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 */
017package org.apache.commons.lang3.concurrent;
018
019import java.util.concurrent.ScheduledExecutorService;
020import java.util.concurrent.ScheduledFuture;
021import java.util.concurrent.ScheduledThreadPoolExecutor;
022import 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 * @since 3.0
126 * @version $Id: TimedSemaphore.java 1436770 2013-01-22 07:09:45Z ggregory $
127 */
128public class TimedSemaphore {
129    /**
130     * Constant for a value representing no limit. If the limit is set to a
131     * value less or equal this constant, the {@code TimedSemaphore} will be
132     * effectively switched off.
133     */
134    public static final int NO_LIMIT = 0;
135
136    /** Constant for the thread pool size for the executor. */
137    private static final int THREAD_POOL_SIZE = 1;
138
139    /** The executor service for managing the timer thread. */
140    private final ScheduledExecutorService executorService;
141
142    /** Stores the period for this timed semaphore. */
143    private final long period;
144
145    /** The time unit for the period. */
146    private final TimeUnit unit;
147
148    /** A flag whether the executor service was created by this object. */
149    private final boolean ownExecutor;
150
151    /** A future object representing the timer task. */
152    private ScheduledFuture<?> task; // @GuardedBy("this")
153
154    /** Stores the total number of invocations of the acquire() method. */
155    private long totalAcquireCount; // @GuardedBy("this")
156
157    /**
158     * The counter for the periods. This counter is increased every time a
159     * period ends.
160     */
161    private long periodCount; // @GuardedBy("this")
162
163    /** The limit. */
164    private int limit; // @GuardedBy("this")
165
166    /** The current counter. */
167    private int acquireCount;  // @GuardedBy("this")
168
169    /** The number of invocations of acquire() in the last period. */
170    private int lastCallsPerPeriod; // @GuardedBy("this")
171
172    /** A flag whether shutdown() was called. */
173    private boolean shutdown;  // @GuardedBy("this")
174
175    /**
176     * Creates a new instance of {@link TimedSemaphore} and initializes it with
177     * the given time period and the limit.
178     *
179     * @param timePeriod the time period
180     * @param timeUnit the unit for the period
181     * @param limit the limit for the semaphore
182     * @throws IllegalArgumentException if the period is less or equals 0
183     */
184    public TimedSemaphore(final long timePeriod, final TimeUnit timeUnit, final int limit) {
185        this(null, timePeriod, timeUnit, limit);
186    }
187
188    /**
189     * Creates a new instance of {@link TimedSemaphore} and initializes it with
190     * an executor service, the given time period, and the limit. The executor
191     * service will be used for creating a periodic task for monitoring the time
192     * period. It can be <b>null</b>, then a default service will be created.
193     *
194     * @param service the executor service
195     * @param timePeriod the time period
196     * @param timeUnit the unit for the period
197     * @param limit the limit for the semaphore
198     * @throws IllegalArgumentException if the period is less or equals 0
199     */
200    public TimedSemaphore(final ScheduledExecutorService service, final long timePeriod,
201            final TimeUnit timeUnit, final int limit) {
202        if (timePeriod <= 0) {
203            throw new IllegalArgumentException("Time period must be greater 0!");
204        }
205
206        period = timePeriod;
207        unit = timeUnit;
208
209        if (service != null) {
210            executorService = service;
211            ownExecutor = false;
212        } else {
213            final ScheduledThreadPoolExecutor s = new ScheduledThreadPoolExecutor(
214                    THREAD_POOL_SIZE);
215            s.setContinueExistingPeriodicTasksAfterShutdownPolicy(false);
216            s.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
217            executorService = s;
218            ownExecutor = true;
219        }
220
221        setLimit(limit);
222    }
223
224    /**
225     * Returns the limit enforced by this semaphore. The limit determines how
226     * many invocations of {@link #acquire()} are allowed within the monitored
227     * period.
228     *
229     * @return the limit
230     */
231    public final synchronized int getLimit() {
232        return limit;
233    }
234
235    /**
236     * Sets the limit. This is the number of times the {@link #acquire()} method
237     * can be called within the time period specified. If this limit is reached,
238     * further invocations of {@link #acquire()} will block. Setting the limit
239     * to a value &lt;= {@link #NO_LIMIT} will cause the limit to be disabled,
240     * i.e. an arbitrary number of{@link #acquire()} invocations is allowed in
241     * the time period.
242     *
243     * @param limit the limit
244     */
245    public final synchronized void setLimit(final int limit) {
246        this.limit = limit;
247    }
248
249    /**
250     * Initializes a shutdown. After that the object cannot be used any more.
251     * This method can be invoked an arbitrary number of times. All invocations
252     * after the first one do not have any effect.
253     */
254    public synchronized void shutdown() {
255        if (!shutdown) {
256
257            if (ownExecutor) {
258                // if the executor was created by this instance, it has
259                // to be shutdown
260                getExecutorService().shutdownNow();
261            }
262            if (task != null) {
263                task.cancel(false);
264            }
265
266            shutdown = true;
267        }
268    }
269
270    /**
271     * Tests whether the {@link #shutdown()} method has been called on this
272     * object. If this method returns <b>true</b>, this instance cannot be used
273     * any longer.
274     *
275     * @return a flag whether a shutdown has been performed
276     */
277    public synchronized boolean isShutdown() {
278        return shutdown;
279    }
280
281    /**
282     * Tries to acquire a permit from this semaphore. This method will block if
283     * the limit for the current period has already been reached. If
284     * {@link #shutdown()} has already been invoked, calling this method will
285     * cause an exception. The very first call of this method starts the timer
286     * task which monitors the time period set for this {@code TimedSemaphore}.
287     * From now on the semaphore is active.
288     *
289     * @throws InterruptedException if the thread gets interrupted
290     * @throws IllegalStateException if this semaphore is already shut down
291     */
292    public synchronized void acquire() throws InterruptedException {
293        if (isShutdown()) {
294            throw new IllegalStateException("TimedSemaphore is shut down!");
295        }
296
297        if (task == null) {
298            task = startTimer();
299        }
300
301        boolean canPass = false;
302        do {
303            canPass = getLimit() <= NO_LIMIT || acquireCount < getLimit();
304            if (!canPass) {
305                wait();
306            } else {
307                acquireCount++;
308            }
309        } while (!canPass);
310    }
311
312    /**
313     * Returns the number of (successful) acquire invocations during the last
314     * period. This is the number of times the {@link #acquire()} method was
315     * called without blocking. This can be useful for testing or debugging
316     * purposes or to determine a meaningful threshold value. If a limit is set,
317     * the value returned by this method won't be greater than this limit.
318     *
319     * @return the number of non-blocking invocations of the {@link #acquire()}
320     * method
321     */
322    public synchronized int getLastAcquiresPerPeriod() {
323        return lastCallsPerPeriod;
324    }
325
326    /**
327     * Returns the number of invocations of the {@link #acquire()} method for
328     * the current period. This may be useful for testing or debugging purposes.
329     *
330     * @return the current number of {@link #acquire()} invocations
331     */
332    public synchronized int getAcquireCount() {
333        return acquireCount;
334    }
335
336    /**
337     * Returns the number of calls to the {@link #acquire()} method that can
338     * still be performed in the current period without blocking. This method
339     * can give an indication whether it is safe to call the {@link #acquire()}
340     * method without risking to be suspended. However, there is no guarantee
341     * that a subsequent call to {@link #acquire()} actually is not-blocking
342     * because in the mean time other threads may have invoked the semaphore.
343     *
344     * @return the current number of available {@link #acquire()} calls in the
345     * current period
346     */
347    public synchronized int getAvailablePermits() {
348        return getLimit() - getAcquireCount();
349    }
350
351    /**
352     * Returns the average number of successful (i.e. non-blocking)
353     * {@link #acquire()} invocations for the entire life-time of this {@code
354     * TimedSemaphore}. This method can be used for instance for statistical
355     * calculations.
356     *
357     * @return the average number of {@link #acquire()} invocations per time
358     * unit
359     */
360    public synchronized double getAverageCallsPerPeriod() {
361        return periodCount == 0 ? 0 : (double) totalAcquireCount
362                / (double) periodCount;
363    }
364
365    /**
366     * Returns the time period. This is the time monitored by this semaphore.
367     * Only a given number of invocations of the {@link #acquire()} method is
368     * possible in this period.
369     *
370     * @return the time period
371     */
372    public long getPeriod() {
373        return period;
374    }
375
376    /**
377     * Returns the time unit. This is the unit used by {@link #getPeriod()}.
378     *
379     * @return the time unit
380     */
381    public TimeUnit getUnit() {
382        return unit;
383    }
384
385    /**
386     * Returns the executor service used by this instance.
387     *
388     * @return the executor service
389     */
390    protected ScheduledExecutorService getExecutorService() {
391        return executorService;
392    }
393
394    /**
395     * Starts the timer. This method is called when {@link #acquire()} is called
396     * for the first time. It schedules a task to be executed at fixed rate to
397     * monitor the time period specified.
398     *
399     * @return a future object representing the task scheduled
400     */
401    protected ScheduledFuture<?> startTimer() {
402        return getExecutorService().scheduleAtFixedRate(new Runnable() {
403            @Override
404            public void run() {
405                endOfPeriod();
406            }
407        }, getPeriod(), getPeriod(), getUnit());
408    }
409
410    /**
411     * The current time period is finished. This method is called by the timer
412     * used internally to monitor the time period. It resets the counter and
413     * releases the threads waiting for this barrier.
414     */
415    synchronized void endOfPeriod() {
416        lastCallsPerPeriod = acquireCount;
417        totalAcquireCount += acquireCount;
418        periodCount++;
419        acquireCount = 0;
420        notifyAll();
421    }
422}