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 * <ol>
026 *  <li>
027 *   {@link #makeObject} is called whenever a new instance is needed.
028 *  </li>
029 *  <li>
030 *   {@link #activateObject} is invoked on every instance that has been
031 *   {@link #passivateObject passivated} before it is
032 *   {@link ObjectPool#borrowObject borrowed} from the pool.
033 *  </li>
034 *  <li>
035 *   {@link #validateObject} may be invoked on {@link #activateObject activated}
036 *   instances to make sure they can be {@link ObjectPool#borrowObject borrowed}
037 *   from the pool. {@link #validateObject} may also be used to
038 *   test an instance being {@link ObjectPool#returnObject returned} to the pool
039 *   before it is {@link #passivateObject passivated}. It will only be invoked
040 *   on an activated instance.
041 *  </li>
042 *  <li>
043 *   {@link #passivateObject} is invoked on every instance when it is returned
044 *   to the pool.
045 *  </li>
046 *  <li>
047 *   {@link #destroyObject} is invoked on every instance when it is being
048 *   "dropped" from the pool (whether due to the response from
049 *   {@link #validateObject}, or for reasons specific to the pool
050 *   implementation.) There is no guarantee that the instance being destroyed
051 *   will be considered active, passive or in a generally consistent state.
052 *  </li>
053 * </ol>
054 * {@link PooledObjectFactory} must be thread-safe. The only promise
055 * an {@link ObjectPool} makes is that the same instance of an object will not
056 * be passed to more than one method of a <code>PoolableObjectFactory</code>
057 * at a time.
058 * <p>
059 * While clients of a {@link KeyedObjectPool} borrow and return instances of
060 * the underlying value type {@code V}, the factory methods act on instances of
061 * {@link PooledObject PooledObject&lt;V&gt;}.  These are the object wrappers that
062 * pools use to track and maintain state information about the objects that
063 * they manage.
064 *
065 * @param <T> Type of element managed in this factory.
066 *
067 * @see ObjectPool
068 *
069 * @since 2.0
070 */
071public interface PooledObjectFactory<T> {
072  /**
073   * Create an instance that can be served by the pool and wrap it in a
074   * {@link PooledObject} to be managed by the pool.
075   *
076   * @return a {@code PooledObject} wrapping an instance that can be served by the pool
077   *
078   * @throws Exception if there is a problem creating a new instance,
079   *    this will be propagated to the code requesting an object.
080   */
081  PooledObject<T> makeObject() throws Exception;
082
083  /**
084   * Destroys an instance no longer needed by the pool.
085   * <p>
086   * It is important for implementations of this method to be aware that there
087   * is no guarantee about what state <code>obj</code> will be in and the
088   * implementation should be prepared to handle unexpected errors.
089   * <p>
090   * Also, an implementation must take in to consideration that instances lost
091   * to the garbage collector may never be destroyed.
092   * </p>
093   *
094   * @param p a {@code PooledObject} wrapping the instance to be destroyed
095   *
096   * @throws Exception should be avoided as it may be swallowed by
097   *    the pool implementation.
098   *
099   * @see #validateObject
100   * @see ObjectPool#invalidateObject
101   */
102  void destroyObject(PooledObject<T> p) throws Exception;
103
104  /**
105   * Ensures that the instance is safe to be returned by the pool.
106   *
107   * @param p a {@code PooledObject} wrapping the instance to be validated
108   *
109   * @return <code>false</code> if <code>obj</code> is not valid and should
110   *         be dropped from the pool, <code>true</code> otherwise.
111   */
112  boolean validateObject(PooledObject<T> p);
113
114  /**
115   * Reinitialize an instance to be returned by the pool.
116   *
117   * @param p a {@code PooledObject} wrapping the instance to be activated
118   *
119   * @throws Exception if there is a problem activating <code>obj</code>,
120   *    this exception may be swallowed by the pool.
121   *
122   * @see #destroyObject
123   */
124  void activateObject(PooledObject<T> p) throws Exception;
125
126  /**
127   * Uninitialize an instance to be returned to the idle object pool.
128   *
129   * @param p a {@code PooledObject} wrapping the instance to be passivated
130   *
131   * @throws Exception if there is a problem passivating <code>obj</code>,
132   *    this exception may be swallowed by the pool.
133   *
134   * @see #destroyObject
135   */
136  void passivateObject(PooledObject<T> p) throws Exception;
137}