001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements.  See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache License, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License.  You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the License for the specific language governing permissions and
015     * limitations under the License.
016     */
017    package org.apache.commons.lang3.concurrent;
018    
019    import java.util.concurrent.ScheduledExecutorService;
020    import java.util.concurrent.ScheduledFuture;
021    import java.util.concurrent.ScheduledThreadPoolExecutor;
022    import java.util.concurrent.TimeUnit;
023    
024    /**
025     * <p>
026     * A specialized <em>semaphore</em> implementation that provides a number of
027     * permits in a given time frame.
028     * </p>
029     * <p>
030     * This class is similar to the {@code java.util.concurrent.Semaphore} class
031     * provided by the JDK in that it manages a configurable number of permits.
032     * Using the {@link #acquire()} method a permit can be requested by a thread.
033     * However, there is an additional timing dimension: there is no {@code
034     * release()} method for freeing a permit, but all permits are automatically
035     * released at the end of a configurable time frame. If a thread calls
036     * {@link #acquire()} and the available permits are already exhausted for this
037     * time frame, the thread is blocked. When the time frame ends all permits
038     * requested so far are restored, and blocking threads are waked up again, so
039     * that they can try to acquire a new permit. This basically means that in the
040     * specified time frame only the given number of operations is possible.
041     * </p>
042     * <p>
043     * A use case for this class is to artificially limit the load produced by a
044     * process. As an example consider an application that issues database queries
045     * on a production system in a background process to gather statistical
046     * information. This background processing should not produce so much database
047     * load that the functionality and the performance of the production system are
048     * impacted. Here a {@code TimedSemaphore} could be installed to guarantee that
049     * only a given number of database queries are issued per second.
050     * </p>
051     * <p>
052     * A thread class for performing database queries could look as follows:
053     *
054     * <pre>
055     * public class StatisticsThread extends Thread {
056     *     // The semaphore for limiting database load.
057     *     private final TimedSemaphore semaphore;
058     *     // Create an instance and set the semaphore
059     *     public StatisticsThread(TimedSemaphore timedSemaphore) {
060     *         semaphore = timedSemaphore;
061     *     }
062     *     // Gather statistics
063     *     public void run() {
064     *         try {
065     *             while(true) {
066     *                 semaphore.acquire();   // limit database load
067     *                 performQuery();        // issue a query
068     *             }
069     *         } catch(InterruptedException) {
070     *             // fall through
071     *         }
072     *     }
073     *     ...
074     * }
075     * </pre>
076     *
077     * The following code fragment shows how a {@code TimedSemaphore} is created
078     * that allows only 10 operations per second and passed to the statistics
079     * thread:
080     *
081     * <pre>
082     * TimedSemaphore sem = new TimedSemaphore(1, TimeUnit.SECOND, 10);
083     * StatisticsThread thread = new StatisticsThread(sem);
084     * thread.start();
085     * </pre>
086     *
087     * </p>
088     * <p>
089     * When creating an instance the time period for the semaphore must be
090     * specified. {@code TimedSemaphore} uses an executor service with a
091     * corresponding period to monitor this interval. The {@code
092     * ScheduledExecutorService} to be used for this purpose can be provided at
093     * construction time. Alternatively the class creates an internal executor
094     * service.
095     * </p>
096     * <p>
097     * Client code that uses {@code TimedSemaphore} has to call the
098     * {@link #acquire()} method in aach processing step. {@code TimedSemaphore}
099     * keeps track of the number of invocations of the {@link #acquire()} method and
100     * blocks the calling thread if the counter exceeds the limit specified. When
101     * the timer signals the end of the time period the counter is reset and all
102     * waiting threads are released. Then another cycle can start.
103     * </p>
104     * <p>
105     * It is possible to modify the limit at any time using the
106     * {@link #setLimit(int)} method. This is useful if the load produced by an
107     * operation has to be adapted dynamically. In the example scenario with the
108     * thread collecting statistics it may make sense to specify a low limit during
109     * day time while allowing a higher load in the night time. Reducing the limit
110     * takes effect immediately by blocking incoming callers. If the limit is
111     * increased, waiting threads are not released immediately, but wake up when the
112     * timer runs out. Then, in the next period more processing steps can be
113     * performed without blocking. By setting the limit to 0 the semaphore can be
114     * switched off: in this mode the {@link #acquire()} method never blocks, but
115     * lets all callers pass directly.
116     * </p>
117     * <p>
118     * When the {@code TimedSemaphore} is no more needed its {@link #shutdown()}
119     * method should be called. This causes the periodic task that monitors the time
120     * interval to be canceled. If the {@code ScheduledExecutorService} has been
121     * created by the semaphore at construction time, it is also shut down.
122     * resources. After that {@link #acquire()} must not be called any more.
123     * </p>
124     *
125     * @version $Id: TimedSemaphore.java 895466 2010-01-03 19:04:08Z oheger $
126     */
127    public class TimedSemaphore {
128        /**
129         * Constant for a value representing no limit. If the limit is set to a
130         * value less or equal this constant, the {@code TimedSemaphore} will be
131         * effectively switched off.
132         */
133        public static final int NO_LIMIT = 0;
134    
135        /** Constant for the thread pool size for the executor. */
136        private static final int THREAD_POOL_SIZE = 1;
137    
138        /** The executor service for managing the timer thread. */
139        private final ScheduledExecutorService executorService;
140    
141        /** Stores the period for this timed semaphore. */
142        private final long period;
143    
144        /** The time unit for the period. */
145        private final TimeUnit unit;
146    
147        /** A flag whether the executor service was created by this object. */
148        private final boolean ownExecutor;
149    
150        /** A future object representing the timer task. */
151        private ScheduledFuture<?> task;
152    
153        /** Stores the total number of invocations of the acquire() method. */
154        private long totalAcquireCount;
155    
156        /**
157         * The counter for the periods. This counter is increased every time a
158         * period ends.
159         */
160        private long periodCount;
161    
162        /** The limit. */
163        private int limit;
164    
165        /** The current counter. */
166        private int acquireCount;
167    
168        /** The number of invocations of acquire() in the last period. */
169        private int lastCallsPerPeriod;
170    
171        /** A flag whether shutdown() was called. */
172        private boolean shutdown;
173    
174        /**
175         * Creates a new instance of {@link TimedSemaphore} and initializes it with
176         * the given time period and the limit.
177         *
178         * @param timePeriod the time period
179         * @param timeUnit the unit for the period
180         * @param limit the limit for the semaphore
181         * @throws IllegalArgumentException if the period is less or equals 0
182         */
183        public TimedSemaphore(long timePeriod, TimeUnit timeUnit, int limit) {
184            this(null, timePeriod, timeUnit, limit);
185        }
186    
187        /**
188         * Creates a new instance of {@link TimedSemaphore} and initializes it with
189         * an executor service, the given time period, and the limit. The executor
190         * service will be used for creating a periodic task for monitoring the time
191         * period. It can be <b>null</b>, then a default service will be created.
192         *
193         * @param service the executor service
194         * @param timePeriod the time period
195         * @param timeUnit the unit for the period
196         * @param limit the limit for the semaphore
197         * @throws IllegalArgumentException if the period is less or equals 0
198         */
199        public TimedSemaphore(ScheduledExecutorService service, long timePeriod,
200                TimeUnit timeUnit, int limit) {
201            if (timePeriod <= 0) {
202                throw new IllegalArgumentException("Time period must be greater 0!");
203            }
204    
205            period = timePeriod;
206            unit = timeUnit;
207    
208            if (service != null) {
209                executorService = service;
210                ownExecutor = false;
211            } else {
212                ScheduledThreadPoolExecutor s = new ScheduledThreadPoolExecutor(
213                        THREAD_POOL_SIZE);
214                s.setContinueExistingPeriodicTasksAfterShutdownPolicy(false);
215                s.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
216                executorService = s;
217                ownExecutor = true;
218            }
219    
220            setLimit(limit);
221        }
222    
223        /**
224         * Returns the limit enforced by this semaphore. The limit determines how
225         * many invocations of {@link #acquire()} are allowed within the monitored
226         * period.
227         *
228         * @return the limit
229         */
230        public final synchronized int getLimit() {
231            return limit;
232        }
233    
234        /**
235         * Sets the limit. This is the number of times the {@link #acquire()} method
236         * can be called within the time period specified. If this limit is reached,
237         * further invocations of {@link #acquire()} will block. Setting the limit
238         * to a value &lt;= {@link #NO_LIMIT} will cause the limit to be disabled,
239         * i.e. an arbitrary number of{@link #acquire()} invocations is allowed in
240         * the time period.
241         *
242         * @param limit the limit
243         */
244        public final synchronized void setLimit(int limit) {
245            this.limit = limit;
246        }
247    
248        /**
249         * Initializes a shutdown. After that the object cannot be used any more.
250         * This method can be invoked an arbitrary number of times. All invocations
251         * after the first one do not have any effect.
252         */
253        public synchronized void shutdown() {
254            if (!shutdown) {
255    
256                if (ownExecutor) {
257                    // if the executor was created by this instance, it has
258                    // to be shutdown
259                    getExecutorService().shutdownNow();
260                }
261                if (task != null) {
262                    task.cancel(false);
263                }
264    
265                shutdown = true;
266            }
267        }
268    
269        /**
270         * Tests whether the {@link #shutdown()} method has been called on this
271         * object. If this method returns <b>true</b>, this instance cannot be used
272         * any longer.
273         *
274         * @return a flag whether a shutdown has been performed
275         */
276        public synchronized boolean isShutdown() {
277            return shutdown;
278        }
279    
280        /**
281         * Tries to acquire a permit from this semaphore. This method will block if
282         * the limit for the current period has already been reached. If
283         * {@link #shutdown()} has already been invoked, calling this method will
284         * cause an exception. The very first call of this method starts the timer
285         * task which monitors the time period set for this {@code TimedSemaphore}.
286         * From now on the semaphore is active.
287         *
288         * @throws InterruptedException if the thread gets interrupted
289         * @throws IllegalStateException if this semaphore is already shut down
290         */
291        public synchronized void acquire() throws InterruptedException {
292            if (isShutdown()) {
293                throw new IllegalStateException("TimedSemaphore is shut down!");
294            }
295    
296            if (task == null) {
297                task = startTimer();
298            }
299    
300            boolean canPass = false;
301            do {
302                canPass = getLimit() <= NO_LIMIT || acquireCount < getLimit();
303                if (!canPass) {
304                    wait();
305                } else {
306                    acquireCount++;
307                }
308            } while (!canPass);
309        }
310    
311        /**
312         * Returns the number of (successful) acquire invocations during the last
313         * period. This is the number of times the {@link #acquire()} method was
314         * called without blocking. This can be useful for testing or debugging
315         * purposes or to determine a meaningful threshold value. If a limit is set,
316         * the value returned by this method won't be greater than this limit.
317         *
318         * @return the number of non-blocking invocations of the {@link #acquire()}
319         * method
320         */
321        public synchronized int getLastAcquiresPerPeriod() {
322            return lastCallsPerPeriod;
323        }
324    
325        /**
326         * Returns the number of invocations of the {@link #acquire()} method for
327         * the current period. This may be useful for testing or debugging purposes.
328         *
329         * @return the current number of {@link #acquire()} invocations
330         */
331        public synchronized int getAcquireCount() {
332            return acquireCount;
333        }
334    
335        /**
336         * Returns the number of calls to the {@link #acquire()} method that can
337         * still be performed in the current period without blocking. This method
338         * can give an indication whether it is safe to call the {@link #acquire()}
339         * method without risking to be suspended. However, there is no guarantee
340         * that a subsequent call to {@link #acquire()} actually is not-blocking
341         * because in the mean time other threads may have invoked the semaphore.
342         *
343         * @return the current number of available {@link #acquire()} calls in the
344         * current period
345         */
346        public synchronized int getAvailablePermits() {
347            return getLimit() - getAcquireCount();
348        }
349    
350        /**
351         * Returns the average number of successful (i.e. non-blocking)
352         * {@link #acquire()} invocations for the entire life-time of this {@code
353         * TimedSemaphore}. This method can be used for instance for statistical
354         * calculations.
355         *
356         * @return the average number of {@link #acquire()} invocations per time
357         * unit
358         */
359        public synchronized double getAverageCallsPerPeriod() {
360            return (periodCount == 0) ? 0 : (double) totalAcquireCount
361                    / (double) periodCount;
362        }
363    
364        /**
365         * Returns the time period. This is the time monitored by this semaphore.
366         * Only a given number of invocations of the {@link #acquire()} method is
367         * possible in this period.
368         *
369         * @return the time period
370         */
371        public long getPeriod() {
372            return period;
373        }
374    
375        /**
376         * Returns the time unit. This is the unit used by {@link #getPeriod()}.
377         *
378         * @return the time unit
379         */
380        public TimeUnit getUnit() {
381            return unit;
382        }
383    
384        /**
385         * Returns the executor service used by this instance.
386         *
387         * @return the executor service
388         */
389        protected ScheduledExecutorService getExecutorService() {
390            return executorService;
391        }
392    
393        /**
394         * Starts the timer. This method is called when {@link #acquire()} is called
395         * for the first time. It schedules a task to be executed at fixed rate to
396         * monitor the time period specified.
397         *
398         * @return a future object representing the task scheduled
399         */
400        protected ScheduledFuture<?> startTimer() {
401            return getExecutorService().scheduleAtFixedRate(new Runnable() {
402                public void run() {
403                    endOfPeriod();
404                }
405            }, getPeriod(), getPeriod(), getUnit());
406        }
407    
408        /**
409         * The current time period is finished. This method is called by the timer
410         * used internally to monitor the time period. It resets the counter and
411         * releases the threads waiting for this barrier.
412         */
413        synchronized void endOfPeriod() {
414            lastCallsPerPeriod = acquireCount;
415            totalAcquireCount += acquireCount;
416            periodCount++;
417            acquireCount = 0;
418            notifyAll();
419        }
420    }