Date post: | 22-Dec-2015 |
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Threads
Thread = independent flow of control
e.g. a server needs to communicate with manycustomers => each customer is served by aseparate thread
Threads – toy example
increments x
increments y
calls repaint MyApplet
Threads – toy exampleimport java.applet.Applet;import java.awt.*;
public class FirstApplet extends Applet { public int x,y;
public void init() {
....
}
public void paint(Graphics g) { g.setColor(Color.white); g.setFont(new Font("SansSerif",Font.BOLD,32)); g.drawString("x="+x,10,40); g.drawString("y="+y,10,80); g.drawString("x-y="+(x-y),10,120); }}
public void init() { setBackground(Color.black);
new Thread(new Runnable () { public void run() { while (true) { x++; } }}).start();
new Thread(new Runnable () { public void run() { while (true) { y++; } }}).start();
new Thread(new Runnable () { public void run() { while (true) { try { Thread.currentThread().sleep(40); } catch (InterruptedException e) {}; repaint(); } }}).start(); }
1. create2. start
Runnable interface
run() - method
Thread(Runnable r); a constructorof the Threadclass
Anonymous classesnew Thread(new Runnable () { public void run() { while (true) { y++; } }}).start(); new ClassName() {
{ ? } ?}
instanceinitializer
local variables
Time slicing
Thread 1Thread 2Thread 3Thread 1Thread 2Thread 3Thread 2Thread 3
time
Threads - priorities new Thread(new Runnable () { public void run() { try { Thread.currentThread().setPriority(Thread.MIN_PRIORITY); } catch (IllegalArgumentException e) {} catch (SecurityException e) {}
while (true) { x++; } }}).start();
MyApplet
Synchronization problems
object
Thread 1 Thread 2
Extremely difficult to debug...
Synchronization problems –toy example
x,dx
Thread 1 Thread 2
while (true) { x+=1000; dx=-1; for (i=0;i<1000;i++) x+=dx;}
while (true) { x-=1000; dx=+1; for (i=0;i<1000;i++) x+=dx;}
MyApplet
public void init() { new Thread(new Runnable () { public void run() { while (true) { x+=1000; dx=-1; for (i=0;i<1000;i++) x+=dx; } }}).start();
new Thread(new Runnable () { public void run() { while (true) { x-=1000; dx=+1; for (i=0;i<1000;i++) x+=dx; } }}).start();
..... }
Synchronization problems –toy example
synchronized blocks of code
object
Thread 1 Thread 2
Do not touch the object.
synchronized blocks of codepublic void init() { new Thread(new Runnable () { public void run() { while (true) { synchronized(lock) { x+=1000; dx=-1; for (i=0;i<1000;i++) x+=dx; } } }}).start();
new Thread(new Runnable () { public void run() { while (true) { synchronized(lock) { x-=1000; dx=+1; for (i=0;i<1000;i++) x+=dx; } } }}).start();
} MyApplet
synchronized blocks of codepublic void init() { new Thread(new Runnable () { public void run() { synchronized(lock) { while (true) { x+=1000; t1++; dx=-1; for (i=0;i<1000;i++) x+=dx; } } }}).start();
new Thread(new Runnable () { public void run() { synchronized(lock) { while(true) { x-=1000; t2++; dx=+1; for (i=0;i<1000;i++) x+=dx; } } }}).start();
} MyApplet
synchronized methods
synchronized ? method(?) { ?}
? method(?) { synchronized(this) { ? }}
stop()
deprecated – can leave objects it is manipulating in inconsistent state
pleaseStop()define your own
wait() and notify()class Pipe { LinkedList l=new LinkedList(); public synchronized void Push(Object o) { l.add(o); this.notify(); } public synchronized Object Pop() { while (l.size()==0) { try { this.wait(); } catch (InterruptedException e) { } } return l.remove(0); }}
Sleeping threadstry { Thread.currentThread().sleep(40);} catch (InterruptedException e) {};
Exceptions
Exceptions
method() some unexpected situation can occur
throw an exception
try { method();} catch (TypeOfTheException e) { // deal with the exception}
Exceptions
try {
} catch (TypeOfTheException e) {
}
Code dealing with “normal” execution
Code dealing with “exceptions”
Exceptions are objectsclass NoElectricityException extends Exception { NoElectricityException() { super(“No electricity!”); } NoElectricityException(String message) { super(message); }}
getMessage() method
Exceptions are objectsClass Chef { ... Food makeRoastBeef(Kitchen k) { if (!k.lightOn()) { k.turnLightOn(); if (!k.lightOn()) throw new NoElectricityException(); ... } ... }}
Exceptions are objects
Food[] prepareDinner() { try { f=makeRoastBeef(k); } catch (NoElectricityException e) { ? }}
Exceptions
Throwable
Exception Error
RuntimeException
checked unchecked
Exceptions examplesInterruptedException.........RuntimeException ArithmeticException IndexOutOfBoundException ArrayIndexOutOfBoundException NullPointerException ...........
Error OutOfMemoryError ............
ThrowsFood makeRoastBeef(Kitchen k) throws NoElectricityException, NoMeatException,... { ...
}
checked exceptions
try/catch/finallytry {
}catch (TypeOfTheException e){
}finally {
}
try { System.out.println(“1”); if (x==0) throw(new MyException());}catch (MyException e){ System.out.println(“3”);}finally { System.out.println(“2”);}
EXERCISE: what will bethe ouput for a) x=1 b) x=0
void Test(int x) throws YourException { try { System.out.println(“1”); if (x==0) throw(new MyException()); if (x==1) throw(new YourException()); if (x==2) return; System.out.println(“4”); } catch (MyException e){ System.out.println(“3”); } finally { System.out.println(“2”); }}void MyTest(int y) { try { System.out.prinln(“6”); Test(y); System.out.println(“7”); } catch (YourException e) { System.out.println(“5”); }} EXERCISE:
what will bethe ouput for a) MyTest(0); b) MyTest(1); c) MyTest(2); d) MyTest(3);
Applets cont.
Applet parameters<APPLET CODE=“MyApplet.class" WIDTH="200" HEIGHT="200"> <PARAM NAME=“BALLS” VALUE=50> <PARAM NAME=“MINDIAM” VALUE=20><PARAM NAEM=“MAXDIAM” VALUE=50></APPLET>
....balls=Integer.parseInt(getParameter(“BALLS"));....
BouncingBallBouncingBall
Why is it flickering?
“repaint” threadrepaintrequests
update() clears screen
calls paint()
Double buffering
“repaint” threadrepaintrequests
update() calls paint()
clears second-screendraws everything on the second-screencopies second-screen to the screen
BouncingBall revisited Image offscreenImage; Graphics offscreenGraphics;
offscreenImage = createImage(width,height); offscreenGraphics = offscreenImage.getGraphics();
offscreenGraphics.setColor(Color.black); offscreenGraphics.fillRect(0,0,width,height);
g.drawImage(offscreenImage, 0, 0, this);
BouncingBallBouncingBall