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top examplets

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A Simple Semaphore Class

Thread A needs to wait for thread B's OK before continuing--Perhaps thread B is producing some resource that thread A is consuming. How do you do it? Use a Semaphore.

In this example, SemaphoreDemo's constructor spawns off a thread that jams a number into a variable. It needs to wait until the constructor has finished jamming the number into the variable.

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SemaphoreDemo

// SemaphoreDemo.java


class SemaphoreDemo extends Thread
  {


  public static void main (String[] args)
    {
    new SemaphoreDemo();
    }


  // The thread will shove a number into here, then the constructor will
  // display it.

  int x;


  // Start a thread, wait for it to shove a number into x, then display x.

  public SemaphoreDemo()
    {
    start();
    sem.waitForEvent();
    System.out.println (x);
    }


  public void run()
    {
    x = 12345;
    sem.postEvent();
    }


  Semaphore sem = new Semaphore();


  };

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Semaphore

// A simple semaphore class.

class Semaphore
  {


  // Constructor.

  public Semaphore()
    {
    count_ = 0;
    }


  // Wait for an event.  If there are no events posted, wait for one.

  public synchronized void waitForEvent()
    {
    while (count_ == 0)
      try
        {wait();}
      catch (InterruptedException e)
        {}
    count_--;
    }


  // Post an event.  If there are any threads hung in waitForEvent(), one
  // of them will be woken up and allowed to run.

  public synchronized void postEvent()
    {
    count_++;
    notify();
    }


  // The semaphore's count.

  int count_;


  };

See also

• Tutorial: Synchronizing Threads
• Language Reference: Synchronized Methods
• Language Reference: Threads and Locks

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Copyright (c) 1997, 1998 by Wayne E. Conrad, All Rights Reserved
Last Updated May 6, 1998
This page has been accidentally visited times since May 1st, 1998.

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