    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
    019import java.util.NoSuchElementException;
    020
    021/**
    022 * A pooling simple interface.
    023 * <p>
    024 * Example of use:
    025 * <pre style="border:solid thin; padding: 1ex;"
    026 * > Object obj = <code style="color:#00C">null</code>;
    027 *
    028 * <code style="color:#00C">try</code> {
    029 *     obj = pool.borrowObject();
    030 *     <code style="color:#00C">try</code> {
    031 *         <code style="color:#0C0">//...use the object...</code>
    032 *     } <code style="color:#00C">catch</code>(Exception e) {
    033 *         <code style="color:#0C0">// invalidate the object</code>
    034 *         pool.invalidateObject(obj);
    035 *         <code style="color:#0C0">// do not return the object to the pool twice</code>
    036 *         obj = <code style="color:#00C">null</code>;
    037 *     } <code style="color:#00C">finally</code> {
    038 *         <code style="color:#0C0">// make sure the object is returned to the pool</code>
    039 *         <code style="color:#00C">if</code>(<code style="color:#00C">null</code> != obj) {
    040 *             pool.returnObject(obj);
    041 *        }
    042 *     }
    043 * } <code style="color:#00C">catch</code>(Exception e) {
    044 *       <code style="color:#0C0">// failed to borrow an object</code>
    045 * }</pre>
    046 * <p>
    047 * See {@link BaseObjectPool} for a simple base implementation.
    048 *
    049 * @param <T> Type of element pooled in this pool.
    050 *
    051 * @see PooledObjectFactory
    052 * @see KeyedObjectPool
    053 * @see BaseObjectPool
    054 *
    055 * @version $Revision: 1533126 $
    056 *
    057 * @since 2.0
    058 */
    059public interface ObjectPool<T> {
    060    /**
    061     * Obtains an instance from this pool.
    062     * <p>
    063     * Instances returned from this method will have been either newly created
    064     * with {@link PooledObjectFactory#makeObject} or will be a previously
    065     * idle object and have been activated with
    066     * {@link PooledObjectFactory#activateObject} and then validated with
    067     * {@link PooledObjectFactory#validateObject}.
    068     * <p>
    069     * By contract, clients <strong>must</strong> return the borrowed instance
    070     * using {@link #returnObject}, {@link #invalidateObject}, or a related
    071     * method as defined in an implementation or sub-interface.
    072     * <p>
    073     * The behaviour of this method when the pool has been exhausted
    074     * is not strictly specified (although it may be specified by
    075     * implementations).
    076     *
    077     * @return an instance from this pool.
    078     *
    079     * @throws IllegalStateException
    080     *              after {@link #close close} has been called on this pool.
    081     * @throws Exception
    082     *              when {@link PooledObjectFactory#makeObject} throws an
    083     *              exception.
    084     * @throws NoSuchElementException
    085     *              when the pool is exhausted and cannot or will not return
    086     *              another instance.
    087     */
    088    T borrowObject() throws Exception, NoSuchElementException,
    089            IllegalStateException;
    090
    091    /**
    092     * Return an instance to the pool. By contract, <code>obj</code>
    093     * <strong>must</strong> have been obtained using {@link #borrowObject()} or
    094     * a related method as defined in an implementation or sub-interface.
    095     *
    096     * @param obj a {@link #borrowObject borrowed} instance to be returned.
    097     *
    098     * @throws Exception
    099     */
    100    void returnObject(T obj) throws Exception;
    101
    102    /**
    103     * Invalidates an object from the pool.
    104     * <p>
    105     * By contract, <code>obj</code> <strong>must</strong> have been obtained
    106     * using {@link #borrowObject} or a related method as defined in an
    107     * implementation or sub-interface.
    108     * <p>
    109     * This method should be used when an object that has been borrowed is
    110     * determined (due to an exception or other problem) to be invalid.
    111     *
    112     * @param obj a {@link #borrowObject borrowed} instance to be disposed.
    113     *
    114     * @throws Exception
    115     */
    116    void invalidateObject(T obj) throws Exception;
    117
    118    /**
    119     * Create an object using the {@link PooledObjectFactory factory} or other
    120     * implementation dependent mechanism, passivate it, and then place it in
    121     * the idle object pool. <code>addObject</code> is useful for "pre-loading"
    122     * a pool with idle objects. (Optional operation).
    123     *
    124     * @throws Exception
    125     *              when {@link PooledObjectFactory#makeObject} fails.
    126     * @throws IllegalStateException
    127     *              after {@link #close} has been called on this pool.
    128     * @throws UnsupportedOperationException
    129     *              when this pool cannot add new idle objects.
    130     */
    131    void addObject() throws Exception, IllegalStateException,
    132            UnsupportedOperationException;
    133
    134    /**
    135     * Return the number of instances currently idle in this pool. This may be
    136     * considered an approximation of the number of objects that can be
    137     * {@link #borrowObject borrowed} without creating any new instances.
    138     * Returns a negative value if this information is not available.
    139     * @return the number of instances currently idle in this pool.
    140     */
    141    int getNumIdle();
    142
    143    /**
    144     * Return the number of instances currently borrowed from this pool. Returns
    145     * a negative value if this information is not available.
    146     * @return the number of instances currently borrowed from this pool.
    147     */
    148    int getNumActive();
    149
    150    /**
    151     * Clears any objects sitting idle in the pool, releasing any associated
    152     * resources (optional operation). Idle objects cleared must be
    153     * {@link PooledObjectFactory#destroyObject(PooledObject)}.
    154     *
    155     * @throws UnsupportedOperationException
    156     *              if this implementation does not support the operation
    157     */
    158    void clear() throws Exception, UnsupportedOperationException;
    159
    160    /**
    161     * Close this pool, and free any resources associated with it.
    162     * <p>
    163     * Calling {@link #addObject} or {@link #borrowObject} after invoking this
    164     * method on a pool will cause them to throw an {@link IllegalStateException}.
    165     * <p>
    166     * Implementations should silently fail if not all resources can be freed.
    167     */
    168    void close();
    169}