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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  
18  package org.apache.commons.pool;
19  
20  import java.util.NoSuchElementException;
21  
22  /**
23   * A pooling interface.
24   * <p>
25   * <code>ObjectPool</code> defines a trivially simple pooling interface. The only
26   * required methods are {@link #borrowObject borrowObject}, {@link #returnObject returnObject}
27   * and {@link #invalidateObject invalidateObject}.
28   * </p>
29   * <p>
30   * Example of use:
31   * <pre style="border:solid thin; padding: 1ex;"
32   * > Object obj = <code style="color:#00C">null</code>;
33   *
34   * <code style="color:#00C">try</code> {
35   *     obj = pool.borrowObject();
36   *     <code style="color:#0C0">//...use the object...</code>
37   * } <code style="color:#00C">catch</code>(Exception e) {
38   *     <code style="color:#0C0">// invalidate the object</code>
39   *     pool.invalidateObject(obj);
40   *     <code style="color:#0C0">// do not return the object to the pool twice</code>
41   *     obj = <code style="color:#00C">null</code>;
42   * } <code style="color:#00C">finally</code> {
43   *     <code style="color:#0C0">// make sure the object is returned to the pool</code>
44   *     <code style="color:#00C">if</code>(<code style="color:#00C">null</code> != obj) {
45   *         pool.returnObject(obj);
46   *    }
47   * }</pre>
48   * </p>
49   *
50   * <p>See {@link BaseObjectPool} for a simple base implementation.</p>
51   *
52   * @author Rodney Waldhoff
53   * @author Sandy McArthur
54   * @version $Revision: 778007 $ $Date: 2009-05-23 15:57:11 -0400 (Sat, 23 May 2009) $
55   * @see PoolableObjectFactory
56   * @see ObjectPoolFactory
57   * @see KeyedObjectPool
58   * @see BaseObjectPool
59   * @since Pool 1.0
60   */
61  public interface ObjectPool {
62      /**
63       * Obtains an instance from this pool.
64       * <p>
65       * Instances returned from this method will have been either newly created with
66       * {@link PoolableObjectFactory#makeObject makeObject} or will be a previously idle object and
67       * have been activated with {@link PoolableObjectFactory#activateObject activateObject} and
68       * then validated with {@link PoolableObjectFactory#validateObject validateObject}.
69       * </p>
70       * <p>
71       * By contract, clients <strong>must</strong> return the borrowed instance using
72       * {@link #returnObject returnObject}, {@link #invalidateObject invalidateObject}, or a related method
73       * as defined in an implementation or sub-interface.
74       * </p>
75       * <p>
76       * The behaviour of this method when the pool has been exhausted
77       * is not strictly specified (although it may be specified by implementations).
78       * Older versions of this method would return <code>null</code> to indicate exhaustion,
79       * newer versions are encouraged to throw a {@link NoSuchElementException}.
80       * </p>
81       *
82       * @return an instance from this pool.
83       * @throws IllegalStateException after {@link #close close} has been called on this pool.
84       * @throws Exception when {@link PoolableObjectFactory#makeObject makeObject} throws an exception.
85       * @throws NoSuchElementException when the pool is exhausted and cannot or will not return another instance.
86       */
87      Object borrowObject() throws Exception, NoSuchElementException, IllegalStateException;
88  
89      /**
90       * Return an instance to the pool.
91       * By contract, <code>obj</code> <strong>must</strong> have been obtained
92       * using {@link #borrowObject() borrowObject}
93       * or a related method as defined in an implementation
94       * or sub-interface.
95       *
96       * @param obj a {@link #borrowObject borrowed} instance to be returned.
97       * @throws Exception 
98       */
99      void returnObject(Object obj) throws Exception;
100 
101     /**
102      * Invalidates an object from the pool.
103      * By contract, <code>obj</code> <strong>must</strong> have been obtained
104      * using {@link #borrowObject borrowObject}
105      * or a related method as defined in an implementation
106      * or sub-interface.
107      * <p>
108      * This method should be used when an object that has been borrowed
109      * is determined (due to an exception or other problem) to be invalid.
110      * </p>
111      *
112      * @param obj a {@link #borrowObject borrowed} instance to be disposed.
113      * @throws Exception
114      */
115     void invalidateObject(Object obj) throws Exception;
116 
117     /**
118      * Create an object using the {@link PoolableObjectFactory factory} or other
119      * implementation dependent mechanism, passivate it, and then place it in the idle object pool.
120      * <code>addObject</code> is useful for "pre-loading" a pool with idle objects.
121      * (Optional operation).
122      *
123      * @throws Exception when {@link PoolableObjectFactory#makeObject} fails.
124      * @throws IllegalStateException after {@link #close} has been called on this pool.
125      * @throws UnsupportedOperationException when this pool cannot add new idle objects.
126      */
127     void addObject() throws Exception, IllegalStateException, UnsupportedOperationException;
128 
129     /**
130      * Return the number of instances
131      * currently idle in this pool (optional operation).
132      * This may be considered an approximation of the number
133      * of objects that can be {@link #borrowObject borrowed}
134      * without creating any new instances.
135      * Returns a negative value if this information is not available.
136      *
137      * @return the number of instances currently idle in this pool or a negative value if unsupported
138      * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
139      */
140     int getNumIdle() throws UnsupportedOperationException;
141 
142     /**
143      * Return the number of instances
144      * currently borrowed from this pool
145      * (optional operation).
146      * Returns a negative value if this information is not available.
147      *
148      * @return the number of instances currently borrowed from this pool or a negative value if unsupported
149      * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
150      */
151     int getNumActive() throws UnsupportedOperationException;
152 
153     /**
154      * Clears any objects sitting idle in the pool, releasing any
155      * associated resources (optional operation).
156      * Idle objects cleared must be {@link PoolableObjectFactory#destroyObject(Object) destroyed}.
157      *
158      * @throws UnsupportedOperationException if this implementation does not support the operation
159      */
160     void clear() throws Exception, UnsupportedOperationException;
161 
162     /**
163      * Close this pool, and free any resources associated with it.
164      * <p>
165      * Calling {@link #addObject} or {@link #borrowObject} after invoking
166      * this method on a pool will cause them to throw an
167      * {@link IllegalStateException}.
168      * </p>
169      *
170      * @throws Exception <strong>deprecated</strong>: implementations should silently fail if not all resources can be freed.
171      */
172     void close() throws Exception;
173 
174     /**
175      * Sets the {@link PoolableObjectFactory factory} this pool uses
176      * to create new instances (optional operation). Trying to change
177      * the <code>factory</code> after a pool has been used will frequently
178      * throw an {@link UnsupportedOperationException}. It is up to the pool
179      * implementation to determine when it is acceptable to call this method.
180      *
181      * @param factory the {@link PoolableObjectFactory} used to create new instances.
182      * @throws IllegalStateException when the factory cannot be set at this time
183      * @throws UnsupportedOperationException if this implementation does not support the operation
184      */
185     void setFactory(PoolableObjectFactory factory) throws IllegalStateException, UnsupportedOperationException;
186 }