Java Threads 1

1. Web programming Winter 2006 Java Threads 1 Web Programming course Dan Goldwasser dgoldwas@cs.haifa.ac.il

2. Web programming Winter 2006 What are threads? A thread in computer science is short for a thread of execution or a sequence of instructions. Multiple threads can be executed in parallel on many computer systems. This multithreading generally occurs by time slicing (where a single processor switches between different threads) or by multiprocessing (where threads are executed on separate processors). Threads are similar to processes, but differ in the way that they share resources. (From Wikipedia, the free encyclopedia)

3. Web programming Winter 2006 Thread operation

4. Web programming Winter 2006 Some Buzz words… Multithreading vs. Multitasking Concurrency vs. parallelism

5. Web programming Winter 2006 What is good for? Maintain responsiveness of an application during a long running task Enable cancellation of separable tasks Some problems are intrinsically parallel To monitor status of some resource (DB) Some APIs and systems demand it: Swing To take advantage of multiple processors

6. Web programming Winter 2006 Java implementation of threads We shall look at three version of thread instances Interface Runnable Class TimerTask Class Thread

7. Web programming Winter 2006 java.lang.Runnable An adapter which enables any given class to operate as a thread Requires one method be implemented: public void run() Most common method for adding threads to an application

8. Web programming Winter 2006 Example 1 class Counter implements Runnable { private int maxNumber; public Counter(int max){ maxNumber=max; } public void run(){ int i=0 while(i<maxNumber) i++; } …

9. Web programming Winter 2006 Timer and TimerTask A simple way to use threads is to schedule a timer and let the thread work periodically Class Timer Class TimerTask

10. Web programming Winter 2006 class TimerTask A task that can be scheduled for one-time or repeated execution by a Timer.

11. Web programming Winter 2006 class Timer facility for threads to schedule tasks for future execution in a background thread.

12. Web programming Winter 2006 Example 2 public class Reminder { Timer timer; public Reminder(int seconds) { timer = new Timer(); timer.schedule(new RemindTask(),seconds*1000); } class RemindTask extends TimerTask { public void run() { System.out.println("Time's up!"); cancel(); //Terminate the timer thread } } public static void main(String args[]) { new Reminder(5); } }

13. Web programming Winter 2006 class Thread A base class with maximum threading functionality. While less common, it is far more powerful. Forces the focus of your class to its “threadedness”. Minimum requirements are like Runnable: public void run()

14. Web programming Winter 2006 class Thread

15. Web programming Winter 2006 Example 3 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()); System.out.println("DONE! " + getName()); } … public static void main (String[] args) { new SimpleThread("Jamaica").start(); new SimpleThread("Fiji").start(); } }

16. Web programming Winter 2006 What is the output of this code?

17. Web programming Winter 2006

18. Web programming Winter 2006 Example 4 class Counter implements Runnable { private int maxNumber; public Counter(int max){ maxNumber=max; } public void run(){ int i=0 while(i++<maxNumber) } } public static void main(String[] args){ runnable r=new Counter(5); Thread t = new Thread(r); t.start(); }

19. Web programming Winter 2006 Thread life cycle

20. Web programming Winter 2006 Thread states A thread is not runnable if – Its sleep method is invoked The thread calls the wait method to wait for a specific condition to be satisfied The thread is blocking on I/O A thread will be runnable again when- If a thread has been put to sleep, the specified number of milliseconds must elapse If a thread is waiting for a condition, then another object must notify the waiting thread of a change in condition by calling notify or notifyAll If a thread is blocked on I/O, the I/O must complete

21. Web programming Winter 2006 Interrupting a thread If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException. If this thread is blocked in an I/O operation upon an interruptible channel then the channel will be closed, the thread's interrupt status will be set, and the thread will receive a ClosedByInterruptException. If none of the previous conditions hold then this thread's interrupt status will be set

22. Web programming Winter 2006 Interrupting a thread The program creates two threads and interrupts them

23. Web programming Winter 2006 class Counter extends Thread { public void run ( ) { long count = 0 ; while (true) { if(Thread.interrupted ( ) ) { System.out.println (“Counter was interrupted”) ; System.out.println (“count = “ + count) ; break ; } ++count ; } } } class Sleeper extends Thread { public void run ( ) { try { Thread.sleep (5000) ; } catch (InterruptedException e) { System.out.println (“Sleeper was interrupted”) ; } } }

24. Web programming Winter 2006 Thread Identification The identity of the currently running thread can be found using the currentThread method This has a static modifier, which means that there is only one method for all instances of Thread objects The method can always be called using the Thread class

25. Web programming Winter 2006 Thread Scheduling Execution of multiple threads on a single CPU in some order is called scheduling The JRE supports a very simple, deterministic scheduling algorithm called fixed-priority scheduling. schedules threads on the basis of their priority relative to other Runnable threads. The thread scheduling algorithm is also preemptive. If at any time a thread with a higher priority than all other Runnable threads becomes Runnable, system chooses the new higher-priority thread for execution. The new thread is said to preempt the other threads

26. Web programming Winter 2006 Thread priorities and scheduling Every thread has a priority which can be increased or decreased by calling the setPriority() method Thread.MIN_PRIORITY is the minimum priority (defined 1) Thread.MAX_PRIORITY is the maximum priority (defined as 10) When a thread is created, it is given a priority of 5 – defined as Thread.NORM_PRIORITY

27. Web programming Winter 2006 Time slicing Time slicing comes into play when multiple Runnable threads of equal priority are the highest-priority threads competing for the CPU  The Java platform does not implement time slicing. However, some platforms do support time slicing. Your programs should not rely on time slicing, as it may produce different results on different systems A thread can voluntarily yield the CPU by calling the yield method. The yield method gives other threads of the same priority a chance to run. If no equal-priority threads are Runnable, the yield is ignored.

28. Web programming Winter 2006 Thread scheduling Differs depending on the operating system Windows NT Each thread is given a time slice after which it is pre-empted by another thread of higher or equal priority Solaris (UNIX) A thread can only be pre-empted by a thread of higher priority. If one is not available, the thread runs to completion In both cases, lower priority threads can only run if all higher priority threads are blocked

29. Web programming Winter 2006 What is the output of this code?

30. Web programming Winter 2006 Hmmm… Scheduling multithreaded programs introduces some new concepts non-determinism race condition concurrency A mechanism for synchronizing the threads could be useful.. (next session)

31. Web programming Winter 2006 Summary What are threads? What are they good for? How are threads implemented in Java? In what states could a thread be? How does a thread move between states? How is the threads execution order determined? What are the risks of using threads?

32. Web programming Winter 2006 Questions?

33. Web programming Winter 2006 Example 3 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()); System.out.println("DONE! " + getName()); } … public static void main (String[] args) { new SimpleThread("Jamaica").start(); new SimpleThread("Fiji").start(); } }