1. Threads CSCE 190 – Java Instructor: Joel Gompert Wed, Aug 3, 2004

2. What is a thread? A sequential flow of instructions Multiple threads are used to do multiple things at once.

3. Example import java.util.Timer; import java.util.TimerTask; class MyTask extends TimerTask { public void run() { System.out.println("Time's up!"); TimerExample.timer.cancel(); // Terminate the timer thread } public class TimerExample { public static Timer timer; public static void main(String args[]) { System.out.println("About to schedule task."); timer = new Timer(); timer.schedule(new MyTask(), 5*1000); System.out.println("Task scheduled."); }

4. Timers run as separate threads By default, a program keeps running as long as its timer threads are running.

5. Creating a custom thread Extend the Thread class Override the run() method.

6. Example public class SimpleThread extends Thread { public SimpleThread(String str) { super(str); } public void run() { for (int i = 0; i < 10; i++) { System.out.println(i + " " + getName()); try { sleep((long)(Math.random() * 1000)); } catch (InterruptedException e) {} } System.out.println("DONE! " + getName()); }

7. How to start a thread public class ThreadsExample { public static void main (String[] args) { new SimpleThread("Jamaica").start(); new SimpleThread("Fiji").start(); }

8. A different way to create a thread Implement the Runnable interface Override the run() method.

9. Does it matter which you choose? Extend Thread Implement Runnable What if you are creating an applet?

10. Two ways to start a thread: myThread.start(); Thread newThread = new Thread(myThread); newThread.start();

11. Implementing Runnable class SimpleThread implements Runnable { Thread newThread; public void start() { newThread = new Thread(this); newThread.start(); } public void run() { // do some stuff } public void stop() { newThread.stop(); }

12. Just a quick note The following: MyClass mc = new MyClass(); mc.doSomething(); Is equivalent to this: (new MyClass()).doSomething();

13. Priority Most Computers have only 1 CPU. Set priority with setPriority() Using integers ranging between Thread.MIN_PRIORITY and Thread.MAX_PRIORITY. Usually the thread with the highest priority is running.

14. Selfish Threads Suppose we have the following: public int tick = 1; public void run() { while (tick < 10000000) tick++; }

15. Time sliced system Thread #1, tick = 50000 Thread #0, tick = 50000 Thread #0, tick = 100000 Thread #1, tick = 100000 Thread #1, tick = 150000 Thread #1, tick = 200000 Thread #0, tick = 150000 Thread #0, tick = 200000 Thread #1, tick = 250000 Thread #0, tick = 250000 Thread #0, tick = 300000 Thread #1, tick = 300000 Thread #1, tick = 350000 Thread #0, tick = 350000 Thread #0, tick = 400000 Thread #1, tick = 400000

16. Non-time-sliced system Thread #0, tick = 50000 Thread #0, tick = 100000 Thread #0, tick = 150000 Thread #0, tick = 200000 Thread #0, tick = 250000 Thread #0, tick = 300000 Thread #0, tick = 350000 Thread #0, tick = 400000 Thread #1, tick = 50000 Thread #1, tick = 100000 Thread #1, tick = 150000 Thread #1, tick = 200000 Thread #1, tick = 250000 Thread #1, tick = 300000 Thread #1, tick = 350000 Thread #1, tick = 400000

17. The solution: make ‘polite’ threads Periodically call the yield() method. For Example: public int tick = 1; public void run() { while (tick < 10000000) tick++; this.yield(); } Or perhaps: public int tick = 1; public void run() { while (tick < 10000000) tick++; if(tick % 1000 == 0) this.yield(); }