CISC 4700 L01 Network & Client-Server Programming Spring Cowell Chapter 15: Writing Threaded Applications

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1 CISC 4700 L01 Network & Client-Server Programming Spring 2016 Cowell Chapter 15: Writing Threaded Applications

2 Idea: application does simultaneous activities. Example: web browsers download text and graphics simultaneously. Such appliations are multi-threaded. Thread: "lightweight" process. Launching multiple threads: cheaper than launching multiple processes.

3 Pre-emptive scheduling Process states: To create multi-threaded applications, either: (1) Create a class that extends the Thread class. (2) Create a class that implements the Runnable interface. Must do this if class is already an extension of another class!

4 Subclassing Thread and Overriding run Thread class: subclass of Runnable class Override its run method, e.g.: 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());

5 SimpleThread ctor: takes String as its argument. Calls its superclass ctor. Sets the name of the SimpleThread, obtainable via getname() method. run()method: does the real work of the SimpleThread

6 TwoThreadsDemo: race between two SimpleThreads... public class TwoThreadsDemo { public static void main (String[] args){ new SimpleThread("Jamaica").start(); new SimpleThread("Fiji").start(); Compile: $ javac TwoThreadsDemo.java

7 Sample run yields intermingled output: $ java TwoThreadsDemo 0 Jamaica 0 Fiji 1 Fiji 1 Jamaica 2 Jamaica 2 Fiji 3 Fiji 3 Jamaica 4 Jamaica 4 Fiji 5 Jamaica 5 Fiji 6 Fiji 6 Jamaica 7 Jamaica 7 Fiji 8 Fiji 9 Fiji 8 Jamaica DONE! Fiji 9 Jamaica DONE! Jamaica

8 start():launches thread, which puts it in a runnable state, and then launches thread's run()method. run(): Says what thread does once started. If you want the thread to do something, you must define a useful run() method.

9 The Runnable Interface Example: Program to calculate prime numbers. About once per second, a thread runs that prints out elapsed time. Steps required: Create the class: public class TimeThreadAndPrime implements Runnable {... Create ctor for the class that creates a Thread object and calls its start() method. The start()method calls the run() method: public TimeThreadAndPrime() { Thread FirstThread = new Thread(this); FirstThread.start();

10 Create the run() method: This is executed when the thread is started, i.e., when start()is called: public void run() { int elapsedtime = 0; while (true) { System.out.println("Time taken: " + elaspedtime + " seconds"); elapsedtime++; try { Thread.sleep(1000); // one second catch (InterruptedException e) { System.out.println( "Error: terminating application"); System.exit(1);

11 Write the Java code for calculating and printing primes in the main() method: public static void main(string args[]) { new TimeThreadAndPrime(); // calculate all primes up to 1,000,000 System.out.println(2); // special case for (long sofar = 3; sofar < ; sofar += 2) { boolean prime = true; for (long c = 3; c < sofar/2; c+= 2) if (sofar %c == 0) { prime = false; break; if (prime) System.out.println(soFar);

Partial output: 2 3 5 7... 1949 Time taken: 0 seconds.

12 Partial output: Time taken: 0 seconds Time taken: 1 seconds Time taken: 2 seconds...

13 More on the Thread Class Methods include: l static Thread currentthread() l void destroy() l String getname() l int getpriority() l boolean isalive() l void wait() l void wait(long m) l void wait(long m, int n) l void join() l void join(long) l void join(long m, int n) l void setname(string) l void setpriority(int) l static void sleep(long) l static void sleep(long m, int n) l void notify() l void notifyall()

14 Priorities: If not specified, it's Thread.NORM_PRIORITY. Thread.MIN_PRIORITY: minimal priority Thread.MAX_PRIORITY: maximal priority Note that thread priorities work the opposite of Unix "nice" value, i.e., larger is better. Default thread name: Thread-n, where n is an int.

15 Synchronizing Threads Problem: avoiding race conditions Solution: Java allows programmer to set a lock (A/K/A "monitor") on a resource. Both object and class locks are available. A lock is specified by synchronized keyword. Lock a chunk of code: synchronized (foo) {... Lock a method: public synchronized int bar() {... Only one thread at a time can have access to synchronized code.

16 Example: The Producer/Consumer Problem Producer: generates an integer between 0 and 9 (inclusive), stores it in a CubbyHole object, and prints the generated number. To make the synchronization problem more interesting, the Producer sleeps for a random amount of time between 0 and 100 milliseconds before repeating the number-generating cycle:

17 public class Producer extends Thread { private CubbyHole cubbyhole; private int number; public Producer(CubbyHole c, int number) { cubbyhole = c; this.number = number; public void run() { for (int i = 0; i < 10; i++) { cubbyhole.put(i); System.out.println("Producer #" + this.number + " put: " + i); try { sleep((int)(math.random() * 100)); catch (InterruptedException e) {

18 Naive implementation of the CubbyHoleclass: public class CubbyHole { private int contents; public int get() { return contents; public void put(int value) { contents = value;

19 Consumer: consumes integers from the CubbyHole as quickly as they become available: public class Consumer extends Thread { private CubbyHole cubbyhole; private int number; public Consumer(CubbyHole c, int number) { cubbyhole = c; this.number = number; public void run() { int value = 0; for (int i = 0; i < 10; i++) { value = cubbyhole.get(); System.out.println("Consumer #" + this.number + " got: " + value);

20 To test the Producer/CubbyHole/Consumer classes: public class ProducerConsumerTest { public static void main(string[] args) { CubbyHole cub = new CubbyHole(); Producer producer = new Producer(cub, 1); Consumer consumer = new Consumer(cub, 1); producer.start(); consumer.start();

21 We want behavior such as Producer #1 put: 1 Consumer #1 got: 1 Producer #1 put: 2 Consumer #1 got: 2 Producer #1 put: 3 Consumer #1 got: 3 What if Producer is faster than Consumer? Consumer #1 got: 3 Producer #1 put: 4 Producer #1 put: 5 Consumer #1 got: 5 What if Consumer is faster than Producer? Producer #1 put: 4 Consumer #1 got: 4 Consumer #1 got: 4 Producer #1 put: 5 Race condition!! Threads must be synchronized somehow.

22 Locking an Object Critical sections of threads: access common resource. Lock via synchronized keyword. Critical sections of Producer/Consumer example: put() and get() methods of CubbyHole. Code skeleton for the CubbyHole class: public class CubbyHole { private int contents; private boolean available = false; public synchronized int get() {... public synchronized void put(int value) {...

23 When Producer calls CubbyHole's put() method, it locks the CubbyHole, preventing the Consumer from calling the CubbyHole's get() method: public synchronized void put(int value) { // CubbyHole locked by the Producer... // CubbyHole unlocked by the Producer When the put() method returns, the Producer unlocks the CubbyHole.

24 Similarly, when Consumer calls CubbyHole's get() method, it locks the CubbyHole, preventing the Producer from calling put(): public synchronized int get() { // CubbyHole locked by the Consumer... // CubbyHole unlocked by the Consumer Lock acquisition/release: atomic No race condition in Thread implementation. Synchronization isn't the whole story. The two threads must also be able to notify one another when they've done their job.

25 Reaquiring a Lock Java locks are reentrant: a thread can reenter, i.e., enter a dynamically nested critical region guarded by the same lock. Reentrant thread: only acquirable by one thread at at time. Concurrent threads that attempt to get the same lock are queued. When the lock is released, it is granted to the thread standing first in line, etc.

26 Example: The class public class Reentrant { public synchronized void a() { b(); System.out.println("here I am, in a()"); public synchronized void b() { System.out.println("here I am, in b()"); contains two synchronized methods: a() and b(). The first synchronized method, a(), calls the other synchronized method, b(). Control enters method a(): l Current thread acquires lock for Reentrant object. l Now, a() calls b(); because b() is also synchronized, thread attempts to acquire the same lock again. Because Java supports reentrant locks, this works. Output: here I am, in b() here I am, in a()

27 Using the notifyall() and wait() Methods CubbyHole private member variables: int contents: holds CubbyHole's value boolean available: true when value has been put but not yet gotten false when the value has been gotten but not yet put Naive implementation of put() and get() methods: public synchronized int get() { if (available) { available = false; return contents; public synchronized void put(int value) { if (!available) { available = true; contents = value;

28 Problem: If Producerhasn't put anything in the CubbyHole and available isn't true, get() does nothing. If the Producer calls put() before the Consumer got the value, put() doesn't do anything. Consumer must wait until Producer puts something into CubbyHole; Producermust notify Consumer that this has happened. Producer must wait until Consumer takes something from CubbyHole; Consumermust notify Producer that this has happened.

29 Need to use Object s notifyall() method: public synchronized int get() { while (!available) { try { // wait for Producer to put value wait(); catch (InterruptedException e) { available = false; // notify Producer: value has been retrieved notifyall(); return contents;

30 public synchronized void put(int value) { while (available) { try { // wait for Consumer to get value wait(); catch (InterruptedException e) { contents = value; available = true; // notify Consumer that value has been set notifyall();

Definition: A thread is a single sequential flow of control within a program.

Definition: A thread is a single sequential flow of control within a program. What Is a Thread? All programmers are familiar with writing sequential programs. You've probably written a program that displays "Hello World!" or sorts a list of names or computes a list of prime numbers.