TimedSemaphore
Date: 2017-02-08

org.apache.commons.lang3.concurrent

## Class TimedSemaphore

- java.lang.Object
- - org.apache.commons.lang3.concurrent.TimedSemaphore

-     public class TimedSemaphore
      extends java.lang.Object

  A specialized semaphore implementation that provides a number of permits in a given time frame.

  This class is similar to the `java.util.concurrent.Semaphore` class provided by the JDK in that it manages a configurable number of permits. Using the `acquire()` method a permit can be requested by a thread. However, there is an additional timing dimension: there is no `release()` method for freeing a permit, but all permits are automatically released at the end of a configurable time frame. If a thread calls `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.

  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 `TimedSemaphore` could be installed to guarantee that only a given number of database queries are issued per second.

  A thread class for performing database queries could look as follows:

       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
               }
           }
           ...
       }

  The following code fragment shows how a `TimedSemaphore` is created that allows only 10 operations per second and passed to the statistics thread:

       TimedSemaphore sem = new TimedSemaphore(1, TimeUnit.SECOND, 10);
       StatisticsThread thread = new StatisticsThread(sem);
       thread.start();

  When creating an instance the time period for the semaphore must be specified. `TimedSemaphore` uses an executor service with a corresponding period to monitor this interval. The `ScheduledExecutorService` to be used for this purpose can be provided at construction time. Alternatively the class creates an internal executor service.

  Client code that uses `TimedSemaphore` has to call the `acquire()` method in each processing step. `TimedSemaphore` keeps track of the number of invocations of the `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.

  An alternative to `acquire()` is the `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 true, and the calling thread can continue with its action. If the semaphore is already at its limit, `tryAcquire()` immediately returns false without blocking; the calling thread must then abort its action. This usage scenario prevents blocking of threads.

  It is possible to modify the limit at any time using the `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 `acquire()` method never blocks, but lets all callers pass directly.

  When the `TimedSemaphore` is no more needed its `shutdown()` method should be called. This causes the periodic task that monitors the time interval to be canceled. If the `ScheduledExecutorService` has been created by the semaphore at construction time, it is also shut down. resources. After that `acquire()` must not be called any more.

  Since:
    3.0

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static int` | `NO_LIMIT` Constant for a value representing no limit.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `TimedSemaphore(long timePeriod, java.util.concurrent.TimeUnit timeUnit, int limit)` Creates a new instance of `TimedSemaphore` and initializes it with the given time period and the limit.
    `TimedSemaphore(java.util.concurrent.ScheduledExecutorService service, long timePeriod, java.util.concurrent.TimeUnit timeUnit, int limit)` Creates a new instance of `TimedSemaphore` and initializes it with an executor service, the given time period, and the limit.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `acquire()` Acquires a permit from this semaphore.
    `int` | `getAcquireCount()` Returns the number of invocations of the `acquire()` method for the current period.
    `int` | `getAvailablePermits()` Returns the number of calls to the `acquire()` method that can still be performed in the current period without blocking.
    `double` | `getAverageCallsPerPeriod()` Returns the average number of successful (i.e.
    `protected java.util.concurrent.ScheduledExecutorService` | `getExecutorService()` Returns the executor service used by this instance.
    `int` | `getLastAcquiresPerPeriod()` Returns the number of (successful) acquire invocations during the last period.
    `int` | `getLimit()` Returns the limit enforced by this semaphore.
    `long` | `getPeriod()` Returns the time period.
    `java.util.concurrent.TimeUnit` | `getUnit()` Returns the time unit.
    `boolean` | `isShutdown()` Tests whether the `shutdown()` method has been called on this object.
    `void` | `setLimit(int limit)` Sets the limit.
    `void` | `shutdown()` Initializes a shutdown.
    `protected java.util.concurrent.ScheduledFuture<?>` | `startTimer()` Starts the timer.
    `boolean` | `tryAcquire()` Tries to acquire a permit from this semaphore.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### NO_LIMIT

          public static final int NO_LIMIT

      Constant for a value representing no limit. If the limit is set to a value less or equal this constant, the `TimedSemaphore` will be effectively switched off.
      See Also:
        Constant Field Values

  - ### Constructor Detail

    - #### TimedSemaphore

          public TimedSemaphore(long timePeriod,
                        java.util.concurrent.TimeUnit timeUnit,
                        int limit)

      Creates a new instance of `TimedSemaphore` and initializes it with the given time period and the limit.
      Parameters:
        `timePeriod` - the time period
        `timeUnit` - the unit for the period
        `limit` - the limit for the semaphore
      Throws:
        `java.lang.IllegalArgumentException` - if the period is less or equals 0

    - #### TimedSemaphore

          public TimedSemaphore(java.util.concurrent.ScheduledExecutorService service,
                        long timePeriod,
                        java.util.concurrent.TimeUnit timeUnit,
                        int limit)

      Creates a new instance of `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 null, then a default service will be created.
      Parameters:
        `service` - the executor service
        `timePeriod` - the time period
        `timeUnit` - the unit for the period
        `limit` - the limit for the semaphore
      Throws:
        `java.lang.IllegalArgumentException` - if the period is less or equals 0

  - ### Method Detail

    - #### getLimit

          public final int getLimit()

      Returns the limit enforced by this semaphore. The limit determines how many invocations of `acquire()` are allowed within the monitored period.
      Returns:
        the limit

    - #### setLimit

          public final void setLimit(int limit)

      Sets the limit. This is the number of times the `acquire()` method can be called within the time period specified. If this limit is reached, further invocations of `acquire()` will block. Setting the limit to a value <= `NO_LIMIT` will cause the limit to be disabled, i.e. an arbitrary number of`acquire()` invocations is allowed in the time period.
      Parameters:
        `limit` - the limit

    - #### shutdown

          public void shutdown()

      Initializes a shutdown. After that the object cannot be used any more. This method can be invoked an arbitrary number of times. All invocations after the first one do not have any effect.

    - #### isShutdown

          public boolean isShutdown()

      Tests whether the `shutdown()` method has been called on this object. If this method returns true, this instance cannot be used any longer.
      Returns:
        a flag whether a shutdown has been performed

    - #### acquire

          public void acquire()
                       throws java.lang.InterruptedException

      Acquires a permit from this semaphore. This method will block if the limit for the current period has already been reached. If `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 `TimedSemaphore`. From now on the semaphore is active.
      Throws:
        `java.lang.InterruptedException` - if the thread gets interrupted
        `java.lang.IllegalStateException` - if this semaphore is already shut down

    - #### tryAcquire

          public boolean tryAcquire()

      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 true. Otherwise, this method returns immediately with the result false.
      Returns:
        true if a permit could be acquired; false otherwise
      Throws:
        `java.lang.IllegalStateException` - if this semaphore is already shut down
      Since:
        3.5

    - #### getLastAcquiresPerPeriod

          public int getLastAcquiresPerPeriod()

      Returns the number of (successful) acquire invocations during the last period. This is the number of times the `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.
      Returns:
        the number of non-blocking invocations of the `acquire()` method

    - #### getAcquireCount

          public int getAcquireCount()

      Returns the number of invocations of the `acquire()` method for the current period. This may be useful for testing or debugging purposes.
      Returns:
        the current number of `acquire()` invocations

    - #### getAvailablePermits

          public int getAvailablePermits()

      Returns the number of calls to the `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 `acquire()` method without risking to be suspended. However, there is no guarantee that a subsequent call to `acquire()` actually is not-blocking because in the mean time other threads may have invoked the semaphore.
      Returns:
        the current number of available `acquire()` calls in the current period

    - #### getAverageCallsPerPeriod

          public double getAverageCallsPerPeriod()

      Returns the average number of successful (i.e. non-blocking) `acquire()` invocations for the entire life-time of this `TimedSemaphore`. This method can be used for instance for statistical calculations.
      Returns:
        the average number of `acquire()` invocations per time unit

    - #### getPeriod

          public long getPeriod()

      Returns the time period. This is the time monitored by this semaphore. Only a given number of invocations of the `acquire()` method is possible in this period.
      Returns:
        the time period

    - #### getUnit

          public java.util.concurrent.TimeUnit getUnit()

      Returns the time unit. This is the unit used by `getPeriod()`.
      Returns:
        the time unit

    - #### getExecutorService

          protected java.util.concurrent.ScheduledExecutorService getExecutorService()

      Returns the executor service used by this instance.
      Returns:
        the executor service

    - #### startTimer

          protected java.util.concurrent.ScheduledFuture<?> startTimer()

      Starts the timer. This method is called when `acquire()` is called for the first time. It schedules a task to be executed at fixed rate to monitor the time period specified.
      Returns:
        a future object representing the task scheduled