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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.impl;
19  
20  import java.lang.ref.SoftReference;
21  import java.lang.ref.ReferenceQueue;
22  import java.lang.ref.Reference;
23  import java.util.ArrayList;
24  import java.util.Iterator;
25  import java.util.List;
26  import java.util.NoSuchElementException;
27  
28  import org.apache.commons.pool.BaseObjectPool;
29  import org.apache.commons.pool.ObjectPool;
30  import org.apache.commons.pool.PoolableObjectFactory;
31  import org.apache.commons.pool.PoolUtils;
32  
33  /**
34   * A {@link java.lang.ref.SoftReference SoftReference} based
35   * {@link ObjectPool}.
36   *
37   * @author Rodney Waldhoff
38   * @author Sandy McArthur
39   * @version $Revision: 1206501 $ $Date: 2011-11-26 10:11:40 -0700 (Sat, 26 Nov 2011) $
40   * @since Pool 1.0
41   */
42  public class SoftReferenceObjectPool extends BaseObjectPool implements ObjectPool {
43      /**
44       * Create a <code>SoftReferenceObjectPool</code> without a factory.
45       * {@link #setFactory(PoolableObjectFactory) setFactory} should be called
46       * before any attempts to use the pool are made.
47       * Generally speaking you should prefer the {@link #SoftReferenceObjectPool(PoolableObjectFactory)} constructor.
48       *
49       * @see #SoftReferenceObjectPool(PoolableObjectFactory)
50       * @deprecated to be removed in pool 2.0.  Use {@link #SoftReferenceObjectPool(PoolableObjectFactory)}.
51       */
52      public SoftReferenceObjectPool() {
53          _pool = new ArrayList();
54          _factory = null;
55      }
56  
57      /**
58       * Create a <code>SoftReferenceObjectPool</code> with the specified factory.
59       *
60       * @param factory object factory to use.
61       */
62      public SoftReferenceObjectPool(PoolableObjectFactory factory) {
63          _pool = new ArrayList();
64          _factory = factory;
65      }
66  
67      /**
68       * Create a <code>SoftReferenceObjectPool</code> with the specified factory and initial idle object count.
69       *
70       * @param factory object factory to use.
71       * @param initSize initial size to attempt to prefill the pool.
72       * @throws Exception when there is a problem prefilling the pool.
73       * @throws IllegalArgumentException when <code>factory</code> is <code>null</code>.
74       * @deprecated because this is a SoftReference pool, prefilled idle obejects may be garbage collected before they are used.
75       *      To be removed in Pool 2.0.
76       */
77      public SoftReferenceObjectPool(PoolableObjectFactory factory, int initSize) throws Exception, IllegalArgumentException {
78          if (factory == null) {
79              throw new IllegalArgumentException("factory required to prefill the pool.");
80          }
81          _pool = new ArrayList(initSize);
82          _factory = factory;
83          PoolUtils.prefill(this, initSize);
84      }
85  
86      /**
87       * <p>Borrow an object from the pool.  If there are no idle instances available in the pool, the configured
88       * factory's {@link PoolableObjectFactory#makeObject()} method is invoked to create a new instance.</p>
89       * 
90       * <p>All instances are {@link PoolableObjectFactory#activateObject(Object) activated} and
91       * {@link PoolableObjectFactory#validateObject(Object) validated} before being returned by this
92       * method.  If validation fails or an exception occurs activating or validating an idle instance,
93       * the failing instance is {@link PoolableObjectFactory#destroyObject(Object) destroyed} and another
94       * instance is retrieved from the pool, validated and activated.  This process continues until either the
95       * pool is empty or an instance passes validation.  If the pool is empty on activation or
96       * it does not contain any valid instances, the factory's <code>makeObject</code> method is used
97       * to create a new instance.  If the created instance either raises an exception on activation or
98       * fails validation, <code>NoSuchElementException</code> is thrown. Exceptions thrown by <code>MakeObject</code>
99       * are propagated to the caller; but other than <code>ThreadDeath</code> or <code>VirtualMachineError</code>,
100      * exceptions generated by activation, validation or destroy methods are swallowed silently.</p>
101      * 
102      * @throws NoSuchElementException if a valid object cannot be provided
103      * @throws IllegalStateException if invoked on a {@link #close() closed} pool
104      * @throws Exception if an exception occurs creating a new instance
105      * @return a valid, activated object instance
106      */
107     public synchronized Object borrowObject() throws Exception {
108         assertOpen();
109         Object obj = null;
110         boolean newlyCreated = false;
111         while(null == obj) {
112             if(_pool.isEmpty()) {
113                 if(null == _factory) {
114                     throw new NoSuchElementException();
115                 } else {
116                     newlyCreated = true;
117                     obj = _factory.makeObject();
118                 }
119             } else {
120                 SoftReference ref = (SoftReference)(_pool.remove(_pool.size() - 1));
121                 obj = ref.get();
122                 ref.clear(); // prevent this ref from being enqueued with refQueue.
123             }
124             if (null != _factory && null != obj) {
125                 try {
126                     _factory.activateObject(obj);
127                     if (!_factory.validateObject(obj)) {
128                         throw new Exception("ValidateObject failed");
129                     }
130                 } catch (Throwable t) {
131                     PoolUtils.checkRethrow(t);
132                     try {
133                         _factory.destroyObject(obj);
134                     } catch (Throwable t2) {
135                         PoolUtils.checkRethrow(t2);
136                         // Swallowed
137                     } finally {
138                         obj = null;
139                     }
140                     if (newlyCreated) {
141                         throw new NoSuchElementException(
142                             "Could not create a validated object, cause: " +
143                             t.getMessage());
144                     }
145                 }
146             }
147         }
148         _numActive++;
149         return obj;
150     }
151 
152     /**
153      * <p>Returns an instance to the pool after successful validation and passivation. The returning instance
154      * is destroyed if any of the following are true:<ul>
155      *   <li>the pool is closed</li>
156      *   <li>{@link PoolableObjectFactory#validateObject(Object) validation} fails</li>
157      *   <li>{@link PoolableObjectFactory#passivateObject(Object) passivation} throws an exception</li>
158      * </ul>
159      *</p>
160      * 
161      * <p>Exceptions passivating or destroying instances are silently swallowed.  Exceptions validating
162      * instances are propagated to the client.</p>
163      * 
164      * @param obj instance to return to the pool
165      */
166     public synchronized void returnObject(Object obj) throws Exception {
167         boolean success = !isClosed();
168         if (_factory != null) {
169             if(!_factory.validateObject(obj)) {
170                 success = false;
171             } else {
172                 try {
173                     _factory.passivateObject(obj);
174                 } catch(Exception e) {
175                     success = false;
176                 }
177             }
178         }
179 
180         boolean shouldDestroy = !success;
181         _numActive--;
182         if(success) {
183             _pool.add(new SoftReference(obj, refQueue));
184         }
185         notifyAll(); // _numActive has changed
186 
187         if (shouldDestroy && _factory != null) {
188             try {
189                 _factory.destroyObject(obj);
190             } catch(Exception e) {
191                 // ignored
192             }
193         }
194     }
195 
196     /**
197      * {@inheritDoc}
198      */
199     public synchronized void invalidateObject(Object obj) throws Exception {
200         _numActive--;
201         if (_factory != null) {
202             _factory.destroyObject(obj);
203         }
204         notifyAll(); // _numActive has changed
205     }
206 
207     /**
208      * <p>Create an object, and place it into the pool.
209      * addObject() is useful for "pre-loading" a pool with idle objects.</p>
210      * 
211      * <p>Before being added to the pool, the newly created instance is
212      * {@link PoolableObjectFactory#validateObject(Object) validated} and 
213      * {@link PoolableObjectFactory#passivateObject(Object) passivated}.  If validation
214      * fails, the new instance is {@link PoolableObjectFactory#destroyObject(Object) destroyed}.
215      * Exceptions generated by the factory <code>makeObject</code> or <code>passivate</code> are
216      * propagated to the caller. Exceptions destroying instances are silently swallowed.</p>
217      * 
218      * @throws IllegalStateException if invoked on a {@link #close() closed} pool
219      * @throws Exception when the {@link #getFactory() factory} has a problem creating or passivating an object.
220      */
221     public synchronized void addObject() throws Exception {
222         assertOpen();
223         if (_factory == null) {
224             throw new IllegalStateException("Cannot add objects without a factory.");
225         }
226         Object obj = _factory.makeObject();
227 
228         boolean success = true;
229         if(!_factory.validateObject(obj)) {
230             success = false;
231         } else {
232             _factory.passivateObject(obj);
233         }
234 
235         boolean shouldDestroy = !success;
236         if(success) {
237             _pool.add(new SoftReference(obj, refQueue));
238             notifyAll(); // _numActive has changed
239         }
240 
241         if(shouldDestroy) {
242             try {
243                 _factory.destroyObject(obj);
244             } catch(Exception e) {
245                 // ignored
246             }
247         }
248     }
249 
250     /**
251      * Returns an approximation not less than the of the number of idle instances in the pool.
252      * 
253      * @return estimated number of idle instances in the pool
254      */
255     public synchronized int getNumIdle() {
256         pruneClearedReferences();
257         return _pool.size();
258     }
259 
260     /**
261      * Return the number of instances currently borrowed from this pool.
262      *
263      * @return the number of instances currently borrowed from this pool
264      */
265     public synchronized int getNumActive() {
266         return _numActive;
267     }
268 
269     /**
270      * Clears any objects sitting idle in the pool.
271      */
272     public synchronized void clear() {
273         if(null != _factory) {
274             Iterator iter = _pool.iterator();
275             while(iter.hasNext()) {
276                 try {
277                     Object obj = ((SoftReference)iter.next()).get();
278                     if(null != obj) {
279                         _factory.destroyObject(obj);
280                     }
281                 } catch(Exception e) {
282                     // ignore error, keep destroying the rest
283                 }
284             }
285         }
286         _pool.clear();
287         pruneClearedReferences();
288     }
289 
290     /**
291      * <p>Close this pool, and free any resources associated with it. Invokes
292      * {@link #clear()} to destroy and remove instances in the pool.</p>
293      * 
294      * <p>Calling {@link #addObject} or {@link #borrowObject} after invoking
295      * this method on a pool will cause them to throw an
296      * {@link IllegalStateException}.</p>
297      *
298      * @throws Exception never - exceptions clearing the pool are swallowed
299      */
300     public void close() throws Exception {
301         super.close();
302         clear();
303     }
304 
305     /**
306      * Sets the {@link PoolableObjectFactory factory} this pool uses
307      * to create new instances. Trying to change
308      * the <code>factory</code> while there are borrowed objects will
309      * throw an {@link IllegalStateException}.
310      *
311      * @param factory the {@link PoolableObjectFactory} used to create new instances.
312      * @throws IllegalStateException when the factory cannot be set at this time
313      * @deprecated to be removed in pool 2.0
314      */
315     public synchronized void setFactory(PoolableObjectFactory factory) throws IllegalStateException {
316         assertOpen();
317         if(0 < getNumActive()) {
318             throw new IllegalStateException("Objects are already active");
319         } else {
320             clear();
321             _factory = factory;
322         }
323     }
324 
325     /**
326      * If any idle objects were garbage collected, remove their
327      * {@link Reference} wrappers from the idle object pool.
328      */
329     private void pruneClearedReferences() {
330         Reference ref;
331         while ((ref = refQueue.poll()) != null) {
332             try {
333                 _pool.remove(ref);
334             } catch (UnsupportedOperationException uoe) {
335                 // ignored
336             }
337         }
338     }
339     
340     /**
341      * Returns the {@link PoolableObjectFactory} used by this pool to create and manage object instances.
342      * 
343      * @return the factory
344      * @since 1.5.5
345      */
346     public synchronized PoolableObjectFactory getFactory() { 
347         return _factory;
348     }
349 
350     /** My pool. */
351     private final List _pool;
352 
353     /** My {@link PoolableObjectFactory}. */
354     private PoolableObjectFactory _factory = null;
355 
356     /**
357      * Queue of broken references that might be able to be removed from <code>_pool</code>.
358      * This is used to help {@link #getNumIdle()} be more accurate with minimial
359      * performance overhead.
360      */
361     private final ReferenceQueue refQueue = new ReferenceQueue();
362 
363     /** Number of active objects. */
364     private int _numActive = 0; //@GuardeBy("this")
365 }