CountDown Latch
In this article we will discuss about CountDownLatch . Same is introduced in java 5.
CountDownLatch works very same way as Object’s wait() and
notify() does ,but it is a Toplevel class
And very easy to use than wait/notify.
CountDownLatch VS wait()/
notify()
Spurious
Wake up:
look above code snippet, in case of
spurious wake up, Thread goes to JobQueue.remove(i) line as it is suddenly
wakes up due to spurious phenomena, but there is no job in queue (as producers
does not produce a job), so above code throws NoSuchElementException.
To solve this problem, we are going
to make a slight change in above code.
put, while(JobQueue.empty()) instead of ,if(JobQueue.empty()) , Now in case of spurious wakes up ,while(JobQueue.empty())
always holds true as producer does not produces a job and put the same in
JobQueue.
While using CountDownlatch,
developers don’t have to bother about spurious wake-up as CountDownlatch
implements on gate principle.
CountDownlatch initializes with a
positive number and blocking thread has to wait until this number becomes
Zero. If a Spurious wake up occurs and count is greater than zero
obviously blocking thread must has to wait as count is not zero, blocking
threads only got chance, when running thread invoke countdown() and count
decreases to zero.
Important Methods of CountDownlatch:
CountDownLatch has one disadvantage ,once it’s counter is zero it can’t be reset again .For reset one should use CyclicBarrier.
Example: In a Restaurant, Food only be served once same is prepared. Preparing and Serving foods are two different services but they have to
maintain a sequence, unless food is prepared it can’t be served.
We try to solve this problem by CountDownlatch.
We try to solve this problem by CountDownlatch.
Code :
package
com.example.concurrency.countdownlatch;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class PrepareServeProblem
{
public static void main(String[] args) {
ExecutorService
service = Executors.newFixedThreadPool(2);
CountDownLatch
latch = new CountDownLatch(1);
Runnable
prepareService = new Runnable()
{
public void run()
{
try {
Thread.sleep(1000);
}
catch
(InterruptedException e) {
// TODO Auto-generated
catch block
e.printStackTrace();
}
System.out.println("Preparing
Food");
latch.countDown();
}
};
Runnable
serveService = new Runnable()
{
public void run()
{
try {
latch.await();
System.out.println("wating for
prepareService to finish");
}
catch
(InterruptedException e) {
// TODO Auto-generated
catch block
e.printStackTrace();
}
System.out.println("Serving the
Food");
}
};
service.execute(serveService);
service.execute(prepareService);
}
}
Output:
Preparing Food
wating for prepareService to finish
Serving the Food
Posts
