Computers have been my hobby since I was 12. Now I'm a freelance Java developer. Like many other developers I am working on various private projects. Some are open source components (Butterfly Components - DI container, web ui, persistence api, mock test api etc.). Some are the tutorials at tutorials.jenkov.com. Yet others are web projects. I hold a bachelor degree in computer science and a master degree in IT focused on P2P networks. Jakob has posted 35 posts at DZone. You can read more from them at their website. View Full User Profile

Java Concurrency: Semaphores

06.19.2008
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This text is part of my blog on Java Concurrency which by now contains 18 texts.

A Semaphore is a thread synchronization construct that can be used either to send signals between threads to avoid missed signals, or to guard a critical section like you would with a lock. Java 5 comes with semaphore implementations in the java.util.concurrent package so you don't have to implement your own semaphores. Still, it can be useful to know the theory behind their implementation and use.

Here is a list of the topics covered in this text:

  1. Simple Semaphore
  2. Using Semaphores for Signaling
  3. Counting Semaphore
  4. Bounded Semaphore
  5. Using Semaphores as Locks

Simple Semaphore

Here is a simple Semaphore implementation:

public class Semaphore {
private boolean signal = false;

public synchronized void take() {
this.signal = true;
this.notify();
}

public synchronized void release() throws InterruptedException{
while(!this.signal) wait();
this.signal = false;
}

}

The take() method sends a signal which is stored internally in the Semaphore. The release() method waits for a signal. When received the signal flag is cleared again, and the release() method exited.

Using a semaphore like this you can avoid missed signals. You will call take() instead of notify() and release() instead of wait(). If the call to take() happens before the call to release() the thread calling release() will still know that take() was called, because the signal is stored internally in the signal variable. This is not the case with wait() and notify().

The names take() and release() may seem a bit odd when using a semaphore for signaling. The names origin from the use of semaphores as locks, as explained later in this text. In that case the names make more sense.


Using Semaphores for Signaling

Here is a simplified example of two threads signaling each other using a Semaphore:

Semaphore semaphore = new Semaphore();

SendingThread sender = new SendingThread(semaphore);

ReceivingThread receiver = new ReceivingThread(semaphore);

receiver.start();
sender.start();
public class SendingThread {
Semaphore semaphore = null;

public SendingThread(Semaphore semaphore){
this.semaphore = semaphore;
}

public void run(){
while(true){
//do something, then signal
this.semaphore.take();

}
}
}
public class RecevingThread {
Semaphore semaphore = null;

public ReceivingThread(Semaphore semaphore){
this.semaphore = semaphore;
}

public void run(){
while(true){
this.semaphore.release();
//receive signal, then do something...
}
}
}

Counting Semaphore

The Semaphore implementation in the previous section does not count the number of signals sent to it by take() method calls. We can change the Semaphore to do so. This is called a counting semaphore. Here is a simple implementation of a counting semaphore:

public class CountingSemaphore {
private int signals = 0;

public synchronized void take() {
this.signals++;
this.notify();
}

public synchronized void release() throws InterruptedException{
while(this.signals == 0) wait();
this.signals--;
}

}

Bounded Semaphore

The CoutingSemaphore has no upper bound on how many signals it can store. We can change the semaphore implementation to have an upper bound, like this:

public class BoundedSemaphore {
private int signals = 0;
private int bound = 0;

public BoundedSemaphore(int upperBound){
this.bound = upperBound;
}

public synchronized void take() throws InterruptedException{
while(this.signals == bound) wait();
this.signal++;
this.notify();
}

public synchronized void release() throws InterruptedException{
while(this.signal == 0) wait();
this.signal--;
}
}

Notice how the take() method now blocks if the number of signals is equal to the upper bound. Not until a thread has called receive will the thread calling take() be allowed to deliver its signal, if the BoundedSemaphore has reached its upper signal limit.


Using Semaphores as Locks

It is possible to use a bounded semaphore as a lock. To do so, set the upper bound to 1, and have the call to take() and release() guard the critical section. Here is an example:

BoundedSemaphore semaphore = new BoundedSemaphore(1);

...

semaphore.take();

try{
//critical section
} finally {
semaphore.release();
}

In contrast to the signaling use case the methods take() and release() are now called by the same thread. Since only one thread is allowed to take the semaphore, all other threads calling take() will be blocked until release() is called. The call to release() will never block since there has always been a call to take() first.

You can also use a bounded semaphore to limit the number of threads allowed into a section of code. For instance, in the example above, what would happen if you set the limit of the BoundedSemaphore to 5? 5 threads would be allowed to enter the critical section at a time. You would have to make sure though, that the thread operations do not conflict for these 5 threads, or you application will fail.

The relase() method is called from inside a finally-block to make sure it is called even if an exception is thrown from the critical section.

References
Published at DZone with permission of its author, Jakob Jenkov. (source)

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

Comments

David Karr replied on Thu, 2008/06/19 - 10:59am

Don't you have "take" and "release" reversed?  Even if you just look at the meanings of the words, it's clear that a typical semaphore would block on "take", because it's trying to "take" something, and "release" releases what you took. The Java 5 Semaphore class has "acquire()" and "release()", where "acquire()" blocks until a resource (a "permit", in their terms) is available, and "release()" releases a permit.

Jakob Jenkov replied on Thu, 2008/06/19 - 4:45pm

Hmmm, you might be right about that. I think that glitch happened because I started out with different names. I used send() and receive(),  and then I translated send into take, and receive into release. You are right, that should probably have been reversed. Anyways, the bounded semaphore can actually block in both the take() and release() method. 

Slava Imeshev replied on Thu, 2008/06/19 - 8:00pm

 

And don't forget Doug Lea's Concurrent Programming in Java. It explores locks, semaphores and other concurrency primitives in great detail. This is a must-read for anyone working with threads in Java.

Hope this helps.

Regards,

Slava Imeshev

Cacheonix - Distrubuted Concurrent Map for Java

Jakob Jenkov replied on Fri, 2008/06/20 - 3:35am in response to: Slava Imeshev

I have read it :-) Personally I prefer Allen Holubs book "Taming Java Threads". I found it easier to read than Doug Lea's book.

Anyways, I'll try to cover all the basics in my blog before diverting into a few more advanced topics not found in either book, like synchronizer templates, the anatomy of a synchronizer etc. T

Finally I'll get into multithreaded servers in Java. Not that I am an expert, but I hope the texts will spawn some interesting and learningful discussions that I can integreate into the blog later.

Jakob Jenkov replied on Fri, 2008/06/20 - 3:39am

BTW the reason I am posting the texts here too, is because JavaLobby asked me to. Before I just put the links in the dzone.com/links link queue.

Slava Imeshev replied on Mon, 2008/06/23 - 10:08pm

Jacob, you are doing a great job. Keep it up. I will check out Taming Java Threads, thanks for the pointer!

Slava

 

Slim Ouertani replied on Tue, 2008/10/07 - 5:13pm

Great post. But I think you have forget to extend thread forRecevingThread and SendingThread.

Jakob Jenkov replied on Wed, 2008/10/08 - 4:34am

haha, true!! ... well spotted!

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