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Threads

Threads. Thread = independent flow of control. e.g. a server needs to communicate with many customers => each customer is served by a separate thread. Threads – toy example. increments x. increments y. calls repaint. MyApplet. Threads – toy example. import java.applet.Applet;

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Threads

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  1. Threads

  2. Thread = independent flow of control e.g. a server needs to communicate with many customers => each customer is served by a separate thread

  3. Threads – toy example increments x increments y calls repaint MyApplet

  4. Threads – toy example import 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); } }

  5. 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(); } • create • start

  6. Runnable interface run() - method Thread(Runnable r); a constructor of the Thread class

  7. Anonymous classes new Thread(new Runnable () { public void run() { while (true) { y++; } } }).start(); instance initializer new ClassName() { { ? } ? } local variables

  8. Time slicing time Thread 1 Thread 2 Thread 3 Thread 1 Thread 2 Thread 3 Thread 2 Thread 3

  9. 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

  10. Synchronization problems Thread 2 Thread 1 object Extremely difficult to debug...

  11. Synchronization problems – toy example Thread 2 Thread 1 x,dx MyApplet 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; }

  12. Synchronization problems – toy example 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(); ..... }

  13. synchronized blocks of code Thread 2 Thread 1 object Do not touch the object.

  14. synchronized blocks of code public 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

  15. synchronized blocks of code public 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

  16. synchronized methods synchronized ? method(?) { ? } ? method(?) { synchronized(this) { ? } }

  17. stop() deprecated – can leave objects it is manipulating in inconsistent state pleaseStop() define your own

  18. 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); } }

  19. Sleeping threads try { Thread.currentThread().sleep(40); } catch (InterruptedException e) {};

  20. Exceptions

  21. Exceptions method() some unexpected situation can occur throw an exception try { method(); } catch (TypeOfTheException e) { // deal with the exception }

  22. Exceptions Code dealing with “normal” execution try { } catch (TypeOfTheException e) { } Code dealing with “exceptions”

  23. Exceptions are objects class NoElectricityException extends Exception { NoElectricityException() { super(“No electricity!”); } NoElectricityException(String message) { super(message); } } getMessage() method

  24. Exceptions are objects Class Chef { ... Food makeRoastBeef(Kitchen k) { if (!k.lightOn()) { k.turnLightOn(); if (!k.lightOn()) throw new NoElectricityException(); ... } ... } }

  25. Exceptions are objects Food[] prepareDinner() { try { f=makeRoastBeef(k); } catch (NoElectricityException e) { ? } }

  26. Exceptions Throwable Error Exception RuntimeException checked unchecked

  27. Exceptions examples InterruptedException ......... RuntimeException ArithmeticException IndexOutOfBoundException ArrayIndexOutOfBoundException NullPointerException ........... Error OutOfMemoryError ............

  28. Throws Food makeRoastBeef(Kitchen k) throws NoElectricityException, NoMeatException,... { ... } checked exceptions

  29. try/catch/finally try { } catch (TypeOfTheException e){ } finally { }

  30. EXERCISE: what will be the ouput for a) x=1 b) x=0 try { System.out.println(“1”); if (x==0) throw(new MyException()); } catch (MyException e){ System.out.println(“3”); } finally { System.out.println(“2”); }

  31. what will be the ouput for a) MyTest(0); b) MyTest(1); c) MyTest(2); d) MyTest(3); 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:

  32. Applets cont.

  33. 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")); .... BouncingBall BouncingBall

  34. Why is it flickering? repaint requests “repaint” thread update() clears screen calls paint()

  35. Double buffering repaint requests “repaint” thread calls paint() update() clears second-screen draws everything on the second-screen copies second-screen to the screen

  36. 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); BouncingBall BouncingBall

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