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 * @see PooledObjectFactory
055 * @see KeyedObjectPool
056 * @see BaseObjectPool
057 *
058 * @since 2.0
059 */
060public interface ObjectPool<T> extends Closeable {
061
062    /**
063     * Creates an object using the {@link PooledObjectFactory factory} or other
064     * implementation dependent mechanism, passivate it, and then place it in
065     * the idle object pool. {@code addObject} is useful for "pre-loading"
066     * a pool with idle objects. (Optional operation).
067     *
068     * @throws Exception
069     *              when {@link PooledObjectFactory#makeObject} fails.
070     * @throws IllegalStateException
071     *              after {@link #close} has been called on this pool.
072     * @throws UnsupportedOperationException
073     *              when this pool cannot add new idle objects.
074     */
075    void addObject() throws Exception, IllegalStateException,
076            UnsupportedOperationException;
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
085     *             when {@link ObjectPool#addObject()} fails.
086     * @since 2.8.0
087     */
088    default void addObjects(final int count) throws Exception {
089        for (int i = 0; i < count; i++) {
090            addObject();
091        }
092    }
093
094    /**
095     * Obtains an instance from this pool.
096     * <p>
097     * Instances returned from this method will have been either newly created
098     * with {@link PooledObjectFactory#makeObject} or will be a previously
099     * idle object and have been activated with
100     * {@link PooledObjectFactory#activateObject} and then validated with
101     * {@link PooledObjectFactory#validateObject}.
102     * </p>
103     * <p>
104     * By contract, clients <strong>must</strong> return the borrowed instance
105     * using {@link #returnObject}, {@link #invalidateObject}, or a related
106     * method as defined in an implementation or sub-interface.
107     * </p>
108     * <p>
109     * The behavior of this method when the pool has been exhausted
110     * is not strictly specified (although it may be specified by
111     * implementations).
112     * </p>
113     *
114     * @return an instance from this pool.
115     *
116     * @throws IllegalStateException
117     *              after {@link #close close} has been called on this pool.
118     * @throws Exception
119     *              when {@link PooledObjectFactory#makeObject} throws an
120     *              exception.
121     * @throws NoSuchElementException
122     *              when the pool is exhausted and cannot or will not return
123     *              another instance.
124     */
125    T borrowObject() throws Exception, NoSuchElementException,
126            IllegalStateException;
127
128    /**
129     * Clears any objects sitting idle in the pool, releasing any associated
130     * resources (optional operation). Idle objects cleared must be
131     * {@link PooledObjectFactory#destroyObject(PooledObject)}.
132     *
133     * @throws UnsupportedOperationException
134     *              if this implementation does not support the operation
135     *
136     * @throws Exception if the pool cannot be cleared
137     */
138    void clear() throws Exception, UnsupportedOperationException;
139
140    /**
141     * Closes this pool, and free any resources associated with it.
142     * <p>
143     * Calling {@link #addObject} or {@link #borrowObject} after invoking this
144     * method on a pool will cause them to throw an {@link IllegalStateException}.
145     * </p>
146     * <p>
147     * Implementations should silently fail if not all resources can be freed.
148     * </p>
149     */
150    @Override
151    void close();
152
153    /**
154     * Returns the number of instances currently borrowed from this pool. Returns
155     * a negative value if this information is not available.
156     * @return the number of instances currently borrowed from this pool.
157     */
158    int getNumActive();
159
160    /**
161     * Returns the number of instances currently idle in this pool. This may be
162     * considered an approximation of the number of objects that can be
163     * {@link #borrowObject borrowed} without creating any new instances.
164     * Returns a negative value if this information is not available.
165     * @return the number of instances currently idle in this pool.
166     */
167    int getNumIdle();
168
169    /**
170     * Invalidates an object from the pool.
171     * <p>
172     * By contract, {@code obj} <strong>must</strong> have been obtained
173     * using {@link #borrowObject} or a related method as defined in an
174     * implementation or sub-interface.
175     * </p>
176     * <p>
177     * This method should be used when an object that has been borrowed is
178     * determined (due to an exception or other problem) to be invalid.
179     * </p>
180     *
181     * @param obj a {@link #borrowObject borrowed} instance to be disposed.
182     *
183     * @throws Exception if the instance cannot be invalidated
184     */
185    void invalidateObject(T obj) throws Exception;
186
187    /**
188     * Invalidates an object from the pool, using the provided
189     * {@link DestroyMode}
190     * <p>
191     * By contract, {@code obj} <strong>must</strong> have been obtained
192     * using {@link #borrowObject} or a related method as defined in an
193     * implementation or sub-interface.
194     * </p>
195     * <p>
196     * This method should be used when an object that has been borrowed is
197     * determined (due to an exception or other problem) to be invalid.
198     * </p>
199     *
200     * @param obj a {@link #borrowObject borrowed} instance to be disposed.
201     * @param mode destroy activation context provided to the factory
202     *
203     * @throws Exception if the instance cannot be invalidated
204     * @since 2.9.0
205     */
206    default void invalidateObject(final T obj, final DestroyMode mode) throws Exception {
207        invalidateObject(obj);
208    }
209
210    /**
211     * Returns an instance to the pool. By contract, {@code obj}
212     * <strong>must</strong> have been obtained using {@link #borrowObject()} or
213     * a related method as defined in an implementation or sub-interface.
214     *
215     * @param obj a {@link #borrowObject borrowed} instance to be returned.
216     *
217     * @throws IllegalStateException
218     *              if an attempt is made to return an object to the pool that
219     *              is in any state other than allocated (i.e. borrowed).
220     *              Attempting to return an object more than once or attempting
221     *              to return an object that was never borrowed from the pool
222     *              will trigger this exception.
223     *
224     * @throws Exception if an instance cannot be returned to the pool
225     */
226    void returnObject(T obj) throws Exception;
227
228}