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Diffstat (limited to 'src/main/java/org/apache/commons/lang3/concurrent/TimedSemaphore.java')
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diff --git a/src/main/java/org/apache/commons/lang3/concurrent/TimedSemaphore.java b/src/main/java/org/apache/commons/lang3/concurrent/TimedSemaphore.java new file mode 100644 index 000000000..93b1fa385 --- /dev/null +++ b/src/main/java/org/apache/commons/lang3/concurrent/TimedSemaphore.java @@ -0,0 +1,464 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one or more + * contributor license agreements. See the NOTICE file distributed with + * this work for additional information regarding copyright ownership. + * The ASF licenses this file to You under the Apache License, Version 2.0 + * (the "License"); you may not use this file except in compliance with + * the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ +package org.apache.commons.lang3.concurrent; + +import java.util.concurrent.ScheduledExecutorService; +import java.util.concurrent.ScheduledFuture; +import java.util.concurrent.ScheduledThreadPoolExecutor; +import java.util.concurrent.TimeUnit; + +import org.apache.commons.lang3.Validate; + +/** + * A specialized <em>semaphore</em> implementation that provides a number of + * permits in a given time frame. + * + * <p> + * This class is similar to the {@code java.util.concurrent.Semaphore} class + * provided by the JDK in that it manages a configurable number of permits. + * Using the {@link #acquire()} method a permit can be requested by a thread. + * However, there is an additional timing dimension: there is no {@code + * release()} method for freeing a permit, but all permits are automatically + * released at the end of a configurable time frame. If a thread calls + * {@link #acquire()} and the available permits are already exhausted for this + * time frame, the thread is blocked. When the time frame ends all permits + * requested so far are restored, and blocking threads are waked up again, so + * that they can try to acquire a new permit. This basically means that in the + * specified time frame only the given number of operations is possible. + * </p> + * <p> + * A use case for this class is to artificially limit the load produced by a + * process. As an example consider an application that issues database queries + * on a production system in a background process to gather statistical + * information. This background processing should not produce so much database + * load that the functionality and the performance of the production system are + * impacted. Here a {@link TimedSemaphore} could be installed to guarantee that + * only a given number of database queries are issued per second. + * </p> + * <p> + * A thread class for performing database queries could look as follows: + * </p> + * + * <pre> + * public class StatisticsThread extends Thread { + * // The semaphore for limiting database load. + * private final TimedSemaphore semaphore; + * // Create an instance and set the semaphore + * public StatisticsThread(TimedSemaphore timedSemaphore) { + * semaphore = timedSemaphore; + * } + * // Gather statistics + * public void run() { + * try { + * while (true) { + * semaphore.acquire(); // limit database load + * performQuery(); // issue a query + * } + * } catch(InterruptedException) { + * // fall through + * } + * } + * ... + * } + * </pre> + * + * <p> + * The following code fragment shows how a {@link TimedSemaphore} is created + * that allows only 10 operations per second and passed to the statistics + * thread: + * </p> + * + * <pre> + * TimedSemaphore sem = new TimedSemaphore(1, TimeUnit.SECOND, 10); + * StatisticsThread thread = new StatisticsThread(sem); + * thread.start(); + * </pre> + * + * <p> + * When creating an instance the time period for the semaphore must be + * specified. {@link TimedSemaphore} uses an executor service with a + * corresponding period to monitor this interval. The {@code + * ScheduledExecutorService} to be used for this purpose can be provided at + * construction time. Alternatively the class creates an internal executor + * service. + * </p> + * <p> + * Client code that uses {@link TimedSemaphore} has to call the + * {@link #acquire()} method in each processing step. {@link TimedSemaphore} + * keeps track of the number of invocations of the {@link #acquire()} method and + * blocks the calling thread if the counter exceeds the limit specified. When + * the timer signals the end of the time period the counter is reset and all + * waiting threads are released. Then another cycle can start. + * </p> + * <p> + * An alternative to {@code acquire()} is the {@link #tryAcquire()} method. This + * method checks whether the semaphore is under the specified limit and + * increases the internal counter if this is the case. The return value is then + * <strong>true</strong>, and the calling thread can continue with its action. + * If the semaphore is already at its limit, {@code tryAcquire()} immediately + * returns <strong>false</strong> without blocking; the calling thread must + * then abort its action. This usage scenario prevents blocking of threads. + * </p> + * <p> + * It is possible to modify the limit at any time using the + * {@link #setLimit(int)} method. This is useful if the load produced by an + * operation has to be adapted dynamically. In the example scenario with the + * thread collecting statistics it may make sense to specify a low limit during + * day time while allowing a higher load in the night time. Reducing the limit + * takes effect immediately by blocking incoming callers. If the limit is + * increased, waiting threads are not released immediately, but wake up when the + * timer runs out. Then, in the next period more processing steps can be + * performed without blocking. By setting the limit to 0 the semaphore can be + * switched off: in this mode the {@link #acquire()} method never blocks, but + * lets all callers pass directly. + * </p> + * <p> + * When the {@link TimedSemaphore} is no more needed its {@link #shutdown()} + * method should be called. This causes the periodic task that monitors the time + * interval to be canceled. If the {@link ScheduledExecutorService} has been + * created by the semaphore at construction time, it is also shut down. + * resources. After that {@link #acquire()} must not be called any more. + * </p> + * + * @since 3.0 + */ +public class TimedSemaphore { + /** + * Constant for a value representing no limit. If the limit is set to a + * value less or equal this constant, the {@link TimedSemaphore} will be + * effectively switched off. + */ + public static final int NO_LIMIT = 0; + + /** Constant for the thread pool size for the executor. */ + private static final int THREAD_POOL_SIZE = 1; + + /** The executor service for managing the timer thread. */ + private final ScheduledExecutorService executorService; + + /** Stores the period for this timed semaphore. */ + private final long period; + + /** The time unit for the period. */ + private final TimeUnit unit; + + /** A flag whether the executor service was created by this object. */ + private final boolean ownExecutor; + + /** A future object representing the timer task. */ + private ScheduledFuture<?> task; // @GuardedBy("this") + + /** Stores the total number of invocations of the acquire() method. */ + private long totalAcquireCount; // @GuardedBy("this") + + /** + * The counter for the periods. This counter is increased every time a + * period ends. + */ + private long periodCount; // @GuardedBy("this") + + /** The limit. */ + private int limit; // @GuardedBy("this") + + /** The current counter. */ + private int acquireCount; // @GuardedBy("this") + + /** The number of invocations of acquire() in the last period. */ + private int lastCallsPerPeriod; // @GuardedBy("this") + + /** A flag whether shutdown() was called. */ + private boolean shutdown; // @GuardedBy("this") + + /** + * Creates a new instance of {@link TimedSemaphore} and initializes it with + * the given time period and the limit. + * + * @param timePeriod the time period + * @param timeUnit the unit for the period + * @param limit the limit for the semaphore + * @throws IllegalArgumentException if the period is less or equals 0 + */ + public TimedSemaphore(final long timePeriod, final TimeUnit timeUnit, final int limit) { + this(null, timePeriod, timeUnit, limit); + } + + /** + * Creates a new instance of {@link TimedSemaphore} and initializes it with + * an executor service, the given time period, and the limit. The executor + * service will be used for creating a periodic task for monitoring the time + * period. It can be <b>null</b>, then a default service will be created. + * + * @param service the executor service + * @param timePeriod the time period + * @param timeUnit the unit for the period + * @param limit the limit for the semaphore + * @throws IllegalArgumentException if the period is less or equals 0 + */ + public TimedSemaphore(final ScheduledExecutorService service, final long timePeriod, + final TimeUnit timeUnit, final int limit) { + Validate.inclusiveBetween(1, Long.MAX_VALUE, timePeriod, "Time period must be greater than 0!"); + + period = timePeriod; + unit = timeUnit; + + if (service != null) { + executorService = service; + ownExecutor = false; + } else { + final ScheduledThreadPoolExecutor s = new ScheduledThreadPoolExecutor( + THREAD_POOL_SIZE); + s.setContinueExistingPeriodicTasksAfterShutdownPolicy(false); + s.setExecuteExistingDelayedTasksAfterShutdownPolicy(false); + executorService = s; + ownExecutor = true; + } + + setLimit(limit); + } + + /** + * Returns the limit enforced by this semaphore. The limit determines how + * many invocations of {@link #acquire()} are allowed within the monitored + * period. + * + * @return the limit + */ + public final synchronized int getLimit() { + return limit; + } + + /** + * Sets the limit. This is the number of times the {@link #acquire()} method + * can be called within the time period specified. If this limit is reached, + * further invocations of {@link #acquire()} will block. Setting the limit + * to a value <= {@link #NO_LIMIT} will cause the limit to be disabled, + * i.e. an arbitrary number of{@link #acquire()} invocations is allowed in + * the time period. + * + * @param limit the limit + */ + public final synchronized void setLimit(final int limit) { + this.limit = limit; + } + + /** + * Initializes a shutdown. After that the object cannot be used anymore. + * This method can be invoked an arbitrary number of times. All invocations + * after the first one do not have any effect. + */ + public synchronized void shutdown() { + if (!shutdown) { + + if (ownExecutor) { + // if the executor was created by this instance, it has + // to be shutdown + getExecutorService().shutdownNow(); + } + if (task != null) { + task.cancel(false); + } + + shutdown = true; + } + } + + /** + * Tests whether the {@link #shutdown()} method has been called on this + * object. If this method returns <b>true</b>, this instance cannot be used + * any longer. + * + * @return a flag whether a shutdown has been performed + */ + public synchronized boolean isShutdown() { + return shutdown; + } + + /** + * Acquires a permit from this semaphore. This method will block if + * the limit for the current period has already been reached. If + * {@link #shutdown()} has already been invoked, calling this method will + * cause an exception. The very first call of this method starts the timer + * task which monitors the time period set for this {@link TimedSemaphore}. + * From now on the semaphore is active. + * + * @throws InterruptedException if the thread gets interrupted + * @throws IllegalStateException if this semaphore is already shut down + */ + public synchronized void acquire() throws InterruptedException { + prepareAcquire(); + + boolean canPass; + do { + canPass = acquirePermit(); + if (!canPass) { + wait(); + } + } while (!canPass); + } + + /** + * Tries to acquire a permit from this semaphore. If the limit of this semaphore has + * not yet been reached, a permit is acquired, and this method returns + * <strong>true</strong>. Otherwise, this method returns immediately with the result + * <strong>false</strong>. + * + * @return <strong>true</strong> if a permit could be acquired; <strong>false</strong> + * otherwise + * @throws IllegalStateException if this semaphore is already shut down + * @since 3.5 + */ + public synchronized boolean tryAcquire() { + prepareAcquire(); + return acquirePermit(); + } + + /** + * Returns the number of (successful) acquire invocations during the last + * period. This is the number of times the {@link #acquire()} method was + * called without blocking. This can be useful for testing or debugging + * purposes or to determine a meaningful threshold value. If a limit is set, + * the value returned by this method won't be greater than this limit. + * + * @return the number of non-blocking invocations of the {@link #acquire()} + * method + */ + public synchronized int getLastAcquiresPerPeriod() { + return lastCallsPerPeriod; + } + + /** + * Returns the number of invocations of the {@link #acquire()} method for + * the current period. This may be useful for testing or debugging purposes. + * + * @return the current number of {@link #acquire()} invocations + */ + public synchronized int getAcquireCount() { + return acquireCount; + } + + /** + * Returns the number of calls to the {@link #acquire()} method that can + * still be performed in the current period without blocking. This method + * can give an indication whether it is safe to call the {@link #acquire()} + * method without risking to be suspended. However, there is no guarantee + * that a subsequent call to {@link #acquire()} actually is not-blocking + * because in the meantime other threads may have invoked the semaphore. + * + * @return the current number of available {@link #acquire()} calls in the + * current period + */ + public synchronized int getAvailablePermits() { + return getLimit() - getAcquireCount(); + } + + /** + * Returns the average number of successful (i.e. non-blocking) + * {@link #acquire()} invocations for the entire life-time of this {@code + * TimedSemaphore}. This method can be used for instance for statistical + * calculations. + * + * @return the average number of {@link #acquire()} invocations per time + * unit + */ + public synchronized double getAverageCallsPerPeriod() { + return periodCount == 0 ? 0 : (double) totalAcquireCount + / (double) periodCount; + } + + /** + * Returns the time period. This is the time monitored by this semaphore. + * Only a given number of invocations of the {@link #acquire()} method is + * possible in this period. + * + * @return the time period + */ + public long getPeriod() { + return period; + } + + /** + * Returns the time unit. This is the unit used by {@link #getPeriod()}. + * + * @return the time unit + */ + public TimeUnit getUnit() { + return unit; + } + + /** + * Returns the executor service used by this instance. + * + * @return the executor service + */ + protected ScheduledExecutorService getExecutorService() { + return executorService; + } + + /** + * Starts the timer. This method is called when {@link #acquire()} is called + * for the first time. It schedules a task to be executed at fixed rate to + * monitor the time period specified. + * + * @return a future object representing the task scheduled + */ + protected ScheduledFuture<?> startTimer() { + return getExecutorService().scheduleAtFixedRate(this::endOfPeriod, getPeriod(), getPeriod(), getUnit()); + } + + /** + * The current time period is finished. This method is called by the timer + * used internally to monitor the time period. It resets the counter and + * releases the threads waiting for this barrier. + */ + synchronized void endOfPeriod() { + lastCallsPerPeriod = acquireCount; + totalAcquireCount += acquireCount; + periodCount++; + acquireCount = 0; + notifyAll(); + } + + /** + * Prepares an acquire operation. Checks for the current state and starts the internal + * timer if necessary. This method must be called with the lock of this object held. + */ + private void prepareAcquire() { + if (isShutdown()) { + throw new IllegalStateException("TimedSemaphore is shut down!"); + } + + if (task == null) { + task = startTimer(); + } + } + + /** + * Internal helper method for acquiring a permit. This method checks whether currently + * a permit can be acquired and - if so - increases the internal counter. The return + * value indicates whether a permit could be acquired. This method must be called with + * the lock of this object held. + * + * @return a flag whether a permit could be acquired + */ + private boolean acquirePermit() { + if (getLimit() <= NO_LIMIT || acquireCount < getLimit()) { + acquireCount++; + return true; + } + return false; + } +} |