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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:#00C">try</code> {
37   *         <code style="color:#0C0">//...use the object...</code>
38   *     } <code style="color:#00C">catch</code>(Exception e) {
39   *         <code style="color:#0C0">// invalidate the object</code>
40   *         pool.invalidateObject(obj);
41   *         <code style="color:#0C0">// do not return the object to the pool twice</code>
42   *         obj = <code style="color:#00C">null</code>;
43   *     } <code style="color:#00C">finally</code> {
44   *         <code style="color:#0C0">// make sure the object is returned to the pool</code>
45   *         <code style="color:#00C">if</code>(<code style="color:#00C">null</code> != obj) {
46   *             pool.returnObject(obj);
47   *        }
48   *     }
49   * } <code style="color:#00C">catch</code>(Exception e) {
50   *       <code style="color:#0C0">// failed to borrow an object</code>
51   * }</pre>
52   * </p>
53   *
54   * <p>See {@link BaseObjectPool} for a simple base implementation.</p>
55   *
56   * @author Rodney Waldhoff
57   * @author Sandy McArthur
58   * @version $Revision: 962893 $ $Date: 2010-07-10 10:37:27 -0700 (Sat, 10 Jul 2010) $
59   * @see PoolableObjectFactory
60   * @see ObjectPoolFactory
61   * @see KeyedObjectPool
62   * @see BaseObjectPool
63   * @since Pool 1.0
64   */
65  public interface ObjectPool {
66      /**
67       * Obtains an instance from this pool.
68       * <p>
69       * Instances returned from this method will have been either newly created with
70       * {@link PoolableObjectFactory#makeObject makeObject} or will be a previously idle object and
71       * have been activated with {@link PoolableObjectFactory#activateObject activateObject} and
72       * then validated with {@link PoolableObjectFactory#validateObject validateObject}.
73       * </p>
74       * <p>
75       * By contract, clients <strong>must</strong> return the borrowed instance using
76       * {@link #returnObject returnObject}, {@link #invalidateObject invalidateObject}, or a related method
77       * as defined in an implementation or sub-interface.
78       * </p>
79       * <p>
80       * The behaviour of this method when the pool has been exhausted
81       * is not strictly specified (although it may be specified by implementations).
82       * Older versions of this method would return <code>null</code> to indicate exhaustion,
83       * newer versions are encouraged to throw a {@link NoSuchElementException}.
84       * </p>
85       *
86       * @return an instance from this pool.
87       * @throws IllegalStateException after {@link #close close} has been called on this pool.
88       * @throws Exception when {@link PoolableObjectFactory#makeObject makeObject} throws an exception.
89       * @throws NoSuchElementException when the pool is exhausted and cannot or will not return another instance.
90       */
91      Object borrowObject() throws Exception, NoSuchElementException, IllegalStateException;
92  
93      /**
94       * Return an instance to the pool.
95       * By contract, <code>obj</code> <strong>must</strong> have been obtained
96       * using {@link #borrowObject() borrowObject}
97       * or a related method as defined in an implementation
98       * or sub-interface.
99       *
100      * @param obj a {@link #borrowObject borrowed} instance to be returned.
101      * @throws Exception 
102      */
103     void returnObject(Object obj) throws Exception;
104 
105     /**
106      * <p>Invalidates an object from the pool.</p>
107      * 
108      * <p>By contract, <code>obj</code> <strong>must</strong> have been obtained
109      * using {@link #borrowObject borrowObject} or a related method as defined in
110      * an implementation or sub-interface.</p>
111      *
112      * <p>This method should be used when an object that has been borrowed
113      * is determined (due to an exception or other problem) to be invalid.</p>
114      *
115      * @param obj a {@link #borrowObject borrowed} instance to be disposed.
116      * @throws Exception
117      */
118     void invalidateObject(Object obj) throws Exception;
119 
120     /**
121      * Create an object using the {@link PoolableObjectFactory factory} or other
122      * implementation dependent mechanism, passivate it, and then place it in the idle object pool.
123      * <code>addObject</code> is useful for "pre-loading" a pool with idle objects.
124      * (Optional operation).
125      *
126      * @throws Exception when {@link PoolableObjectFactory#makeObject} fails.
127      * @throws IllegalStateException after {@link #close} has been called on this pool.
128      * @throws UnsupportedOperationException when this pool cannot add new idle objects.
129      */
130     void addObject() throws Exception, IllegalStateException, UnsupportedOperationException;
131 
132     /**
133      * Return the number of instances
134      * currently idle in this pool (optional operation).
135      * This may be considered an approximation of the number
136      * of objects that can be {@link #borrowObject borrowed}
137      * without creating any new instances.
138      * Returns a negative value if this information is not available.
139      *
140      * @return the number of instances currently idle in this pool or a negative value if unsupported
141      * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
142      */
143     int getNumIdle() throws UnsupportedOperationException;
144 
145     /**
146      * Return the number of instances
147      * currently borrowed from this pool
148      * (optional operation).
149      * Returns a negative value if this information is not available.
150      *
151      * @return the number of instances currently borrowed from this pool or a negative value if unsupported
152      * @throws UnsupportedOperationException <strong>deprecated</strong>: if this implementation does not support the operation
153      */
154     int getNumActive() throws UnsupportedOperationException;
155 
156     /**
157      * Clears any objects sitting idle in the pool, releasing any
158      * associated resources (optional operation).
159      * Idle objects cleared must be {@link PoolableObjectFactory#destroyObject(Object) destroyed}.
160      *
161      * @throws UnsupportedOperationException if this implementation does not support the operation
162      */
163     void clear() throws Exception, UnsupportedOperationException;
164 
165     /**
166      * Close this pool, and free any resources associated with it.
167      * <p>
168      * Calling {@link #addObject} or {@link #borrowObject} after invoking
169      * this method on a pool will cause them to throw an
170      * {@link IllegalStateException}.
171      * </p>
172      *
173      * @throws Exception <strong>deprecated</strong>: implementations should silently fail if not all resources can be freed.
174      */
175     void close() throws Exception;
176 
177     /**
178      * Sets the {@link PoolableObjectFactory factory} this pool uses
179      * to create new instances (optional operation). Trying to change
180      * the <code>factory</code> after a pool has been used will frequently
181      * throw an {@link UnsupportedOperationException}. It is up to the pool
182      * implementation to determine when it is acceptable to call this method.
183      *
184      * @param factory the {@link PoolableObjectFactory} used to create new instances.
185      * @throws IllegalStateException when the factory cannot be set at this time
186      * @throws UnsupportedOperationException if this implementation does not support the operation
187      * @deprecated to be removed in pool 2.0
188      */
189     void setFactory(PoolableObjectFactory factory) throws IllegalStateException, UnsupportedOperationException;
190 }