path: root/src/main/java/org/apache/commons/lang3/concurrent/package-info.java

/*
 * 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.
 */
/**
 * Provides support classes for multi-threaded programming. This package is intended to be an extension to
 * {@link java.util.concurrent}. These classes are thread-safe.
 *
 * <p>
 * A group of classes deals with the correct creation and initialization of objects that are accessed by multiple
 * threads. All these classes implement the {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer} interface
 * which provides just a single method:
 * </p>
 *
 * <pre>
 * <code>
 * public interface ConcurrentInitializer&lt;T&gt; {
 *    T get() throws ConcurrentException;
 * }
 * </code>
 * </pre>
 *
 * <p>
 * A {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer} produces an object. By calling the
 * {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer#get() get()} method the object managed by the
 * initializer can be obtained. There are different implementations of the interface available addressing various use
 * cases:
 * </p>
 *
 * <p>
 * {@link org.apache.commons.lang3.concurrent.ConstantInitializer} is a very straightforward implementation of the
 * {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer} interface: An instance is passed an object when it
 * is constructed. In its {@code get()} method it simply returns this object. This is useful, for instance in unit tests
 * or in cases when you want to pass a specific object to a component which expects a
 * {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer}.
 * </p>
 *
 * <p>
 * The {@link org.apache.commons.lang3.concurrent.LazyInitializer} class can be used to defer the creation of an object
 * until it is actually used. This makes sense, for instance, if the creation of the object is expensive and would slow
 * down application startup or if the object is needed only for special executions.
 * {@link org.apache.commons.lang3.concurrent.LazyInitializer} implements the <em>double-check idiom for an instance
 * field</em> as discussed in Joshua Bloch's "Effective Java", 2nd edition, item 71. It uses <strong>volatile</strong>
 * fields to reduce the amount of synchronization. Note that this idiom is appropriate for instance fields only. For
 * <strong>static</strong> fields there are superior alternatives.
 * </p>
 *
 * <p>
 * We provide an example use case to demonstrate the usage of this class: A server application uses multiple worker
 * threads to process client requests. If such a request causes a fatal error, an administrator is to be notified using
 * a special messaging service. We assume that the creation of the messaging service is an expensive operation. So it
 * should only be performed if an error actually occurs. Here is where
 * {@link org.apache.commons.lang3.concurrent.LazyInitializer} comes into play. We create a specialized subclass for
 * creating and initializing an instance of our messaging service.
 * {@link org.apache.commons.lang3.concurrent.LazyInitializer} declares an abstract
 * {@link org.apache.commons.lang3.concurrent.LazyInitializer#initialize() initialize()} method which we have to
 * implement to create the messaging service object:
 * </p>
 *
 * <pre>
 * <code>
 * public class MessagingServiceInitializer extends LazyInitializer&lt;MessagingService&gt; {
 *   protected MessagingService initialize() throws ConcurrentException {
 *     // Do all necessary steps to create and initialize the service object
 *     MessagingService service = ...
 *     return service;
 *   }
 * }
 * </code>
 * </pre>
 *
 * <p>
 * Now each server thread is passed a reference to a shared instance of our new {@code MessagingServiceInitializer}
 * class. The threads run in a loop processing client requests. If an error is detected, the messaging service is
 * obtained from the initializer, and the administrator is notified:
 * </p>
 *
 * <pre>
 * <code>
 * public class ServerThread implements Runnable {
 *  // The initializer for obtaining the messaging service.
 *  private final ConcurrentInitializer&lt;MessagingService&gt; initializer;
 *
 *  public ServerThread(ConcurrentInitializer&lt;MessagingService&gt; init) {
 *    initializer = init;
 *  }
 *
 *  public void run() {
 *    while (true) {
 *      try {
 *        // wait for request
 *        // process request
 *      } catch (FatalServerException ex) {
 *        // get messaging service
 *        try {
 *          MessagingService svc = initializer.get();
 *          svc.notifyAdministrator(ex);
 *        } catch (ConcurrentException cex) {
 *          cex.printStackTrace();
 *        }
 *      }
 *    }
 *  }
 * }
 * </code>
 * </pre>
 *
 * <p>
 * The {@link org.apache.commons.lang3.concurrent.AtomicInitializer} class is very similar to
 * {@link org.apache.commons.lang3.concurrent.LazyInitializer}. It serves the same purpose: to defer the creation of an
 * object until it is needed. The internal structure is also very similar. Again there is an abstract
 * {@link org.apache.commons.lang3.concurrent.AtomicInitializer#initialize() initialize()} method which has to be
 * implemented by concrete subclasses in order to create and initialize the managed object. Actually, in our example
 * above we can turn the {@code MessagingServiceInitializer} into an atomic initializer by simply changing the
 * <strong>extends</strong> declaration to refer to {@code AtomicInitializer&lt;MessagingService&gt;} as super class.
 * </p>
 *
 * <p>
 * With {@link org.apache.commons.lang3.concurrent.AtomicSafeInitializer} there is yet another variant implementing the
 * lazy initializing pattern. Its implementation is close to
 * {@link org.apache.commons.lang3.concurrent.AtomicInitializer}; it also uses atomic variables internally and therefore
 * does not need synchronization. The name &quot;Safe&quot; is derived from the fact that it implements an additional
 * check which guarantees that the {@link org.apache.commons.lang3.concurrent.AtomicSafeInitializer#initialize()
 * initialize()} method is called only once. So it behaves exactly in the same way as
 * {@link org.apache.commons.lang3.concurrent.LazyInitializer}.
 * </p>
 *
 * <p>
 * Now, which one of the lazy initializer implementations should you use? First of all we have to state that is
 * problematic to give general recommendations regarding the performance of these classes. The initializers make use of
 * low-level functionality whose efficiency depends on multiple factors including the target platform and the number of
 * concurrent threads. So developers should make their own benchmarks in scenarios close to their specific use cases.
 * The following statements are rules of thumb which have to be verified in practice.
 * </p>
 *
 * <p>
 * {@link org.apache.commons.lang3.concurrent.AtomicInitializer} is probably the most efficient implementation due to
 * its lack of synchronization and further checks. Its main drawback is that the {@code initialize()} method can be
 * called multiple times. In cases where this is not an issue
 * {@link org.apache.commons.lang3.concurrent.AtomicInitializer} is a good choice.
 * {@link org.apache.commons.lang3.concurrent.AtomicSafeInitializer} and
 * {@link org.apache.commons.lang3.concurrent.LazyInitializer} both guarantee that the initialization method is called
 * only once. Because {@link org.apache.commons.lang3.concurrent.AtomicSafeInitializer} does not use synchronization it
 * is probably slightly more efficient than {@link org.apache.commons.lang3.concurrent.LazyInitializer}, but the
 * concrete numbers might depend on the level of concurrency.
 * </p>
 *
 * <p>
 * Another implementation of the {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer} interface is
 * {@link org.apache.commons.lang3.concurrent.BackgroundInitializer}. It is again an abstract base class with an
 * {@link org.apache.commons.lang3.concurrent.BackgroundInitializer#initialize() initialize()} method that has to be
 * defined by concrete subclasses. The idea of {@link org.apache.commons.lang3.concurrent.BackgroundInitializer} is that
 * it calls the {@code initialize()} method in a separate worker thread. An application creates a background initializer
 * and starts it. Then it can continue with its work while the initializer runs in parallel. When the application needs
 * the results of the initializer it calls its {@code get()} method. {@code get()} blocks until the initialization is
 * complete. This is useful for instance at application startup. Here initialization steps (e.g. reading configuration
 * files, opening a database connection, etc.) can be run in background threads while the application shows a splash
 * screen and constructs its UI.
 * </p>
 *
 * <p>
 * As a concrete example consider an application that has to read the content of a URL - maybe a page with news - which
 * is to be displayed to the user after login. Because loading the data over the network can take some time a
 * specialized implementation of {@link org.apache.commons.lang3.concurrent.BackgroundInitializer} can be created for
 * this purpose:
 * </p>
 *
 * <pre>
 * <code>
 * public class URLLoader extends BackgroundInitializer&lt;String&gt; {
 *   // The URL to be loaded.
 *   private final URL url;
 *
 *   public URLLoader(URL u) {
 *     url = u;
 *   }
 *
 *   protected String initialize() throws ConcurrentException {
 *     try {
 *       InputStream in = url.openStream();
 *       // read content into string
 *       ...
 *       return content;
 *     } catch (IOException ioex) {
 *       throw new ConcurrentException(ioex);
 *     }
 *   }
 * }
 * </code>
 * </pre>
 *
 * <p>
 * An application creates an instance of {@code URLLoader} and starts it. Then it can do other things. When it needs the
 * content of the URL it calls the initializer's {@code get()} method:
 * </p>
 *
 * <pre>
 * <code>
 * URL url = new URL("http://www.application-home-page.com/");
 * URLLoader loader = new URLLoader(url);
 * loader.start();  // this starts the background initialization
 *
 * // do other stuff
 * ...
 * // now obtain the content of the URL
 * String content;
 * try {
 *   content = loader.get();  // this may block
 * } catch (ConcurrentException cex) {
 *   content = "Error when loading URL " + url;
 * }
 * // display content
 * </code>
 * </pre>
 *
 * <p>
 * Related to {@link org.apache.commons.lang3.concurrent.BackgroundInitializer} is the
 * {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer} class. As the name implies, this class can
 * handle multiple initializations in parallel. The basic usage scenario is that a
 * {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer} instance is created. Then an arbitrary number
 * of {@link org.apache.commons.lang3.concurrent.BackgroundInitializer} objects is added using the
 * {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer#addInitializer(String, BackgroundInitializer)}
 * method. When adding an initializer a string has to be provided which is later used to obtain the result for this
 * initializer. When all initializers have been added the
 * {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer#start()} method is called. This starts
 * processing of all initializers. Later the {@code get()} method can be called. It waits until all initializers have
 * finished their initialization. {@code get()} returns an object of type
 * {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer.MultiBackgroundInitializerResults}. This object
 * provides information about all initializations that have been performed. It can be checked whether a specific
 * initializer was successful or threw an exception. Of course, all initialization results can be queried.
 * </p>
 *
 * <p>
 * With {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer} we can extend our example to perform
 * multiple initialization steps. Suppose that in addition to loading a web site we also want to create a JPA entity
 * manager factory and read a configuration file. We assume that corresponding
 * {@link org.apache.commons.lang3.concurrent.BackgroundInitializer} implementations exist. The following example
 * fragment shows the usage of {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer} for this purpose:
 * </p>
 *
 * <pre>
 * <code>
 * MultiBackgroundInitializer initializer = new MultiBackgroundInitializer();
 * initializer.addInitializer("url", new URLLoader(url));
 * initializer.addInitializer("jpa", new JPAEMFInitializer());
 * initializer.addInitializer("config", new ConfigurationInitializer());
 * initializer.start();  // start background processing
 *
 * // do other interesting things in parallel
 * ...
 * // evaluate the results of background initialization
 * MultiBackgroundInitializer.MultiBackgroundInitializerResults results =
 * initializer.get();
 * String urlContent = (String) results.getResultObject("url");
 * EntityManagerFactory emf =
 * (EntityManagerFactory) results.getResultObject("jpa");
 * ...
 * </code>
 * </pre>
 *
 * <p>
 * The child initializers are added to the multi initializer and are assigned a unique name. The object returned by the
 * {@code get()} method is then queried for the single results using these unique names.
 * </p>
 *
 * <p>
 * If background initializers - including {@link org.apache.commons.lang3.concurrent.MultiBackgroundInitializer} - are
 * created using the standard constructor, they create their own {@link java.util.concurrent.ExecutorService} which is
 * used behind the scenes to execute the worker tasks. It is also possible to pass in an
 * {@link java.util.concurrent.ExecutorService} when the initializer is constructed. That way client code can configure
 * the {@link java.util.concurrent.ExecutorService} according to its specific needs; for instance, the number of threads
 * available could be limited.
 * </p>
 *
 * <h2>Utility Classes</h2>
 *
 * <p>
 * Another group of classes in the new {@code concurrent} package offers some generic functionality related to
 * concurrency. There is the {@link org.apache.commons.lang3.concurrent.ConcurrentUtils} class with a bunch of static
 * utility methods. One focus of this class is dealing with exceptions thrown by JDK classes. Many JDK classes of the
 * executor framework throw exceptions of type {@link java.util.concurrent.ExecutionException} if something goes wrong.
 * The root cause of these exceptions can also be a runtime exception or even an error. In typical Java programming you
 * often do not want to deal with runtime exceptions directly; rather you let them fall through the hierarchy of method
 * invocations until they reach a central exception handler. Checked exceptions in contrast are usually handled close to
 * their occurrence. With {@link java.util.concurrent.ExecutionException} this principle is violated. Because it is a
 * checked exception, an application is forced to handle it even if the cause is a runtime exception. So you typically
 * have to inspect the cause of the {@link java.util.concurrent.ExecutionException} and test whether it is a checked
 * exception which has to be handled. If this is not the case, the causing exception can be rethrown.
 * </p>
 *
 * <p>
 * The {@link org.apache.commons.lang3.concurrent.ConcurrentUtils#extractCause(java.util.concurrent.ExecutionException)}
 * method does this work for you. It is passed an {@link java.util.concurrent.ExecutionException} and tests its root
 * cause. If this is an error or a runtime exception, it is directly rethrown. Otherwise, an instance of
 * {@link org.apache.commons.lang3.concurrent.ConcurrentException} is created and initialized with the root cause
 * ({@link org.apache.commons.lang3.concurrent.ConcurrentException} is a new exception class in the
 * {@code o.a.c.l.concurrent} package). So if you get such a
 * {@link org.apache.commons.lang3.concurrent.ConcurrentException}, you can be sure that the original cause for the
 * {@link java.util.concurrent.ExecutionException} was a checked exception. For users who prefer runtime exceptions in
 * general there is also an
 * {@link org.apache.commons.lang3.concurrent.ConcurrentUtils#extractCauseUnchecked(java.util.concurrent.ExecutionException)}
 * method which behaves like {@code extractCause()}, but returns the unchecked exception
 * {@link org.apache.commons.lang3.concurrent.ConcurrentRuntimeException} instead.
 * </p>
 *
 * <p>
 * In addition to the {@code extractCause()} methods there are corresponding
 * {@link org.apache.commons.lang3.concurrent.ConcurrentUtils#handleCause(java.util.concurrent.ExecutionException)} and
 * {@link org.apache.commons.lang3.concurrent.ConcurrentUtils#handleCauseUnchecked(java.util.concurrent.ExecutionException)}
 * methods. These methods extract the cause of the passed in {@link java.util.concurrent.ExecutionException} and throw
 * the resulting {@link org.apache.commons.lang3.concurrent.ConcurrentException} or
 * {@link org.apache.commons.lang3.concurrent.ConcurrentRuntimeException}. This makes it easy to transform an
 * {@link java.util.concurrent.ExecutionException} into a
 * {@link org.apache.commons.lang3.concurrent.ConcurrentException} ignoring unchecked exceptions:
 * </p>
 *
 * <pre>
 * <code>
 * Future&lt;Object&gt; future = ...;
 * try {
 *   Object result = future.get();
 *   ...
 * } catch (ExecutionException eex) {
 *   ConcurrentUtils.handleCause(eex);
 * }
 * </code>
 * </pre>
 *
 * <p>
 * There is also some support for the concurrent initializers introduced in the last sub section. The
 * {@code initialize()} method is passed a {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer} object and
 * returns the object created by this initializer. It is null-safe. The {@code initializeUnchecked()} method works
 * analogously, but a {@link org.apache.commons.lang3.concurrent.ConcurrentException} throws by the initializer is
 * rethrown as a {@link org.apache.commons.lang3.concurrent.ConcurrentRuntimeException}. This is especially useful if
 * the specific {@link org.apache.commons.lang3.concurrent.ConcurrentInitializer} does not throw checked exceptions.
 * Using this method the code for requesting the object of an initializer becomes less verbose. The direct invocation
 * looks as follows:
 * </p>
 *
 * <pre>
 * <code>
 * ConcurrentInitializer&lt;MyClass&gt; initializer = ...;
 * try {
 *   MyClass obj = initializer.get();
 *   // do something with obj
 * } catch (ConcurrentException cex) {
 *   // exception handling
 * }
 * </code>
 * </pre>
 *
 * <p>
 * Using the {@link org.apache.commons.lang3.concurrent.ConcurrentUtils#initializeUnchecked(ConcurrentInitializer)}
 * method, this becomes:
 * </p>
 *
 * <pre>
 * <code>
 * ConcurrentInitializer&lt;MyClass&gt; initializer = ...;
 * MyClass obj = ConcurrentUtils.initializeUnchecked(initializer);
 * // do something with obj
 * </code>
 * </pre>
 *
 * <p>
 * Another utility class deals with the creation of threads. When using the <em>Executor</em> framework new in JDK 1.5
 * the developer usually does not have to care about creating threads; the executors create the threads they need on
 * demand. However, sometimes it is desired to set some properties of the newly created worker threads. This is possible
 * through the {@link java.util.concurrent.ThreadFactory} interface; an implementation of this interface has to be
 * created and passed to an executor on creation time. Currently, the JDK does not provide an implementation of
 * {@link java.util.concurrent.ThreadFactory}, so one has to start from scratch.
 * </p>
 *
 * <p>
 * With {@link org.apache.commons.lang3.concurrent.BasicThreadFactory} Commons Lang has an implementation of
 * {@link java.util.concurrent.ThreadFactory} that works out of the box for many common use cases. For instance, it is
 * possible to set a naming pattern for the new threads, set the daemon flag and a priority, or install a handler for
 * uncaught exceptions. Instances of {@link org.apache.commons.lang3.concurrent.BasicThreadFactory} are created and
 * configured using the nested {@link org.apache.commons.lang3.concurrent.BasicThreadFactory.Builder} class. The
 * following example shows a typical usage scenario:
 * </p>
 *
 * <pre>
 * <code>
 * BasicThreadFactory factory = new BasicThreadFactory.Builder()
 *   .namingPattern("worker-thread-%d")
 *   .daemon(true)
 *   .uncaughtExceptionHandler(myHandler)
 *   .build();
 * ExecutorService exec = Executors.newSingleThreadExecutor(factory);
 * </code>
 * </pre>
 *
 * <p>
 * The nested {@link org.apache.commons.lang3.concurrent.BasicThreadFactory.Builder} class defines some methods for
 * configuring the new {@link org.apache.commons.lang3.concurrent.BasicThreadFactory} instance. Objects of this class
 * are immutable, so these attributes cannot be changed later. The naming pattern is a string which can be passed to
 * {@link String#format(java.util.Locale, String, Object...)}. The placeholder <em>%d</em> is replaced by an
 * increasing counter value. An instance can wrap another {@link java.util.concurrent.ThreadFactory} implementation;
 * this is achieved by calling the builder's
 * {@link org.apache.commons.lang3.concurrent.BasicThreadFactory.Builder#wrappedFactory(java.util.concurrent.ThreadFactory)
 * wrappedFactory(ThreadFactory)} method. This factory is then used for creating new threads; after that the specific
 * attributes are applied to the new thread. If no wrapped factory is set, the default factory provided by the JDK is
 * used.
 * </p>
 *
 * <h2>Synchronization objects</h2>
 *
 * <p>
 * The {@code concurrent} package also provides some support for specific synchronization problems with threads.
 * </p>
 *
 * <p>
 * {@link org.apache.commons.lang3.concurrent.TimedSemaphore} allows restricted access to a resource in a given time
 * frame. Similar to a semaphore, a number of permits can be acquired. What is new is the fact that the permits
 * available are related to a given time unit. For instance, the timed semaphore can be configured to allow 10 permits
 * in a second. Now multiple threads access the semaphore and call its
 * {@link org.apache.commons.lang3.concurrent.TimedSemaphore#acquire()} method. The semaphore keeps track about the
 * number of granted permits in the current time frame. Only 10 calls are allowed; if there are further callers, they
 * are blocked until the time frame (one second in this example) is over. Then all blocking threads are released, and
 * the counter of available permits is reset to 0. So the game can start anew.
 * </p>
 *
 * <p>
 * What are use cases for {@link org.apache.commons.lang3.concurrent.TimedSemaphore}? One example is to artificially
 * limit the load produced by multiple threads. Consider a batch application accessing a database to extract statistical
 * data. The application runs multiple threads which issue database queries in parallel and perform some calculation on
 * the results. If the database to be processed is huge and is also used by a production system, multiple factors have
 * to be balanced: On one hand, the time required for the statistical evaluation should not take too long. Therefore you
 * will probably use a larger number of threads because most of its life time a thread will just wait for the database
 * to return query results. On the other hand, the load on the database generated by all these threads should be limited
 * so that the responsiveness of the production system is not affected. With a
 * {@link org.apache.commons.lang3.concurrent.TimedSemaphore} object this can be achieved. The semaphore can be
 * configured to allow e.g. 100 queries per second. After these queries have been sent to the database the threads have
 * to wait until the second is over - then they can query again. By fine-tuning the limit enforced by the semaphore a
 * good balance between performance and database load can be established. It is even possible to chang? the number of
 * available permits at runtime. So this number can be reduced during the typical working hours and increased at night.
 * </p>
 *
 * <p>
 * The following code examples demonstrate parts of the implementation of such a scenario. First the batch application
 * has to create an instance of {@link org.apache.commons.lang3.concurrent.TimedSemaphore} and to initialize its
 * properties with default values:
 * </p>
 *
 * {@code TimedSemaphore semaphore = new TimedSemaphore(1, TimeUnit.SECONDS, 100);}
 *
 * <p>
 * Here we specify that the semaphore should allow 100 permits in one second. This is effectively the limit of database
 * queries per second in our example use case. Next the server threads issuing database queries and performing
 * statistical operations can be initialized. They are passed a reference to the semaphore at creation time. Before they
 * execute a query they have to acquire a permit.
 * </p>
 *
 * <pre>
 * <code>
 * public class StatisticsTask implements Runnable {
 * // The semaphore for limiting database load.
 *   private final TimedSemaphore semaphore;
 *
 *   public StatisticsTask(TimedSemaphore sem, Connection con) {
 *     semaphore = sem;
 *      ...
 *   }
 *
 *   //The main processing method. Executes queries and evaluates their results.
 *   public void run() {
 *     try {
 *       while (!isDone()) {
 *         semaphore.acquire();    // enforce the load limit
 *         executeAndEvaluateQuery();
 *       }
 *     } catch (InterruptedException iex) {
 *       // fall through
 *     }
 *   }
 * }
 * </code>
 * </pre>
 *
 * <p>
 * The important line here is the call to {@code semaphore.acquire()}. If the number of permits in the current time
 * frame has not yet been reached, the call returns immediately. Otherwise, it blocks until the end of the time frame.
 * The last piece missing is a scheduler service which adapts the number of permits allowed by the semaphore according
 * to the time of day. We assume that this service is pretty simple and knows only two different time slots: working
 * shift and night shift. The service is triggered periodically. It then determines the current time slot and configures
 * the timed semaphore accordingly.
 * </p>
 *
 * <pre>
 * <code>
 * public class SchedulerService {
 *   // The semaphore for limiting database load.
 *   private final TimedSemaphore semaphore;
 *     ...
 *
 *   // Configures the timed semaphore based on the current time of day. This method is called periodically.
 *   public void configureTimedSemaphore() {
 *      int limit;
 *      if (isWorkshift()) {
 *        limit = 50;    // low database load
 *      } else {
 *        limit = 250;   // high database load
 *      }
 *
 *      semaphore.setLimit(limit);
 *   }
 * }
 * </code>
 * </pre>
 *
 * <p>
 * With the {@link org.apache.commons.lang3.concurrent.TimedSemaphore#setLimit(int)} method the number of permits
 * allowed for a time frame can be changed. There are some other methods for querying the internal state of a timed
 * semaphore. Also some statistical data is available, e.g. the average number of {@code acquire()} calls per time
 * frame. When a timed semaphore is no more needed, its {@code shutdown()} method has to be called.
 * </p>
 */
package org.apache.commons.lang3.concurrent;