the pool.
046 *  </li>
047 *  <li>
048 *   {@link #destroyObject} is invoked on every instance when it is being
049 *   "dropped" from the pool (whether due to the response from
050 *   {@link #validateObject}, or for reasons specific to the pool
051 *   implementation.) There is no guarantee that the instance being destroyed
052 *   will be considered active, passive or in a generally consistent state.
053 *  </li>
054 * </ol>
055 * {@link PooledObjectFactory} must be thread-safe. The only promise
056 * an {@link ObjectPool} makes is that the same instance of an object will not
057 * be passed to more than one method of a <code>PoolableObjectFactory</code>
058 * at a time.
059 * <p>
060 * While clients of a {@link KeyedObjectPool} borrow and return instances of
061 * the underlying value type {@code V}, the factory methods act on instances of
062 * {@link PooledObject PooledObject&lt;V&gt;}.  These are the object wrappers that
063 * pools use to track and maintain state information about the objects that
064 * they manage.
065 * </p>
066 *
067 * @param <T> Type of element managed in this factory.
068 *
069 * @see ObjectPool
070 *
071 * @since 2.0
072 */
073public interface PooledObjectFactory<T> {
074
075  /**
076   * Creates an instance that can be served by the pool and wrap it in a
077   * {@link PooledObject} to be managed by the pool.
078   *
079   * @return a {@code PooledObject} wrapping an instance that can be served by the pool
080   *
081   * @throws Exception if there is a problem creating a new instance,
082   *    this will be propagated to the code requesting an object.
083   */
084  PooledObject<T> makeObject() throws Exception;
085
086  /**
087   * Destroys an instance no longer needed by the pool.
088   * <p>
089   * It is important for implementations of this method to be aware that there
090   * is no guarantee about what state <code>obj</code> will be in and the
091   * implementation should be prepared to handle unexpected errors.
092   * </p>
093   * <p>
094   * Also, an implementation must take in to consideration that instances lost
095   * to the garbage collector may never be destroyed.
096   * </p>
097   *
098   * @param p a {@code PooledObject} wrapping the instance to be destroyed
099   *
100   * @throws Exception should be avoided as it may be swallowed by
101   *    the pool implementation.
102   *
103   * @see #validateObject
104   * @see ObjectPool#invalidateObject
105   */
106  void destroyObject(PooledObject<T> p) throws Exception;
107
108  /**
109   * Ensures that the instance is safe to be returned by the pool.
110   *
111   * @param p a {@code PooledObject} wrapping the instance to be validated
112   *
113   * @return <code>false</code> if <code>obj</code> is not valid and should
114   *         be dropped from the pool, <code>true</code> otherwise.
115   */
116  boolean validateObject(PooledObject<T> p);
117
118  /**
119   * Reinitializes an instance to be returned by the pool.
120   *
121   * @param p a {@code PooledObject} wrapping the instance to be activated
122   *
123   * @throws Exception if there is a problem activating <code>obj</code>,
124   *    this exception may be swallowed by the pool.
125   *
126   * @see #destroyObject
127   */
128  void activateObject(PooledObject<T> p) throws Exception;
129
130  /**
131   * Uninitializes an instance to be returned to the idle object pool.
132   *
133   * @param p a {@code PooledObject} wrapping the instance to be passivated
134   *
135   * @throws Exception if there is a problem passivating <code>obj</code>,
136   *    this exception may be swallowed by the pool.
137   *
138   * @see #destroyObject
139   */
140  void passivateObject(PooledObject<T> p) throws Exception;
141}