Friday, February 25, 2011

Chapter 58: Preventing Thread Execution

In the previous chapters we saw what a thread is and how to run or execute them. But, there is one more important thing that we need to know. Let me give a clue. Lets say you have multiple threads and you want one of your threads to finish before the others. It would definitely be sweet to alter the way our threads run and finish, wouldn't it? That is exactly what we are going to learn in this chapter.

Before we begin, I repeat something I said earlier.
WE CANNOT AND I MEAN CANNOT GUARANTEE THE WAY THREADS EXECUTE IN OUR PROGRAM.

We can only request the scheduler to run the threads the way we want but that by no means is guarantee. Think of the scheduler as your hard headed boss who ends up irritating you even if you try out of your way to please him. Though it's a bad analogy, that's exactly how the scheduler works. Stubborn and all by itself and not listening to anyone.

Lets get started!!!

Stopping a Thread from Executing

A thread that’s been stopped usually means a thread that’s moved to the dead state. But you should be able to recognize when a thread will get kicked out of running but not be sent back to either runnable or dead.

For the purpose of the exam, we aren’t concerned with a thread blocking on I/O (say, waiting for something to arrive from an input stream from the server). We are concerned with the following:
• Sleeping
• Waiting
• Blocked because it needs an object’s lock

Sleeping

The sleep() method is a static method of class Thread. You use it in your code to “slow a thread down” by forcing it to go into a sleep mode before coming back to runnable. When a thread sleeps, it drifts off somewhere and doesn’t return to runnable until it wakes up.

So why would you want a thread to sleep? Well, you might think the thread is moving too quickly through its code. Or you might need to force your threads to take turns, since reasonable turn-taking isn’t guaranteed in the Java specification.

You do this by invoking the static Thread.sleep() method, giving it a time in milliseconds as follows:
try {
Thread.sleep(5*1000); // Sleep for 5 seconds
} catch (InterruptedException ex) { }

Notice that the sleep() method can throw a checked InterruptedException (you’ll usually know if that is a possibility, since another thread has to explicitly do the interrupting), so you must acknowledge the exception with a handle or declare. Typically, you wrap calls to sleep() in a try/catch, as in the preceding code.

Let’s modify our Rocky, Cena, Triple H code by using sleep() to try to force the threads to alternate rather than letting one thread dominate for any period of time. Where do you think the sleep() method should go?

class NameRunnable implements Runnable {

public void run() {

for (int x = 1; x < 4; x++) {

System.out.println("Run by "

+ Thread.currentThread().getName());

try {

Thread.sleep(1000);

} catch (InterruptedException ex) { }

}

}

}


public class SleepExample {

public static void main (String [] args) {


// Make one Runnable

NameRunnable nr = new NameRunnable();


Thread one = new Thread(nr);

one.setName("Rocky");

Thread two = new Thread(nr);

two.setName("Cena");

Thread three = new Thread(nr);

three.setName("Triple H");


one.start();

two.start();

three.start();

}

}

Running this code shows Rocky, Cena, and Triple H alternating nicely:

% java SleepExample

Run by Rocky

Run by Cena

Run by Triple H

Run by Rocky

Run by Cena

Run by Triple H

Run by Rocky

Run by Cena

Run by Triple H


Just keep in mind that the behavior in the preceding output is still not guaranteed. You can’t be certain how long a thread will actually run before it gets put to sleep, so you can’t know with certainty that only one of the three threads will be in the runnable state when the running thread goes to sleep. In other words, if there are two threads awake and in the runnable pool, you can’t know with certainty that the least recently used thread will be the one selected to run. Still, using sleep() is the best way to help all threads get a chance to run! Or at least to guarantee that one thread doesn’t get in and stay until it’s done. When a thread encounters a sleep call, it must go to sleep for at least the specified number of milliseconds (unless it is interrupted before its wake-up time, in which case it immediately throws the InterruptedException).

Exam Tip: Just because a thread’s sleep() expires, and it wakes up, does not mean it will return to running! Remember, when a thread wakes up, it simply goes back to the runnable state. So the time specified in sleep() is the minimum duration in which the thread won’t run, but it is not the exact duration in which the thread won’t run. So you can’t, for example, rely on the sleep() method to give you a perfectly accurate timer. Although in many applications using sleep() as a timer is certainly good enough, you must know that a sleep() time is not a guarantee that the thread will start running again as soon as the time expires and the thread wakes

Remember that sleep() is a static method, so don’t be fooled into thinking that one thread can put another thread to sleep. You can put sleep() code anywhere, since all code is being run by some thread. When the executing code (meaning the currently running thread’s code) hits a sleep() call, it puts the currently running thread to sleep.

Previous Chapter: Chapter 57 - Thread States & Transitions

Next Chapter: Chapter 59 - Thread Priorities, yield and join

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