Internet Software Development

Internet Software Development Concurrent Programming Paul J Krause

Contents • What are threads? • The risks of threads • Threads in Java • Two multithreaded programs • Final comments

Threads • It is the use of multiple threads that makes concurrent programming difficult • Multiple processes executing concurrently is not a problem • Why do I say this? • What is the difference between a thread and a process? • Pause for classroom discussion …

Threads vs. Processes • A process is an independent flow of control • There is no shared memory space among different processes • Processes communicate between each other by specific communication channels (not shared variables) • e.g. Pipes in Unix, e.g. • ls | more

Threads vs. Processes • A thread is a single flow of control within a program • It performs one of the tasks that a program needs to perform to achieve its goals • If a program needs to perform several tasks, these could be handled in different threads (and hence performed concurrently) • These threads will interact and cooperate through a shared memory space

The Risks of Threads • Different threads can interact via shared variables and objects • The execution of each thread may proceed independently of the others • In general, the relative ordering of execution of the different threads is non-deterministic • This can lead to safety and liveness problems

A:Thread Data: B:Thread A:Thread Data: B:Thread set get get set a) Normal b) Abnormal due to delay in thread A Race Conditions

A:Thread O: P: B:Thread lock lock Example of Deadlock waiting to lock O waiting to lock P

Livelock Picture from Lethbridge and Laganière, 2001

<<interface>> Runnable Thread MyThread MyThread run( ) run( ) Threads in Java

Creation of Threads • java.lang.Thread • run( ) - a “hook” method, that must be overridden • start( ) - invoke this to start a new thread of control

Creation of Threads II public class MyThread extends Thread { // fields … public void run() { // body of the thread … } // other methods … } new MyThread().start(); // Don’t invoke run() directly! (why not?)

Simple Counter public class Counter extends Thread { // From Jia, 2002 protected int count; protected int inc; protected int delay; public void run() { for (;;) { System.out.print(count + " "); count += inc; sleep(delay); } }

main method for Counter public static void main(String[] args) { new Counter(0, 1, 33).start(); new Counter(0, -1, 100).start(); }

How many threads? • The thread that executes the main method terminates after creating the following: • The thread that executes Counter with positive increment • The thread that executes Counter with a negative increment these last two execute concurrently for ever!

Implementing Runnable public interface Runnable { public abstract void run(); } public class MyThread extends SomeClass implements Runnable { public void run() { // thread body here } // other methods … }

Starting a thread • A little more complicated. • Notice MyThread has no start() method • So, we first create an instance of MyThread • Then, use this to create an instance of Thread, and invoke itsstart() method: new Thread(new MyThread()).start();

Counter2 public class Counter2 implements Runnable { // From Jia, 2002 protected int count; protected int inc; protected int delay; public void run() { for (;;) { System.out.print(count + " "); count += inc; Thread.sleep(delay); } }

Counter2 main method public static void main(String[] args) { new Thread(new Counter2(0, 2, 33)).start(); new Thread(new Counter(0, -2, 100)).start(); }

And Finally • We have introduced threads as flows of control within a program • Multiple threads may interact through shared memory space • This is both Good News and Bad News • We have seen some simple examples of controlling threads (start, sleep) • Next week we will see how to cope with the Bad News!

Internet Software Development