Concurrent Programming

1. Concurrent Programming CS 3331 Fall 2009

2. Outline • What’re threads? • How to create threads? • How to controlling threads? • Thread safety and synchronization • BoundedQueue example

3. Concurrent Programming • Shared memory vs. message-based (e.g., IPC) • Built-in language features vs. library-based

4. Multithreaded Programming flow of control • Thread • A single sequential flow of control within a program, i.e., a lightweight flow that executes concurrently with other threads within a single process. • Communicates with other threads via shared variables and objects. • Managed by JVM. sequential concurrent

5. Why? • Can take advantages of multiple CPUs. • Allows a server to handle multiple clients simultaneously. • Suitable for developing reactive system. • Makes application more responsive to user inputs (e.g., GUI programs such as BouncingBall applet). • Natural to model real worlds.

6. But, • More difficult to program and debug (e.g., shared access to objects) • Nondeterministic order of thread execution • Safety and liveness problems

7. Creating Threads in Java • Two approaches • By extending the java.lang.Thread class • By implementing the java.lang.Runnable interface

8. Creating Threads (Cont.) • Extending the Thread class flow of control public class MyThread extends Thread { public void run() { // the thread body } // other methods and fields. } // Start a new thread by calling the start method MyThread thread = new MyThread(); thread.start(); // … start() run() // … The method run is the concurrent unit in Java.

9. Creating Threads (Cont.) • Implementing the Runnable interface public interface Runnable { void run(); } public class MyThread implements Runnable { public void run() { // the thread body } // other methods and fields. } // Start a new thread by creating an instance and calling the start method, e.g., new Thread(new MyThread()).start();

10. Example --- Simple Counter public class Counter extends Thead { private int count, inc, delay; public Counter(int init, int inc, int delay) { this.count = init; this.inc = inc; this.delay = delay; } public void run() { try { for (;;) { System.out.print(count + “ “); count += inc; sleep(delay); } } catch (InterruptedException e) {} } public static void main(String[] args) { new Counter(0, 1, 33).start(); new Counter(0, -1, 100).start(); } }

11. Example (Cont.) Output: 0 0 1 2 -1 3 4 5 -2 6 7 8 -3 9 10 -4 11 12 13 -5 14 15 16 -6 17 18 -7 19 20 21 -8 22 23 24 -9 25 26 -10 27 28 -11 29 30 31 -12 32 33 34 -13 35 36 37 -14 38 39 -15 40 41 42 -16 43 44 45 … Q: What will happen if we change the statement “new Counter().start()” to “new Counter().run()” in the main method?

12. Exercise Write a thread class named Talker that prints a hello message, say “Hello, I am Tom!”, continuously and creates two instances of Talker as separate threads. public class Talker _______________________ { private String name; public Talker(String name) { this.name = name; } }

13. Alive yield New Runnable start Dead sleep join end of run Non-runnable Controlling Threads • Life cycle of threads • New • After creation but before start() call • Alive • After the start() call; run() is implicitly called. • Consists of two sub states • Runnable: Ready to run; maybe running or waiting for their turns • Non-runnable: Not ready to run; waiting for some events (e.g., sleeping, blocked, or waiting) • Dead • When run() method returns

14. Methods of Thread Class Method Description start() Enter the Runnable state from the new state *sleep() Enter the Non-runnable state join() Enter the Non-runnable state and wait for another thread to terminate *yield() Release CPU and remain in the Runnable interrupt() Cause InterruptedException isAlive() True if in the alive state isInterrupted() True if interrupted * static method

15. main jobs[0] … jobs[9] Example Coordinating concurrent jobs public class Job extends Thread { public void run() { // do some meaningful work, e.g., writing multithreaded applets. } public static void main(String[] args) { Job[] jobs = new Job[10]; for (int i = 0; i < jobs.length; i++) { jobs[i] = new Job(); jobs[i].start(); } for (int i = 0; i < jobs.length; i++) { jobs[i].join(); } } }

16. Exercise Explain the execution flow of the following program. public class Job extends Thread { public void run() { // do some meaningful work, e.g., writing multithreaded applets. } public static void main(String[] args) { Job[] jobs = new Job[10]; for (int i = 0; i < jobs.length; i++) { jobs[i] = new Job(); jobs[i].start(); jobs[i].join(); } } }

17. Thread Safety • Example --- bank accounts public class Account { private long balance; public boolean withdraw(long amount) { if (amount <= balance) { long newBalance = balance – amount; balance = newBalance; return true; } else { return false; } } // other methods and fields } • What might happen in the presence of multiple threads?

18. Example (Cont.) if (amount <= balance) { long newBalance = balance – amount; balance = newBalance; return true; } • Possible scenario Assume that the initial balance is $1,000,000, and two withdraw requests of $1,000,000 are made almost simultaneously. time balance withdraw 1 withdraw 2 t1 1,000,000 amount<=balance t2 1,000,000 amount<=balance t3 1,000,000 newbalance=...; t4 1,000,000 newbalance=...; t5 0 balance=...; t6 0 balance=...; t7 0 return true; t8 0 return true;

19. Atomicity of Operations • The problem • Several calls of the withdraw method may be interleaved interfering with each others. • Atomic operation • An operation that can’t be interrupted, e.g., reading and assignment to (word-sized) variables. • Non-atomic operations should be explicitly synchronized to ensure atomicity.

20. Making methods atomic • Synchronized methods and statements • A lock is associated with each object. // synchronized method synchronized void aMethod() { // do something } // synchronized statement synchronized (expr) { // expr should be of a reference type! // do somthing }

21. Example public class Account { private long balance; public synchronized boolean withdraw(long amount) { if (amount <= balance) { long newBalance = balance – amount; balance = newBalance; return true; } else { return false; } } // other methods and fields }

22. Exercise Make withdraw method synchronized by using the synchronized statement. public class Account { private long balance; public boolean withdraw(long amount) { // WRITE YOUR CODE HERE… if (amount <= balance) { long newBalance = balance – amount; balance = newBalance; return true; } else { return false; } } }

23. Synchronization • Synchronization • Mechanism to controls the order in which threads execute • Competition vs. cooperative synchronization • Mutual exclusion of threads • Each synchronized method or statement is guarded by an object. • When entering a synchronized method or statement, the object will be locked until the method is finished. • When the object is locked by another thread, the current thread must wait.

24. Example --- Bounded Queue A first-in, first-out queue with a fixed capacity public class BoundedQueue { protected Object rep[]; // circular array protected int front = 0; // front of the queue protected int back = -1; // back of the queue protected int size; // capacity of the queue protected int count = 0; // num of objects in the queue //@ requires size > 0; public BoundedQueue(int size) { this.size = size; rep = new Object[size]; back = size – 1; } public boolean isEmpty() { return (count == 0); }

25. Bounded Queue (Cont.) public boolean isFull() { return (count == size); } public int getCount() { return count; } public void put(/*@ non_null @*/ Object e) { if (!isFull()) { back = (back + 1) % size; rep[back] = e; count++; } } public Object get() { Object result = null; if (!isEmpty()) { result = rep[front]; front = (front + 1) % size; count--; } return result; } }

26. Making Queue Thread-Safe public class SyncBoundedQueue extends BoundedQueue { public SyncBoundedQueue(int size) { super(size); } public synchronized boolean isEmpty() { return super.isEmpty(); } public synchronized boolean isFull() { return super.isFull(); } public synchronized int getCount() { return super.getCount(); } public synchronized void put(Object e) { super.put(e); } public synchronized Object get() { return super.get(); } }

27. SyncBoundedQueue Thread Broker Thief Use of Bounded Queue Typical use in producers and consumers

28. Producer public class Thief extends Thread { private BoundedQueue queue; private int n; // number of items to steal public Thief(BoundedQueue queue, int n) { this.queue = queue; this.n = n; } public void run() { for (int i = 0; i < n; i++) { queue.put(new Integer(i)); System.out.println(“produced: “ + i); try { sleep((int) (Math.random() * 100)); } catch (InterruptedException e) {} } } }

29. Consumer public class Broker extends Thread { private BoundedQueue queue; private int n; // number of stolen items to sell public Broker(BoundedQueue queue, int n) { this.queue = queue; this.n = n; } public void run() { for (int i = 0; i < n; i++) { Object obj = queue.get(); System.out.println(“\tconsumed: “ + obj); try { sleep((int) (Math.random() * 400)); } catch (InterruptedException e) {} } } }

30. Sample Main Program The main method of class SyncBoundedQueue public static void main(String[] args) { SyncBoundedQueue queue = new SyncBoundedQueue(5); new Thief(queue, 15).start(); // produce 15 items new Broker(queue, 10).start(); // consume 10 items } Output Some of the items might be lost, as the producer produces items faster than the consumer consumes them.

31. Cooperation among Threads • Guarded suspension • Before a method is executed, the guard is tested. A guard is the precondition for an action to complete successfully. • Execution continues only when the guard is true, ensuring the successful completion of the method invocation. • Execution is temporarily suspended until the guard becomes true, at which time execution may continue. • Thread controlling methods • wait(), notify(), and notifyAll() defined in the class Object • Can be used to implement guarded suspension

32. Thread Methods of Class Object Method Description wait() Temporarily blocked and placed in the wait queue of the receiving object. notify() Awaken one of the threads in the wait queue associated with the receiving object and remove it from the queue. notifyAll() Same as notify() except that all threads in the wait queue are awakened.

33. Bounded Queue with Guarded Suspension public class BoundedQueueWithGuard extends BoundedQueue { public Producer(int size) { super(size); } public synchronized boolean isEmpty() { return super.isEmpty(); } public synchronized boolean isFull() { return super.isFull(); } public synchronized int getCount() { return super.getCount(); } <<put, get, and main method>> }

34. Put Method public synchronized void put(Object obj) { try { while (isFull()) { wait(); } } catch (InterruptedException e) {} super.put(e); notify(); }

35. Exercise --- Get Method Write the get method with guarded suspension. public synchronized Object get() { // WRITE YOUR CODE HERE. }

36. Smart Thief and Broker public static void main(String args[]) { BoundedQueueWithGuard queue = new BoundedQueueWithGuard(5); new Thief(queue, 15).start(); // produce 15 items. new Broker(queue, 15).start(); // consume 15 times } Output: No lost items!