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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  */
131 public class TimedSemaphore {
132     /**
133      * Constant for a value representing no limit. If the limit is set to a
134      * value less or equal this constant, the {@code TimedSemaphore} will be
135      * effectively switched off.
136      */
137     public static final int NO_LIMIT = 0;
138 
139     /** Constant for the thread pool size for the executor. */
140     private static final int THREAD_POOL_SIZE = 1;
141 
142     /** The executor service for managing the timer thread. */
143     private final ScheduledExecutorService executorService;
144 
145     /** Stores the period for this timed semaphore. */
146     private final long period;
147 
148     /** The time unit for the period. */
149     private final TimeUnit unit;
150 
151     /** A flag whether the executor service was created by this object. */
152     private final boolean ownExecutor;
153 
154     /** A future object representing the timer task. */
155     private ScheduledFuture<?> task; // @GuardedBy("this")
156 
157     /** Stores the total number of invocations of the acquire() method. */
158     private long totalAcquireCount; // @GuardedBy("this")
159 
160     /**
161      * The counter for the periods. This counter is increased every time a
162      * period ends.
163      */
164     private long periodCount; // @GuardedBy("this")
165 
166     /** The limit. */
167     private int limit; // @GuardedBy("this")
168 
169     /** The current counter. */
170     private int acquireCount;  // @GuardedBy("this")
171 
172     /** The number of invocations of acquire() in the last period. */
173     private int lastCallsPerPeriod; // @GuardedBy("this")
174 
175     /** A flag whether shutdown() was called. */
176     private boolean shutdown;  // @GuardedBy("this")
177 
178     /**
179      * Creates a new instance of {@link TimedSemaphore} and initializes it with
180      * the given time period and the limit.
181      *
182      * @param timePeriod the time period
183      * @param timeUnit the unit for the period
184      * @param limit the limit for the semaphore
185      * @throws IllegalArgumentException if the period is less or equals 0
186      */
187     public TimedSemaphore(final long timePeriod, final TimeUnit timeUnit, final int limit) {
188         this(null, timePeriod, timeUnit, limit);
189     }
190 
191     /**
192      * Creates a new instance of {@link TimedSemaphore} and initializes it with
193      * an executor service, the given time period, and the limit. The executor
194      * service will be used for creating a periodic task for monitoring the time
195      * period. It can be <b>null</b>, then a default service will be created.
196      *
197      * @param service the executor service
198      * @param timePeriod the time period
199      * @param timeUnit the unit for the period
200      * @param limit the limit for the semaphore
201      * @throws IllegalArgumentException if the period is less or equals 0
202      */
203     public TimedSemaphore(final ScheduledExecutorService service, final long timePeriod,
204             final TimeUnit timeUnit, final int limit) {
205         Validate.inclusiveBetween(1, Long.MAX_VALUE, timePeriod, "Time period must be greater than 0!");
206 
207         period = timePeriod;
208         unit = timeUnit;
209 
210         if (service != null) {
211             executorService = service;
212             ownExecutor = false;
213         } else {
214             final ScheduledThreadPoolExecutor s = new ScheduledThreadPoolExecutor(
215                     THREAD_POOL_SIZE);
216             s.setContinueExistingPeriodicTasksAfterShutdownPolicy(false);
217             s.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
218             executorService = s;
219             ownExecutor = true;
220         }
221 
222         setLimit(limit);
223     }
224 
225     /**
226      * Returns the limit enforced by this semaphore. The limit determines how
227      * many invocations of {@link #acquire()} are allowed within the monitored
228      * period.
229      *
230      * @return the limit
231      */
232     public final synchronized int getLimit() {
233         return limit;
234     }
235 
236     /**
237      * Sets the limit. This is the number of times the {@link #acquire()} method
238      * can be called within the time period specified. If this limit is reached,
239      * further invocations of {@link #acquire()} will block. Setting the limit
240      * to a value &lt;= {@link #NO_LIMIT} will cause the limit to be disabled,
241      * i.e. an arbitrary number of{@link #acquire()} invocations is allowed in
242      * the time period.
243      *
244      * @param limit the limit
245      */
246     public final synchronized void setLimit(final int limit) {
247         this.limit = limit;
248     }
249 
250     /**
251      * Initializes a shutdown. After that the object cannot be used any more.
252      * This method can be invoked an arbitrary number of times. All invocations
253      * after the first one do not have any effect.
254      */
255     public synchronized void shutdown() {
256         if (!shutdown) {
257 
258             if (ownExecutor) {
259                 // if the executor was created by this instance, it has
260                 // to be shutdown
261                 getExecutorService().shutdownNow();
262             }
263             if (task != null) {
264                 task.cancel(false);
265             }
266 
267             shutdown = true;
268         }
269     }
270 
271     /**
272      * Tests whether the {@link #shutdown()} method has been called on this
273      * object. If this method returns <b>true</b>, this instance cannot be used
274      * any longer.
275      *
276      * @return a flag whether a shutdown has been performed
277      */
278     public synchronized boolean isShutdown() {
279         return shutdown;
280     }
281 
282     /**
283      * Tries to acquire a permit from this semaphore. This method will block if
284      * the limit for the current period has already been reached. If
285      * {@link #shutdown()} has already been invoked, calling this method will
286      * cause an exception. The very first call of this method starts the timer
287      * task which monitors the time period set for this {@code TimedSemaphore}.
288      * From now on the semaphore is active.
289      *
290      * @throws InterruptedException if the thread gets interrupted
291      * @throws IllegalStateException if this semaphore is already shut down
292      */
293     public synchronized void acquire() throws InterruptedException {
294         if (isShutdown()) {
295             throw new IllegalStateException("TimedSemaphore is shut down!");
296         }
297 
298         if (task == null) {
299             task = startTimer();
300         }
301 
302         boolean canPass = false;
303         do {
304             canPass = getLimit() <= NO_LIMIT || acquireCount < getLimit();
305             if (!canPass) {
306                 wait();
307             } else {
308                 acquireCount++;
309             }
310         } while (!canPass);
311     }
312 
313     /**
314      * Returns the number of (successful) acquire invocations during the last
315      * period. This is the number of times the {@link #acquire()} method was
316      * called without blocking. This can be useful for testing or debugging
317      * purposes or to determine a meaningful threshold value. If a limit is set,
318      * the value returned by this method won't be greater than this limit.
319      *
320      * @return the number of non-blocking invocations of the {@link #acquire()}
321      * method
322      */
323     public synchronized int getLastAcquiresPerPeriod() {
324         return lastCallsPerPeriod;
325     }
326 
327     /**
328      * Returns the number of invocations of the {@link #acquire()} method for
329      * the current period. This may be useful for testing or debugging purposes.
330      *
331      * @return the current number of {@link #acquire()} invocations
332      */
333     public synchronized int getAcquireCount() {
334         return acquireCount;
335     }
336 
337     /**
338      * Returns the number of calls to the {@link #acquire()} method that can
339      * still be performed in the current period without blocking. This method
340      * can give an indication whether it is safe to call the {@link #acquire()}
341      * method without risking to be suspended. However, there is no guarantee
342      * that a subsequent call to {@link #acquire()} actually is not-blocking
343      * because in the mean time other threads may have invoked the semaphore.
344      *
345      * @return the current number of available {@link #acquire()} calls in the
346      * current period
347      */
348     public synchronized int getAvailablePermits() {
349         return getLimit() - getAcquireCount();
350     }
351 
352     /**
353      * Returns the average number of successful (i.e. non-blocking)
354      * {@link #acquire()} invocations for the entire life-time of this {@code
355      * TimedSemaphore}. This method can be used for instance for statistical
356      * calculations.
357      *
358      * @return the average number of {@link #acquire()} invocations per time
359      * unit
360      */
361     public synchronized double getAverageCallsPerPeriod() {
362         return periodCount == 0 ? 0 : (double) totalAcquireCount
363                 / (double) periodCount;
364     }
365 
366     /**
367      * Returns the time period. This is the time monitored by this semaphore.
368      * Only a given number of invocations of the {@link #acquire()} method is
369      * possible in this period.
370      *
371      * @return the time period
372      */
373     public long getPeriod() {
374         return period;
375     }
376 
377     /**
378      * Returns the time unit. This is the unit used by {@link #getPeriod()}.
379      *
380      * @return the time unit
381      */
382     public TimeUnit getUnit() {
383         return unit;
384     }
385 
386     /**
387      * Returns the executor service used by this instance.
388      *
389      * @return the executor service
390      */
391     protected ScheduledExecutorService getExecutorService() {
392         return executorService;
393     }
394 
395     /**
396      * Starts the timer. This method is called when {@link #acquire()} is called
397      * for the first time. It schedules a task to be executed at fixed rate to
398      * monitor the time period specified.
399      *
400      * @return a future object representing the task scheduled
401      */
402     protected ScheduledFuture<?> startTimer() {
403         return getExecutorService().scheduleAtFixedRate(new Runnable() {
404             @Override
405             public void run() {
406                 endOfPeriod();
407             }
408         }, getPeriod(), getPeriod(), getUnit());
409     }
410 
411     /**
412      * The current time period is finished. This method is called by the timer
413      * used internally to monitor the time period. It resets the counter and
414      * releases the threads waiting for this barrier.
415      */
416     synchronized void endOfPeriod() {
417         lastCallsPerPeriod = acquireCount;
418         totalAcquireCount += acquireCount;
419         periodCount++;
420         acquireCount = 0;
421         notifyAll();
422     }
423 }