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.io.Closeable;
020import java.util.NoSuchElementException;
021
022/**
023 * A pooling simple interface.
024 * <p>
025 * Example of use:
026 * </p>
027 * <pre style="border:solid thin; padding: 1ex;"
028 * > Object obj = <code style="color:#00C">null</code>;
029 *
030 * <code style="color:#00C">try</code> {
031 *     obj = pool.borrowObject();
032 *     <code style="color:#00C">try</code> {
033 *         <code style="color:#0C0">//...use the object...</code>
034 *     } <code style="color:#00C">catch</code> (Exception e) {
035 *         <code style="color:#0C0">// invalidate the object</code>
036 *         pool.invalidateObject(obj);
037 *         <code style="color:#0C0">// do not return the object to the pool twice</code>
038 *         obj = <code style="color:#00C">null</code>;
039 *     } <code style="color:#00C">finally</code> {
040 *         <code style="color:#0C0">// make sure the object is returned to the pool</code>
041 *         <code style="color:#00C">if</code> (<code style="color:#00C">null</code> != obj) {
042 *             pool.returnObject(obj);
043 *        }
044 *     }
045 * } <code style="color:#00C">catch</code>(Exception e) {
046 *       <code style="color:#0C0">// failed to borrow an object</code>
047 * }</pre>
048 * <p>
049 * See {@link BaseObjectPool} for a simple base implementation.
050 * </p>
051 *
052 * @param <T> Type of element pooled in this pool.
053 * 
054 *
055 * @see PooledObjectFactory
056 * @see KeyedObjectPool
057 * @see BaseObjectPool
058 *
059 * @since 2.0
060 */
061public interface ObjectPool<T> extends Closeable {
062
063    /**
064     * Creates an object using the {@link PooledObjectFactory factory} or other
065     * implementation dependent mechanism, passivate it, and then place it in
066     * the idle object pool. {@code addObject} is useful for "pre-loading"
067     * a pool with idle objects. (Optional operation).
068     *
069     * @throws Exception
070     *              when {@link PooledObjectFactory#makeObject} fails.
071     * @throws IllegalStateException
072     *              after {@link #close} has been called on this pool.
073     * @throws UnsupportedOperationException
074     *              when this pool cannot add new idle objects.
075     */
076    void addObject() throws Exception;
077
078    /**
079     * Calls {@link ObjectPool#addObject()} {@code count}
080     * number of times.
081     *
082     * @param count
083     *            the number of idle objects to add.
084     * @throws Exception See {@link ObjectPool#addObject()}.
085     * @since 2.8.0
086     */
087    default void addObjects(final int count) throws Exception {
088        for (int i = 0; i < count; i++) {
089            addObject();
090        }
091    }
092
093    /**
094     * Borrows an instance from this pool.
095     * <p>
096     * Instances returned from this method will have been either newly created
097     * with {@link PooledObjectFactory#makeObject} or will be a previously
098     * idle object and have been activated with
099     * {@link PooledObjectFactory#activateObject} and then validated with
100     * {@link PooledObjectFactory#validateObject}.
101     * </p>
102     * <p>
103     * By contract, clients <strong>must</strong> return the borrowed instance
104     * using {@link #returnObject}, {@link #invalidateObject}, or a related
105     * method as defined in an implementation or sub-interface.
106     * </p>
107     * <p>
108     * The behavior of this method when the pool has been exhausted
109     * is not strictly specified (although it may be specified by
110     * implementations).
111     * </p>
112     *
113     * @return an instance from this pool.
114     *
115     * @throws IllegalStateException
116     *              after {@link #close close} has been called on this pool.
117     * @throws Exception
118     *              when {@link PooledObjectFactory#makeObject} throws an
119     *              exception.
120     * @throws NoSuchElementException
121     *              when the pool is exhausted and cannot or will not return
122     *              another instance.
123     */
124    T borrowObject() throws Exception;
125
126    /**
127     * Clears any objects sitting idle in the pool, releasing any associated
128     * resources (optional operation). Idle objects cleared must be
129     * {@link PooledObjectFactory#destroyObject(PooledObject)}.
130     *
131     * @throws UnsupportedOperationException
132     *              if this implementation does not support the operation
133     *
134     * @throws Exception if the pool cannot be cleared
135     */
136    void clear() throws Exception;
137
138    /**
139     * Closes this pool, and free any resources associated with it.
140     * <p>
141     * Calling {@link #addObject} or {@link #borrowObject} after invoking this
142     * method on a pool will cause them to throw an {@link IllegalStateException}.
143     * </p>
144     * <p>
145     * Implementations should silently fail if not all resources can be freed.
146     * </p>
147     */
148    @Override
149    void close();
150
151    /**
152     * Gets the number of instances currently borrowed from this pool. Returns
153     * a negative value if this information is not available.
154     * @return the number of instances currently borrowed from this pool.
155     */
156    int getNumActive();
157
158    /**
159     * Gets the number of instances currently idle in this pool. This may be
160     * considered an approximation of the number of objects that can be
161     * {@link #borrowObject borrowed} without creating any new instances.
162     * Returns a negative value if this information is not available.
163     * @return the number of instances currently idle in this pool.
164     */
165    int getNumIdle();
166
167    /**
168     * Invalidates an object from the pool.
169     * <p>
170     * By contract, {@code obj} <strong>must</strong> have been obtained
171     * using {@link #borrowObject} or a related method as defined in an
172     * implementation or sub-interface.
173     * </p>
174     * <p>
175     * This method should be used when an object that has been borrowed is
176     * determined (due to an exception or other problem) to be invalid.
177     * </p>
178     *
179     * @param obj a {@link #borrowObject borrowed} instance to be disposed.
180     *
181     * @throws Exception if the instance cannot be invalidated
182     */
183    void invalidateObject(T obj) throws Exception;
184
185    /**
186     * Invalidates an object from the pool, using the provided
187     * {@link DestroyMode}
188     * <p>
189     * By contract, {@code obj} <strong>must</strong> have been obtained
190     * using {@link #borrowObject} or a related method as defined in an
191     * implementation or sub-interface.
192     * </p>
193     * <p>
194     * This method should be used when an object that has been borrowed is
195     * determined (due to an exception or other problem) to be invalid.
196     * </p>
197     *
198     * @param obj a {@link #borrowObject borrowed} instance to be disposed.
199     * @param destroyMode destroy activation context provided to the factory
200     * @throws Exception if the instance cannot be invalidated
201     * @since 2.9.0
202     */
203    default void invalidateObject(final T obj, final DestroyMode destroyMode) throws Exception {
204        invalidateObject(obj);
205    }
206
207    /**
208     * Returns an instance to the pool. By contract, {@code obj}
209     * <strong>must</strong> have been obtained using {@link #borrowObject()} or
210     * a related method as defined in an implementation or sub-interface.
211     *
212     * @param obj a {@link #borrowObject borrowed} instance to be returned.
213     *
214     * @throws IllegalStateException
215     *              if an attempt is made to return an object to the pool that
216     *              is in any state other than allocated (i.e. borrowed).
217     *              Attempting to return an object more than once or attempting
218     *              to return an object that was never borrowed from the pool
219     *              will trigger this exception.
220     *
221     * @throws Exception if an instance cannot be returned to the pool
222     */
223    void returnObject(T obj) throws Exception;
224
225}