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     */
017    
018    package org.apache.commons.pool;
019    
020    import java.util.NoSuchElementException;
021    
022    /**
023     * A pooling interface.
024     * <p>
025     * <code>ObjectPool</code> defines a trivially simple pooling interface. The only
026     * required methods are {@link #borrowObject borrowObject}, {@link #returnObject returnObject}
027     * and {@link #invalidateObject invalidateObject}.
028     * </p>
029     * <p>
030     * Example of use:
031     * <pre style="border:solid thin; padding: 1ex;"
032     * > Object obj = <code style="color:#00C">null</code>;
033     *
034     * <code style="color:#00C">try</code> {
035     *     obj = pool.borrowObject();
036     *     <code style="color:#00C">try</code> {
037     *         <code style="color:#0C0">//...use the object...</code>
038     *     } <code style="color:#00C">catch</code>(Exception e) {
039     *         <code style="color:#0C0">// invalidate the object</code>
040     *         pool.invalidateObject(obj);
041     *         <code style="color:#0C0">// do not return the object to the pool twice</code>
042     *         obj = <code style="color:#00C">null</code>;
043     *     } <code style="color:#00C">finally</code> {
044     *         <code style="color:#0C0">// make sure the object is returned to the pool</code>
045     *         <code style="color:#00C">if</code>(<code style="color:#00C">null</code> != obj) {
046     *             pool.returnObject(obj);
047     *        }
048     *     }
049     * } <code style="color:#00C">catch</code>(Exception e) {
050     *       <code style="color:#0C0">// failed to borrow an object</code>
051     * }</pre>
052     * </p>
053     *
054     * <p>See {@link BaseObjectPool} for a simple base implementation.</p>
055     *
056     * @param <T> the type of objects held in this pool
057     * 
058     * @author Rodney Waldhoff
059     * @author Sandy McArthur
060     * @version $Revision: 1222396 $ $Date: 2011-12-22 14:02:25 -0500 (Thu, 22 Dec 2011) $
061     * @see PoolableObjectFactory
062     * @see ObjectPoolFactory
063     * @see KeyedObjectPool
064     * @see BaseObjectPool
065     * @since Pool 1.0
066     */
067    public interface ObjectPool<T> {
068        /**
069         * Obtains an instance from this pool.
070         * <p>
071         * Instances returned from this method will have been either newly created with
072         * {@link PoolableObjectFactory#makeObject makeObject} or will be a previously idle object and
073         * have been activated with {@link PoolableObjectFactory#activateObject activateObject} and
074         * then validated with {@link PoolableObjectFactory#validateObject validateObject}.
075         * </p>
076         * <p>
077         * By contract, clients <strong>must</strong> return the borrowed instance using
078         * {@link #returnObject returnObject}, {@link #invalidateObject invalidateObject}, or a related method
079         * as defined in an implementation or sub-interface.
080         * </p>
081         * <p>
082         * The behaviour of this method when the pool has been exhausted
083         * is not strictly specified (although it may be specified by implementations).
084         * Older versions of this method would return <code>null</code> to indicate exhaustion,
085         * newer versions are encouraged to throw a {@link NoSuchElementException}.
086         * </p>
087         *
088         * @return an instance from this pool.
089         * @throws IllegalStateException after {@link #close close} has been called on this pool.
090         * @throws Exception when {@link PoolableObjectFactory#makeObject makeObject} throws an exception.
091         * @throws NoSuchElementException when the pool is exhausted and cannot or will not return another instance.
092         */
093        T borrowObject() throws Exception, NoSuchElementException, IllegalStateException;
094    
095        /**
096         * Return an instance to the pool.
097         * By contract, <code>obj</code> <strong>must</strong> have been obtained
098         * using {@link #borrowObject() borrowObject}
099         * or a related method as defined in an implementation
100         * or sub-interface.
101         *
102         * @param obj a {@link #borrowObject borrowed} instance to be returned.
103         * @throws Exception 
104         */
105        void returnObject(T obj) throws Exception;
106    
107        /**
108         * <p>Invalidates an object from the pool.</p>
109         * 
110         * <p>By contract, <code>obj</code> <strong>must</strong> have been obtained
111         * using {@link #borrowObject borrowObject} or a related method as defined in
112         * an implementation or sub-interface.</p>
113         *
114         * <p>This method should be used when an object that has been borrowed
115         * is determined (due to an exception or other problem) to be invalid.</p>
116         *
117         * @param obj a {@link #borrowObject borrowed} instance to be disposed.
118         * @throws Exception
119         */
120        void invalidateObject(T obj) throws Exception;
121    
122        /**
123         * Create an object using the {@link PoolableObjectFactory factory} or other
124         * implementation dependent mechanism, passivate it, and then place it in the idle object pool.
125         * <code>addObject</code> is useful for "pre-loading" a pool with idle objects.
126         * (Optional operation).
127         *
128         * @throws Exception when {@link PoolableObjectFactory#makeObject} fails.
129         * @throws IllegalStateException after {@link #close} has been called on this pool.
130         * @throws UnsupportedOperationException when this pool cannot add new idle objects.
131         */
132        void addObject() throws Exception, IllegalStateException, UnsupportedOperationException;
133    
134        /**
135         * Return the number of instances
136         * currently idle in this pool (optional operation).
137         * This may be considered an approximation of the number
138         * of objects that can be {@link #borrowObject borrowed}
139         * without creating any new instances.
140         * Returns a negative value if this information is not available.
141         *
142         * @return the number of instances currently idle in this pool or a negative value if unsupported
143         * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
144         */
145        int getNumIdle() throws UnsupportedOperationException;
146    
147        /**
148         * Return the number of instances
149         * currently borrowed from this pool
150         * (optional operation).
151         * Returns a negative value if this information is not available.
152         *
153         * @return the number of instances currently borrowed from this pool or a negative value if unsupported
154         * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
155         */
156        int getNumActive() throws UnsupportedOperationException;
157    
158        /**
159         * Clears any objects sitting idle in the pool, releasing any
160         * associated resources (optional operation).
161         * Idle objects cleared must be {@link PoolableObjectFactory#destroyObject(Object) destroyed}.
162         *
163         * @throws UnsupportedOperationException if this implementation does not support the operation
164         */
165        void clear() throws Exception, UnsupportedOperationException;
166    
167        /**
168         * Close this pool, and free any resources associated with it.
169         * <p>
170         * Calling {@link #addObject} or {@link #borrowObject} after invoking
171         * this method on a pool will cause them to throw an
172         * {@link IllegalStateException}.
173         * </p>
174         *
175         * @throws Exception <strong>deprecated</strong>: implementations should silently fail if not all resources can be freed.
176         */
177        void close() throws Exception;
178    
179        /**
180         * Sets the {@link PoolableObjectFactory factory} this pool uses
181         * to create new instances (optional operation). Trying to change
182         * the <code>factory</code> after a pool has been used will frequently
183         * throw an {@link UnsupportedOperationException}. It is up to the pool
184         * implementation to determine when it is acceptable to call this method.
185         *
186         * @param factory the {@link PoolableObjectFactory} used to create new instances.
187         * @throws IllegalStateException when the factory cannot be set at this time
188         * @throws UnsupportedOperationException if this implementation does not support the operation
189         * @deprecated to be removed in pool 2.0
190         */
191        @Deprecated
192        void setFactory(PoolableObjectFactory<T> factory) throws IllegalStateException, UnsupportedOperationException;
193    }