1. Chapter 11: Threaded Programs Situations where the program is following multiple execution paths (how to stop one?) Thread: a line of execution Thread class –usage: extend Thread implement Runnable interface

2. The Need for Threads Consider a simple program performing a repetitive graphics action (bouncing a ball, scrolling a message, etc.) To stop such a program we want to have the event manager catch “stop” signals But the stop signal may not be caught because the processor concentrates on the other process

3. import java.applet.*; import java.awt.*; public class BigMessage extends Applet { private Label prompt = new Label("Message: "); private TextField message = new TextField(20); private Button start = new Button("Start"); private Button stop = new Button("Stop"); private Panel buttonpanel = new Panel(); private Panel messagepanel = new Panel(); private Panel dummypanel = new Panel(); private MessageFrame messageframe = null; public void init() { messagepanel.add(prompt); messagepanel.add(message); buttonpanel.add(start); buttonpanel.add(stop); dummypanel.setLayout(new GridLayout(2,1)); dummypanel.add(messagepanel); dummypanel.add(buttonpanel); add(dummypanel); }

4. public boolean action(Event e, Object arg) { if (e.target == start) { if (messageframe != null) messageframe.dispose(); messageframe = new MessageFrame("A Message", message.getText()); messageframe.flashMessage(); } else if (e.target == stop) { if (messageframe != null) messageframe.dispose(); messageframe = null; } return true; } class MessageFrame extends Frame { private MessageLine display; MessageFrame(String fname, String newm) { super(fname); setForeground(Color.black); setBackground(Color.white); display = new MessageLine(newm); add(display); pack(); show(); }

5. public void flashMessage() { try { display.initMessage(); Thread.sleep(200L); while (true) { display.advanceMessage(); Thread.sleep(200L); } catch (InterruptedException e) { } class MessageLine extends Canvas { private final static int VGAP = 5; private final static int HGAP = 1; private final static int FSIZE = 48; private String themessage; private Font bigFont = new Font("Courier",Font.PLAIN,FSIZE); private int location; private int lwidth;

6. MessageLine(String newm) { int mlength = Math.max(10,2 * newm.length()); int lheight = FSIZE + 2 * VGAP; lwidth = (FSIZE + HGAP) * mlength + HGAP; resize(lwidth,lheight); themessage = newm; } public void draw() { Graphics g = getGraphics(); g.setXORMode(Color.white); g.setFont(bigFont); g.drawString(themessage,location,FSIZE + 1); } public void initMessage() { location = lwidth; draw(); } public void advanceMessage() { draw(); location -= (FSIZE + HGAP); if (location < (- lwidth)) location = lwidth; draw(); }

7. Executing Without Threads The message printing process, once started, “starves out” the event handler Events may be recorded, but the system is too busy to respond Idea: add “threads” to programs A thread is an executing section of code Threads can be started, stopped, made to sleep, suspended, etc.

8. Thread Class Part of java.lang package Thread connected to execution path in program Constructor: Thread() - makes a basic thread Threads can be run –declare class that extends Thread class, override Thread method run() in class –declare class that implements interface Runnable which provides run() method

9. Thread Methods Methods: static void sleep(long ms) throws InterruptedException - causes executing thread to pause approximately ms milliseconds void start() throws IllegalThreadStateException - starts this thread (calls run() at finish) void run() - begin execution of thread (generally not called directly) final void stop() throws SecurityException - stops execution of this thread

10. First Option: Extend Thread Make “hog” class extend Thread class Add run() method to class Use start() to initiate execution Pause during execution (sleep) to allow others access Use stop() to halt thread

11. public class BigMessage extends Applet { public boolean action(Event e, Object arg) { if (e.target == start) { if (messageframe != null) messageframe.dispose(); messageframe = new MessageFrame("A Message", message.getText()); messageframe.start(); // starts thread running } else if (e.target == stop) { if (messageframe != null) { messageframe.stop(); // use stop to halt thread messageframe.dispose(); } messageframe = null; } return true; }

12. class MessageFrame extends Thread { // Thread not Frame private MessageLine display; private Frame theFrame = null; // has a Frame object MessageFrame(String fname, String newm) { theFrame = new Frame(fname); // build Frame aspects theFrame.setForeground(Color.black); theFrame.setBackground(Color.white); display = new MessageLine(newm); theFrame.add(display); theFrame.pack(); theFrame.show(); } public void dispose() { // Add routine to dispose of Frame if (theFrame != null) theFrame.dispose(); } public void run() { // Routine run, override Thread method try { display.initMessage(); sleep(200L); while (true) { display.advanceMessage(); sleep(200L); } catch (InterruptedException e) { }

13. Second Option: Runnable Interf. Have “hog” implement Runnable interface Add thread instance to class At start of problem routine create thread Have run() implement problem routine Call start() to initiate execution Pause during execution (sleep) Use stop() to halt thread

14. public class BigMessage extends Applet { public boolean action(Event e, Object arg) { if (e.target == start) { // Unchanged } else if (e.target == stop) { messageframe.stopMessage(); // Halt execution // rest unchanged } class MessageFrame extends Frame implements Runnable { private MessageLine display; private Thread displaythread = null; // Add control thread public void flashMessage() { displaythread = new Thread(this); // Create thread displaythread.start(); // Call start to run } public void stopMessage() { // Add routine to stop thread if (displaythread != null) displaythread.stop(); displaythread = null; } public void run() { // flash becomes run // unchanged otherwise }

15. Other Java Thread Capabilities Methods for stopping execution when a browser leaves a page (applet) Methods for synchronizing threads (making process(es) wait for others to complete) Methods for grouping threads (and applying operators to the entire group)