Threads Eivind J. Nordby University of Karlstad Inst. for Information Technology Dept. of Computer Science

Computer Science, University of Karlstad2 Thread Definition: l A thread is a single sequential flow of control within a program.

Computer Science, University of Karlstad3 Introduction A thread l is an instantiation of a code running in a separate time space l is a convenient means for synchronization of asynchronous tasks l is running in the parent process’ address space

Computer Science, University of Karlstad4 Introduction A thread is not l a process, it does not have an address space l an OS task, it is part of its parent task

Computer Science, University of Karlstad5 Multiple Threads can run in a Single Program

Computer Science, University of Karlstad6 The Basic Idea, Example Create some need get information continue Produce something deliver continue transfer wait ProducerConsumer

Computer Science, University of Karlstad7 Threads run public class SimpleThread extends Thread { public SimpleThread(String name) { super(name); } public void run() { for (int i = 0; i < 10; i++) { System.out.println(i + " " + getName()); try { sleep((int)(Math.random() * 1000)); } catch (InterruptedException e) {} } System.out.println("DONE! " + getName()); } Called when the thread is start() -ed

Computer Science, University of Karlstad8 Threads run in Parallel public class TwoThreadsTest { public static void main (String[] args) { new SimpleThread("Jamaica").start(); new SimpleThread("Fiji").start(); }

Computer Science, University of Karlstad9 A Possible Result 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

Computer Science, University of Karlstad10 The Life Cycle of a Thread

Computer Science, University of Karlstad11 Threads

Computer Science, University of Karlstad12 Implementing the Runnable Interface public class Clock extends Applet implements Runnable { private Thread clockThread = null; public void start() { if (clockThread == null) { clockThread = new Thread(this, "Clock"); clockThread.start(); } Applet.start() Thread.start()

Computer Science, University of Karlstad13 Runnables

Computer Science, University of Karlstad14 Implementing the Runnable Interface public void run() { Thread myThread = Thread.currentThread() while (myThread == clockThread) { repaint(); try { Thread.sleep(1000); } catch (InterruptedException e){} // the VM doesn't want us to sleep // anymore, so get back to work } // My creator has nulled out clockThread // to signal that I should stop working } // run

Computer Science, University of Karlstad15 Thread synchronization: example Public class ThreadApplet extends SequentialApplet { public void start() { new Thread(hello).start(); new Thread(goodbye).start(); }

Computer Science, University of Karlstad16 hello and goodbye are two Runnable objects display hello or goodbye in a common text area txt l use the Java awt function appendText for the display l code page 9 –run() {txt.appendText(msg);}

Computer Science, University of Karlstad17 hello and goodbye

Computer Science, University of Karlstad18 Interaction diagrams: Description l Like the GoF patterns, interaction diagrams describe the flow of control l Unlike the GoF patterns the diagrams show discrete steps per thread –no synchronization is implied between the threads

Computer Science, University of Karlstad19 Interaction diagram: Example goodbye applethello start start/run appendText return

Computer Science, University of Karlstad20 Java synchronization primitives l to avoid interference when more than one concurrent thread can execute a function on a shared object the keyword synchronized locks out other synchronized threads from that object

Computer Science, University of Karlstad21 Synchronized wrapper l assume appendText was not already synchronized –could produce l HeGoodlbye lo l GHoeoldlbo ye –or other funny output instead of l Hello Goodbye (or the other way round)

Computer Science, University of Karlstad22 Synchronized wrapper class Appender { // page 17 private TextArea textArea; Appender(TextArea t) { textArea = t; } synchronized void append(String s) { textArea.appendText(s); }

Computer Science, University of Karlstad23 What is synchronized? l Always an object –the object executing a synchronized method –an explicitly mentioned object Object syncher = new Object; void f1() {… synchronized(syncher) {…} … } void f2() {… synchronized(syncher) {…} … } only one block is executed at a time –a synchroinzed method synchronized on the current object ( l synchronized(this) {…}

Computer Science, University of Karlstad24 Giving up a synch wait suspends the current thread and releases synchronization l another waiting thread synchronized on that object can proceed the other thread can resume the waiting thread –normally using notifyAll(); l one of the waiting threads starts when the lock is freed