Package org.apache.commons.lang3.concurrent

Class TimedSemaphore

java.lang.Object
org.apache.commons.lang3.concurrent.TimedSemaphore
public class TimedSemaphore extends Object
A specialized semaphore implementation that provides a number of permits in a given time frame.

This class is similar to the 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 Details

    • 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:

  • Constructor Details

    • TimedSemaphore

      public TimedSemaphore(long timePeriod, 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:
      IllegalArgumentException - if the period is less or equals 0

    • TimedSemaphore

      public TimedSemaphore(ScheduledExecutorService service, long timePeriod, 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:
      IllegalArgumentException - if the period is less or equals 0

  • Method Details

    • acquire

      public void acquire() throws 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:
      InterruptedException - if the thread gets interrupted
      IllegalStateException - if this semaphore is already shut down

    • 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 meantime 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

    • getExecutorService

      protected ScheduledExecutorService getExecutorService()
      Returns the executor service used by this instance.
      Returns:
      the executor service

    • 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

    • 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

    • 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 TimeUnit getUnit()
      Returns the time unit. This is the unit used by getPeriod().
      Returns:
      the time unit

    • 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

    • 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 ofacquire() 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 anymore. This method can be invoked an arbitrary number of times. All invocations after the first one do not have any effect.

    • startTimer

      protected 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

    • 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:
      IllegalStateException - if this semaphore is already shut down
      Since:
      3.5