Code confusion

[Ben Zaidi]

package chapter9;

public class SimpleThreads {

	static void threadMessage(String message) {
		String threadName = Thread.currentThread().getName();
		System.out.format("%s: %s%n", threadName, message);
	}

	private static class MessageLoop implements Runnable {

		public void run() {
			String importantInfo[] = { "Mares eat oats", "Does eat oats",
					"Little lambs eat ivy", "A kid will eat ivy too" };

			try {
				for (int i = 0; i < importantInfo.length; i++) {
					//Pause for 4 seconds
					Thread.sleep(4000);
					//Print a message
					threadMessage(importantInfo[i]);
				}
			} catch (InterruptedException e) {
				threadMessage("I wasn't done!");
			}
		}

	}

	public static void main(String[] args) throws InterruptedException {
		//Delay, in milliseconds before we interrupt MessageLoop
		//thread (default one hour).
		long patience = 1000 * 10;

		//If command line argument present, gives patience in seconds.
		if (args.length > 0) {
			try {
				patience = Long.parseLong(args[0]) * 1000;
			} catch (NumberFormatException e) {
				System.err.println("Argument must be an integer.");
				System.exit(1);
			}

		}

		threadMessage("Starting MessageLoop thread");
		long startTime = System.currentTimeMillis();
		System.out.println("Start time is: " + startTime);
		Thread t = new Thread(new MessageLoop());
		t.start();

		threadMessage("Waiting for MessageLoop thread to finish");
		//loop until MessageLoop thread exits
		while (t.isAlive()) {
			threadMessage("Still waiting...");
			//Wait maximum of 1 second for MessageLoop thread to
			//finish.
			t.join(1000);
			System.out
					.println("Current time is: " + System.currentTimeMillis());
			System.out.println("Calculation: "
					+ (System.currentTimeMillis() - startTime));
			if (((System.currentTimeMillis() - startTime) > patience)
					&& t.isAlive()) {
				threadMessage("Tired of waiting!");
				t.interrupt();
				//Shouldn't be long now -- wait indefinitely
				t.join();
			}

		}
		threadMessage("Finally!");

	}

}

This is an example from the sun's site. I have almost understood the example, but i am confused at line 68. As t is interrupted, it means
the run() method will exit immediately. But what does line 68 signifies in this code? I understood the usage of t.join(1000) at line
58 which means that, currently stop the main thread for and wait for 1 second for MessageLoop to complete. If it doesn't just interrupt the
thread t. But what does line 68 means? Because after throwing the InterruptedException thread t will terminate and main will complete its
execution. Any pointers will be helpful.

[Anastasia Sirotenko]

Ben Zaidi wrote:
As t is interrupted, it means the run() method will exit immediately. But what does line 68 signifies in this code? I understood the usage of t.join(1000) at line
58 which means that, currently stop the main thread for and wait for 1 second for MessageLoop to complete. If it doesn't just interrupt the
thread t. But what does line 68 means? Because after throwing the InterruptedException thread t will terminate and main will complete its
execution. Any pointers will be helpful.

when you call t.interrupt, the interrupted flag will be set, but the thread will be interrupted only if something is watchig this flag. The Thread.sleep(4000); construction in run() method does watch the interrupded flag, so the t thread will be interrupded as soon as it tryies to execute next Thread.sleep(4000);. Between t.interrupt() and Thread.sleep(4000); the main thread teoretically can do something, so if you want to prevent this, you do t.join() after you set t.interrupt() just to wait while the thread will be fully interrupted.
in this particular program you can get this output if comment out t.join():

 main: Tired of waiting!
main: Still waiting...
Current time is: 1251360866275
Calculation: 5015
main: Finally!

and the usage of t.join garantees this output:

main: Tired of waiting!
main: Finally!

And then, if you remove all Thread.sleep() construction with its InterruptedException catch block and include the for loop in some while(true) loop, the t thread will never terminate, just because nothing will check if the interrupted flag is set for the thread.

[Ben Zaidi]

Thanks, it really clarified the confusion.

[Ben Zaidi]

Hello,

It was really helpful. But i want to know how sleep() method is related in checking the status of the Interrupt flag.
My question is, does only sleep() method does that or some other method from Thread class also does that like
wait() or something else. If sleep is related, then how it checking the status of interrupted flag?

[Anastasia Sirotenko]

Ben Zaidi wrote:Hello,

It was really helpful. But i want to know how sleep() method is related in checking the status of the Interrupt flag.
My question is, does only sleep() method does that or some other method from Thread class also does that like
wait() or something else. If sleep is related, then how it checking the status of interrupted flag?

As fas as i know Thread.sleep() and Thread.wait() methods check interrupted flag of a thread and throw InterruptedException if it is set.
But still you can use Thread.interrupted() (it returns true if thread is interrupted and resets the flag to false) and Thread.isInterrupted() (it returns true if thread is interrupted and doesnt reset anything) methods to control your thread's state by yourself.