Mastering Java Multithreading: Concurrency Solutions

1. Threads AdvancedProgramming in Java MehdiEinali

2. Agenda • Concurrency • Threads in java • Problems in concurrency • Solution for problems in concurrency • Problems in Solutions for problems in concurrency • Java High-level API for concurrency

3. concurrency

4. Sequential Programming • Up to this point, we learned sequential programming. • Everything in a program happens one step at a time. • What is wrong with this approach?

5. Multitasking vs single task • A single-tasking system can only run one program at a time, while a multi-tasking operating system allows more than one program to be running in concurrency. • Single task OS: • Multiple task OS:

6. OS Support

7. Concurrency vs. Parallelism CPU CPU1 CPU2

8. Concurrency vs parallelism

9. concurency

10. Thread vsprocess • A process is an executing instance of an program. • Processes contain their own state information, use their own address spaces, and only interact with each other via inter-process communication mechanisms (generally managed by the operating system) • A thread is a single sequence of execution within a program • Threads within a process share the same state and same memory space, and can communicate with each other directly, because they share the same variables

11. Thread vs process

12. Jvm is multi threaded CPU Process 1 Process 2 Process 3 Process 4 main run GC

13. Multithreading • A thread is a single sequence of execution within a program • Multithreadingrefers to multiple threads of control within a single program • each program can run multiple threads of control within it, e.g., Web Browser

14. What are Threads Good For? • To maintain responsiveness of an application during a long running task. • To enable cancellation of separable tasks. • Some problems are intrinsically parallel. • To monitor status of some resource (DB).

15. Parallel Processing • Multi-Processor Systems • Multi-core CPUs • Dual core • Core2duo • Corei7, corei5 • Even with no multi-core processors, Multithreading is useful • How? • I/O bounded tasks • Responsive UI • Simulated multi-threading

16. Language Support • Some languages have no built-in mechanism for muli-threading • C, C++, … • QT as a solution • OS-dependent libraries • pthread in linux • Windows API • Java has multi-threading in its core language • Pros and cons

17. Threads in java

18. Application Thread • When we execute an application: • The JVM creates a Thread object whose task is defined by the main() method • It starts the thread • The thread executes the statements of the program one by one until the method returns and the thread dies

19. Multiple Threads in an Application • Each thread has its private run-time stack • If two threads execute the same method, each will have its own copy of the local variables the methods uses • However, all threads see the same dynamic memory (heap) • Two different threads can act on the same object and same static fields concurrently

20. Creating Threads • There are two ways to create our own Thread object • Subclassing the Thread class and instantiating a new object of that class • Implementing the Runnable interface • In both cases the run() method should be implemented

21. Extending Thread publicclassThreadExampleextends Thread { publicvoid run() { for (int i = 1; i <= 100; i++) { System.out.println("Thread: " + i); } } }

22. Thread Methods void start() • Creates a new thread and makes it runnable • This method can be called only once void run() • The new thread begins its life inside this method

23. Thread Methods • sleep(intm)/sleep(intm,intn) • The thread sleeps for m milliseconds, plus n nanoseconds • yield() • Causes the currently executing thread object to temporarily pause and allow other threads to execute • Allow only threads of the same priority to run • Nothing is guaranteed for this method

24. Implementing Runnable publicclassRunnableExample implements Runnable { publicvoid run () { for (int i = 1; i <= 100; i++) { System.out.println ("Runnable: " + i); } } }

25. A Runnable Object • The Thread object’s run() method calls the Runnable object’s run() method • Allows threads to run inside any object, regardless of inheritance

26. Starting the Threads publicclassThreadsStartExample { publicstaticvoid main (String argv[]) { newThreadExample ().start (); new Thread(newRunnableExample ()).start (); } }

27. Ready queue Newly created threads Currently executed thread • Waiting for I/O operation to be completed • Waiting to be notified • Sleeping • Waiting to enter a synchronized section Scheduling Threads start() I/O operation completes What happens when a program with a ServerSocket calls accept()?

28. More Thread States

29. Thread State Diagram Alive Running new ThreadExample(); while (…) { … } New Thread Runnable Dead Thread thread.start(); run() method returns Blocked Object.wait() Thread.sleep() blocking IO call waiting on a monitor

30. classThreadExampleextends Thread { publicvoid run() { MultiThreading.task("Thread"); } } classRunnableExampleimplements Runnable{ publicvoid run() { MultiThreading.task("Runnable"); } } publicclassMultiThreading { publicstaticvoid task(String taskName){ for (int i = 1; i <= 10; i++) { System.out.println(taskName + ": " + i); try { Thread.sleep(new Random().nextInt(10)); } catch (InterruptedException e) { e.printStackTrace(); }}}

31. Running the Threads ThreadExample thr1 = newThreadExample(); thr1.start(); RunnableExample run1 = newRunnableExample(); new Thread(run1).start(); ThreadExample thr2 = newThreadExample(); thr2.start(); RunnableExample run2 = newRunnableExample(); new Thread(run2).start();

32. Output • First Run • Second Run … • Thread: 7 • Runnable: 7 • Thread: 9 • Runnable: 9 • Thread: 10 • Thread: 8 • Runnable: 8 • Runnable: 10 • Thread: 9 • Runnable: 9 • Runnable: 10 • Thread: 10 … • Thread: 8 • Runnable: 9 • Thread: 9 • Runnable: 7 • Thread: 8 • Runnable: 8 • Thread: 9 • Thread: 10 • Runnable: 10 • Thread: 10 • Runnable: 9 • Runnable: 10

33. responsiveness

34. GUI Example • Start Counting  starts counting the counter • Stop Counting  stops counting the counter

35. Unresponsive UI StartButton: startButton.addActionListener(newActionListener() { publicvoidactionPerformed(ActionEventevt) { this.stop= false; for (int i = 0; i < 100000; i++) { if(this.stop) break; tfCount.setText("" + countValue); countValue++; } } });

36. Unresponsive UI (2) StopButton: stopButton.addActionListener(newActionListener() { publicvoidactionPerformed(ActionEvente) { this.stop= true; } });

37. Responsive UI btnStart.addActionListener(newActionListener() { publicvoidactionPerformed(ActionEventevt) { this.stop= false; Thread counter = newCounterThread(); counter.start(); } }); • Inner class classCounterThreadextends Thread { publicvoid run() { for (int i = 0; i < 100000; i++) { if(this.stop){ break;} tfCount.setText("" + countValue); countValue++; try { sleep(10); } catch (InterruptedException ex) {} } } }

38. Responsive UI btnStart.addActionListener(newActionListener() { publicvoidactionPerformed(ActionEventevt) { this.stop= false; newThread() { publicvoid run() { for (int i = 0; i < 100000; i++) { if(this.stop){ break;} tfCount.setText("" + countValue); countValue++; try { sleep(10); } catch (InterruptedException ex) {} } } }.start(); } }); • Define anonymous inner class • Create object • Start thread

39. Thread scheduling

40. Java Scheduling • Thread scheduling is the mechanism used to determine how runnable threads are allocated CPU time • Scheduler is based on priority of threads • The priority of a thread : the importance of a thread to the scheduler • Uses fixed-priority scheduling: • Threads are scheduled according to their priority • Priority is compared with other threads in the ready queue

41. Thread Priority • The scheduler will lean toward running the waiting thread with the highest priority first • Lower-priority threads just tend to run less often • The exact behavior depends on the platform • Usually, all threads should run at the default priority • Trying to manipulate thread priorities is usually a mistake

42. Thread Priority (2) • Every thread has a priority • When a thread is created, it inherits the priority of the thread that created it • The priority values range from 1 to 10, in increasing priority

43. Thread Priority (3) • The priority can be adjusted subsequently using the setPriority() method • The priority of a thread may be obtained using getPriority() • Priority constants are defined: • MIN_PRIORITY=1 • MAX_PRIORITY=10 • NORM_PRIORITY=5

44. Some Notes • Thread implementation in Java is actually based on operating system support • Some Windows operating systems support only 7 priority levels, so different levels in Java may actually be mapped to the same operating system level

45. Daemon Threads • Daemon threads are “background” threads, that provide services to other threads, e.g., the garbage collection thread • The Java VM will not exit if non-Daemon threads are executing • The Java VM will exit if only Daemon threads are executing • Daemon threads die when the Java VM exits • A thread becomes a daemon with setDaemon() method

46. quiz • Which one is impossible output? • A) 1234567 • B) 3416527 • C) 1334567 • D) 7654321 • E) 0124567 • F) none of them

47. Problems in concurrency

48. Shared data • An object in a program can be changed by more than one thread • Q: Is the order of changes that were preformed on the object important?

49. Race Condition • A race condition –the outcome of a program is affected by the order in which the program's threads are allocated CPU time • Two threads are simultaneously modifying a single object • Both threads “race” to store their value

50. Race Condition Example Read A Decrement A Check Condition(B) Increment(B) Check Condition(C) Increment(C) Read A Decrement A Check Condition(B) Increment(A) B C A