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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  package org.apache.commons.lang3.concurrent;
18  
19  import java.util.concurrent.ScheduledExecutorService;
20  import java.util.concurrent.ScheduledFuture;
21  import java.util.concurrent.ScheduledThreadPoolExecutor;
22  import java.util.concurrent.TimeUnit;
23  
24  import org.apache.commons.lang3.Validate;
25  
26  /**
27   * <p>
28   * A specialized <em>semaphore</em> implementation that provides a number of
29   * permits in a given time frame.
30   * </p>
31   * <p>
32   * This class is similar to the {@code java.util.concurrent.Semaphore} class
33   * provided by the JDK in that it manages a configurable number of permits.
34   * Using the {@link #acquire()} method a permit can be requested by a thread.
35   * However, there is an additional timing dimension: there is no {@code
36   * release()} method for freeing a permit, but all permits are automatically
37   * released at the end of a configurable time frame. If a thread calls
38   * {@link #acquire()} and the available permits are already exhausted for this
39   * time frame, the thread is blocked. When the time frame ends all permits
40   * requested so far are restored, and blocking threads are waked up again, so
41   * that they can try to acquire a new permit. This basically means that in the
42   * specified time frame only the given number of operations is possible.
43   * </p>
44   * <p>
45   * A use case for this class is to artificially limit the load produced by a
46   * process. As an example consider an application that issues database queries
47   * on a production system in a background process to gather statistical
48   * information. This background processing should not produce so much database
49   * load that the functionality and the performance of the production system are
50   * impacted. Here a {@code TimedSemaphore} could be installed to guarantee that
51   * only a given number of database queries are issued per second.
52   * </p>
53   * <p>
54   * A thread class for performing database queries could look as follows:
55   * </p>
56   *
57   * <pre>
58   * public class StatisticsThread extends Thread {
59   *     // The semaphore for limiting database load.
60   *     private final TimedSemaphore semaphore;
61   *     // Create an instance and set the semaphore
62   *     public StatisticsThread(TimedSemaphore timedSemaphore) {
63   *         semaphore = timedSemaphore;
64   *     }
65   *     // Gather statistics
66   *     public void run() {
67   *         try {
68   *             while(true) {
69   *                 semaphore.acquire();   // limit database load
70   *                 performQuery();        // issue a query
71   *             }
72   *         } catch(InterruptedException) {
73   *             // fall through
74   *         }
75   *     }
76   *     ...
77   * }
78   * </pre>
79   *
80   * <p>
81   * The following code fragment shows how a {@code TimedSemaphore} is created
82   * that allows only 10 operations per second and passed to the statistics
83   * thread:
84   * </p>
85   *
86   * <pre>
87   * TimedSemaphore sem = new TimedSemaphore(1, TimeUnit.SECOND, 10);
88   * StatisticsThread thread = new StatisticsThread(sem);
89   * thread.start();
90   * </pre>
91   *
92   * <p>
93   * When creating an instance the time period for the semaphore must be
94   * specified. {@code TimedSemaphore} uses an executor service with a
95   * corresponding period to monitor this interval. The {@code
96   * ScheduledExecutorService} to be used for this purpose can be provided at
97   * construction time. Alternatively the class creates an internal executor
98   * service.
99   * </p>
100  * <p>
101  * Client code that uses {@code TimedSemaphore} has to call the
102  * {@link #acquire()} method in aach processing step. {@code TimedSemaphore}
103  * keeps track of the number of invocations of the {@link #acquire()} method and
104  * blocks the calling thread if the counter exceeds the limit specified. When
105  * the timer signals the end of the time period the counter is reset and all
106  * waiting threads are released. Then another cycle can start.
107  * </p>
108  * <p>
109  * It is possible to modify the limit at any time using the
110  * {@link #setLimit(int)} method. This is useful if the load produced by an
111  * operation has to be adapted dynamically. In the example scenario with the
112  * thread collecting statistics it may make sense to specify a low limit during
113  * day time while allowing a higher load in the night time. Reducing the limit
114  * takes effect immediately by blocking incoming callers. If the limit is
115  * increased, waiting threads are not released immediately, but wake up when the
116  * timer runs out. Then, in the next period more processing steps can be
117  * performed without blocking. By setting the limit to 0 the semaphore can be
118  * switched off: in this mode the {@link #acquire()} method never blocks, but
119  * lets all callers pass directly.
120  * </p>
121  * <p>
122  * When the {@code TimedSemaphore} is no more needed its {@link #shutdown()}
123  * method should be called. This causes the periodic task that monitors the time
124  * interval to be canceled. If the {@code ScheduledExecutorService} has been
125  * created by the semaphore at construction time, it is also shut down.
126  * resources. After that {@link #acquire()} must not be called any more.
127  * </p>
128  *
129  * @since 3.0
130  * @version $Id: TimedSemaphore.java 1593668 2014-05-10 05:58:17Z djones $
131  */
132 public class TimedSemaphore {
133     /**
134      * Constant for a value representing no limit. If the limit is set to a
135      * value less or equal this constant, the {@code TimedSemaphore} will be
136      * effectively switched off.
137      */
138     public static final int NO_LIMIT = 0;
139 
140     /** Constant for the thread pool size for the executor. */
141     private static final int THREAD_POOL_SIZE = 1;
142 
143     /** The executor service for managing the timer thread. */
144     private final ScheduledExecutorService executorService;
145 
146     /** Stores the period for this timed semaphore. */
147     private final long period;
148 
149     /** The time unit for the period. */
150     private final TimeUnit unit;
151 
152     /** A flag whether the executor service was created by this object. */
153     private final boolean ownExecutor;
154 
155     /** A future object representing the timer task. */
156     private ScheduledFuture<?> task; // @GuardedBy("this")
157 
158     /** Stores the total number of invocations of the acquire() method. */
159     private long totalAcquireCount; // @GuardedBy("this")
160 
161     /**
162      * The counter for the periods. This counter is increased every time a
163      * period ends.
164      */
165     private long periodCount; // @GuardedBy("this")
166 
167     /** The limit. */
168     private int limit; // @GuardedBy("this")
169 
170     /** The current counter. */
171     private int acquireCount;  // @GuardedBy("this")
172 
173     /** The number of invocations of acquire() in the last period. */
174     private int lastCallsPerPeriod; // @GuardedBy("this")
175 
176     /** A flag whether shutdown() was called. */
177     private boolean shutdown;  // @GuardedBy("this")
178 
179     /**
180      * Creates a new instance of {@link TimedSemaphore} and initializes it with
181      * the given time period and the limit.
182      *
183      * @param timePeriod the time period
184      * @param timeUnit the unit for the period
185      * @param limit the limit for the semaphore
186      * @throws IllegalArgumentException if the period is less or equals 0
187      */
188     public TimedSemaphore(final long timePeriod, final TimeUnit timeUnit, final int limit) {
189         this(null, timePeriod, timeUnit, limit);
190     }
191 
192     /**
193      * Creates a new instance of {@link TimedSemaphore} and initializes it with
194      * an executor service, the given time period, and the limit. The executor
195      * service will be used for creating a periodic task for monitoring the time
196      * period. It can be <b>null</b>, then a default service will be created.
197      *
198      * @param service the executor service
199      * @param timePeriod the time period
200      * @param timeUnit the unit for the period
201      * @param limit the limit for the semaphore
202      * @throws IllegalArgumentException if the period is less or equals 0
203      */
204     public TimedSemaphore(final ScheduledExecutorService service, final long timePeriod,
205             final TimeUnit timeUnit, final int limit) {
206         Validate.inclusiveBetween(1, Long.MAX_VALUE, timePeriod, "Time period must be greater than 0!");
207 
208         period = timePeriod;
209         unit = timeUnit;
210 
211         if (service != null) {
212             executorService = service;
213             ownExecutor = false;
214         } else {
215             final ScheduledThreadPoolExecutor s = new ScheduledThreadPoolExecutor(
216                     THREAD_POOL_SIZE);
217             s.setContinueExistingPeriodicTasksAfterShutdownPolicy(false);
218             s.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
219             executorService = s;
220             ownExecutor = true;
221         }
222 
223         setLimit(limit);
224     }
225 
226     /**
227      * Returns the limit enforced by this semaphore. The limit determines how
228      * many invocations of {@link #acquire()} are allowed within the monitored
229      * period.
230      *
231      * @return the limit
232      */
233     public final synchronized int getLimit() {
234         return limit;
235     }
236 
237     /**
238      * Sets the limit. This is the number of times the {@link #acquire()} method
239      * can be called within the time period specified. If this limit is reached,
240      * further invocations of {@link #acquire()} will block. Setting the limit
241      * to a value &lt;= {@link #NO_LIMIT} will cause the limit to be disabled,
242      * i.e. an arbitrary number of{@link #acquire()} invocations is allowed in
243      * the time period.
244      *
245      * @param limit the limit
246      */
247     public final synchronized void setLimit(final int limit) {
248         this.limit = limit;
249     }
250 
251     /**
252      * Initializes a shutdown. After that the object cannot be used any more.
253      * This method can be invoked an arbitrary number of times. All invocations
254      * after the first one do not have any effect.
255      */
256     public synchronized void shutdown() {
257         if (!shutdown) {
258 
259             if (ownExecutor) {
260                 // if the executor was created by this instance, it has
261                 // to be shutdown
262                 getExecutorService().shutdownNow();
263             }
264             if (task != null) {
265                 task.cancel(false);
266             }
267 
268             shutdown = true;
269         }
270     }
271 
272     /**
273      * Tests whether the {@link #shutdown()} method has been called on this
274      * object. If this method returns <b>true</b>, this instance cannot be used
275      * any longer.
276      *
277      * @return a flag whether a shutdown has been performed
278      */
279     public synchronized boolean isShutdown() {
280         return shutdown;
281     }
282 
283     /**
284      * Tries to acquire a permit from this semaphore. This method will block if
285      * the limit for the current period has already been reached. If
286      * {@link #shutdown()} has already been invoked, calling this method will
287      * cause an exception. The very first call of this method starts the timer
288      * task which monitors the time period set for this {@code TimedSemaphore}.
289      * From now on the semaphore is active.
290      *
291      * @throws InterruptedException if the thread gets interrupted
292      * @throws IllegalStateException if this semaphore is already shut down
293      */
294     public synchronized void acquire() throws InterruptedException {
295         if (isShutdown()) {
296             throw new IllegalStateException("TimedSemaphore is shut down!");
297         }
298 
299         if (task == null) {
300             task = startTimer();
301         }
302 
303         boolean canPass = false;
304         do {
305             canPass = getLimit() <= NO_LIMIT || acquireCount < getLimit();
306             if (!canPass) {
307                 wait();
308             } else {
309                 acquireCount++;
310             }
311         } while (!canPass);
312     }
313 
314     /**
315      * Returns the number of (successful) acquire invocations during the last
316      * period. This is the number of times the {@link #acquire()} method was
317      * called without blocking. This can be useful for testing or debugging
318      * purposes or to determine a meaningful threshold value. If a limit is set,
319      * the value returned by this method won't be greater than this limit.
320      *
321      * @return the number of non-blocking invocations of the {@link #acquire()}
322      * method
323      */
324     public synchronized int getLastAcquiresPerPeriod() {
325         return lastCallsPerPeriod;
326     }
327 
328     /**
329      * Returns the number of invocations of the {@link #acquire()} method for
330      * the current period. This may be useful for testing or debugging purposes.
331      *
332      * @return the current number of {@link #acquire()} invocations
333      */
334     public synchronized int getAcquireCount() {
335         return acquireCount;
336     }
337 
338     /**
339      * Returns the number of calls to the {@link #acquire()} method that can
340      * still be performed in the current period without blocking. This method
341      * can give an indication whether it is safe to call the {@link #acquire()}
342      * method without risking to be suspended. However, there is no guarantee
343      * that a subsequent call to {@link #acquire()} actually is not-blocking
344      * because in the mean time other threads may have invoked the semaphore.
345      *
346      * @return the current number of available {@link #acquire()} calls in the
347      * current period
348      */
349     public synchronized int getAvailablePermits() {
350         return getLimit() - getAcquireCount();
351     }
352 
353     /**
354      * Returns the average number of successful (i.e. non-blocking)
355      * {@link #acquire()} invocations for the entire life-time of this {@code
356      * TimedSemaphore}. This method can be used for instance for statistical
357      * calculations.
358      *
359      * @return the average number of {@link #acquire()} invocations per time
360      * unit
361      */
362     public synchronized double getAverageCallsPerPeriod() {
363         return periodCount == 0 ? 0 : (double) totalAcquireCount
364                 / (double) periodCount;
365     }
366 
367     /**
368      * Returns the time period. This is the time monitored by this semaphore.
369      * Only a given number of invocations of the {@link #acquire()} method is
370      * possible in this period.
371      *
372      * @return the time period
373      */
374     public long getPeriod() {
375         return period;
376     }
377 
378     /**
379      * Returns the time unit. This is the unit used by {@link #getPeriod()}.
380      *
381      * @return the time unit
382      */
383     public TimeUnit getUnit() {
384         return unit;
385     }
386 
387     /**
388      * Returns the executor service used by this instance.
389      *
390      * @return the executor service
391      */
392     protected ScheduledExecutorService getExecutorService() {
393         return executorService;
394     }
395 
396     /**
397      * Starts the timer. This method is called when {@link #acquire()} is called
398      * for the first time. It schedules a task to be executed at fixed rate to
399      * monitor the time period specified.
400      *
401      * @return a future object representing the task scheduled
402      */
403     protected ScheduledFuture<?> startTimer() {
404         return getExecutorService().scheduleAtFixedRate(new Runnable() {
405             @Override
406             public void run() {
407                 endOfPeriod();
408             }
409         }, getPeriod(), getPeriod(), getUnit());
410     }
411 
412     /**
413      * The current time period is finished. This method is called by the timer
414      * used internally to monitor the time period. It resets the counter and
415      * releases the threads waiting for this barrier.
416      */
417     synchronized void endOfPeriod() {
418         lastCallsPerPeriod = acquireCount;
419         totalAcquireCount += acquireCount;
420         periodCount++;
421         acquireCount = 0;
422         notifyAll();
423     }
424 }