Java threading Multithreaded applets Peter Mozelius DSVUCSC 1

Java threading Multithreaded applets Peter Mozelius DSV/UCSC 1

Robustness q q q An applet should not crash and hang the browser A program should handle its errors Built in error handling in Java q q q try catch finally 2

Catching exceptions q try – catch - finally try { Do something useful that sometimes can go wrong and throw an Exception; } catch(Exception e) { Here will the exeption be caught; } finally { Will always be executed; } 3

Threading o o o A technique for dividing the execution of an applet in parallel tasks As when several instruments play in parallel in an orchestra With synchronized interaction and pauses between the threads 4

Threading o Parallel code execution o Multitasking n o To run several processes in parallel Multithreading n To run several threads in parallel 5

Java threads q q Since the very first ver 1. 0 in Java To save system resources q q Instead of starting a new process Start a new thread Thread = lightweight process 6

Why threading? q q For practical and economical reasons Example: A web browser could fetch several images in parallell An applet could show an animations in separate threads, at the same time as the main thread will handle the GUI 7

How to make threads sleep v So other threads should be able to execute their tasks the thread must sometime sleep for a while v We will now look at how you should make threads rest Do we need to rest as well? 8

How threads sleep v v Threads will be put in bed by sleep() As an argument this method will need the number of milliseconds to sleep Example: sleep(1000); v Makes the thread sleep exactly 1 second 9

Exception handling v To avoid thread trouble: try{ sleep(millisec); }catch(Interrupted. Exception ie){ System. out. print(”Thread error: ”); System. out. print(ie. get. Message()); } } 10

Starting and stopping threads v If you create a thread like: Thread t = new Thread(); v You’ll later start it by: t. start(); 11

Starting and stopping threads v In the class Thread there is also a method named stop() v BUT do NOT stop the thread by stop() v This method is deprecated because it leaves the thread in an unclear condition 12

suspend() och resume() v Two other ways of stopping and restarting threads are by: suspend() och resume() v BUT these methods are deprecated as well because they create deadlocks And they are NOT recommended v 13

How the threads work v Threads execute their tasks in run() v If the iteration in a run-method is trigged by a boolean condition in a loop v The thread will always be able to finish it’s work in a secure manner 14

A stop condition in run() Create a boolean condition like: private boolean stopped; public void run(){ while(!stopped) { move. The. Ball(); } } 15

Thread and Runnable v The two ways of creating threads in Java: 1) By subclassing Thread One. Class extends Thread 2) By implementing Runnable Another. Class implements Runnable 16

Runnable v The fact that Runnable is an interface can be very useful when a class inherits something else like in: My. Class extends JApplet implements Runnable We will now look at some code but first: 15 min PAUS! 17

Let’s have a break! 18

Threading with Thread class Test. Thread extends Thread { private String name; private int sleep. Time; public Test. Thread(String name, int sleep. Time) { this. name = name; this. sleep. Time = sleep. Time; } 19

Threading with Thread public void run() { System. out. println("Hello, from : " + name); try { sleep(sleep. Time); }catch (Interrupted. Exception ie) { System. err. println(“ERROR in " + name); System. err. println(ie. get. Message()); } System. out. println("Hello again, from: " + name); }//run }//Test. Thread 20

Threading with Threads can then be created from an application: (or an applet) public class L 4_example 1 { public static void main(String[] args) { Test. Thread t 1 = new Test. Thread(“T 1", 2000); Test. Thread b 2 = new Test. Thread(“T 2", 20); t 1. start(); t 2. start(); } }//L 4_example 1 21

Threading with Runnable class My. Thread implements Runnable { private String thread. Name; private int sleep. Time; private Thread t; public My. Thread(String name, int time) { thread. Name = name; sleep. Time = time; t = new Thread(this); } 22

Threading with Runnable public void run() { System. out. println("Hello, from: " + name); int i = 0; while (i++ < 2) { try { t. sleep(sleep. Time); } catch (Interrupted. Exception ie) { System. err. println(“ERROR in " + name); System. err. println(ie. get. Message()); } 23

Threading with Runnable System. out. println("Hello again, from: " + thread. Name); }//while }//run public void start. Me() { t. start(); } }//My. Thread 24

Threading with Runnable public class L 4_example 2 { public static void main(String[] args) { My. Thread t 1 = new My. Thread("T 1", 2000); My. Thread t 2 = new My. Thread("T 2", 20); t 1. start. Me(); t 2. start. Me(); } }//L 4_example 2 25

Graphics in threaded classes v If two diffrent thread classes want to write to the same graphical component: 26

Graphics in threaded classes A reference to the graphical component can be passed to the constructor in the thread class : class Pen. Thread extends Thread { private JPanel center. Panel ; public Pen. Thread(JPanel center. Panel) { this. center. Panel = center. Panel; . . . 27

Graphics in threaded classes Then you can write on the panel by: public void write() { Graphics pen = center. Panel. get. Graphics(); pen. draw. String("Hello, from: " + thread. Name, x, y); pen. dispose(); } 28

Graphics in threaded classes o The complete code for L 4_example 3 can be found in the Moodle system o An example that could be a good start for solving Assignment 7 o So let’s run this applet! 29