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Threads II

Threads II. IS 313 5.15.2003. Outline. Quiz Thread review Stopping a thread java.util.Timer Swing threads javax.swing.Timer ProgressMonitor invokeLater. Thread Review I. Threads Java objects Independent paths of execution Share memory and code To define a thread’s behavior run().

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Threads II

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  1. Threads II IS 313 5.15.2003

  2. Outline • Quiz • Thread review • Stopping a thread • java.util.Timer • Swing threads • javax.swing.Timer • ProgressMonitor • invokeLater

  3. Thread Review I • Threads • Java objects • Independent paths of execution • Share memory and code • To define a thread’s behavior • run()

  4. Threads Review II • Threads only appear to run simultaneously • only a single thread executes at a time • each thread runs for a time and then is replaced by another • Priority • determines which available thread is allowed to run

  5. Thread Review III • Java event handling • takes place in a single thread • other system threads • Threads may have resource conflict • share the processor with sleep() and yield() • achieve exclusive use through synchronized methods • coordinate using wait() and notify()

  6. Thread Review IV

  7. Thread Review V • Lifecycle methods • start = transition to runnable • end of run method = transition to dead • transition to non-runnable • wait () • sleep () • blocked • transition back to runnable • notify () • end of sleep • resource unlocked

  8. How to stop a thread • run starts a thread • end of run method terminates • what if I want to stop thread sooner? • Answer • it depends

  9. Why not stop() • Existing method in thread API • Answer • Dead thread drops its locks • synchronized method may be only partially executed • corrupt state

  10. Example public void run () { while (true) { ... do something ... } }

  11. Alternative I (exit variable) private boolean m_isRunning; public synchronized void setIsRunning (boolean newVal) { m_isRunning = newVal; } public synchronized boolean IsRunning () { return m_isRunning; } public void run () { setIsRunning(true); while (isRunning()) { ... do something ... } }

  12. To Stop the Thread thread.setRunning (false)

  13. Requirements • Inner loop • Exit variable checked regularly

  14. Alternative II (interrupt) public void run () { try { while (true) { foo.wait(); ... do something ... } } catch (InterruptedException e) { .. clean up ... } }

  15. To Stop the Thread thread.interrupt ();

  16. Requirements • Thread is in “wait” state in its inner loop • Also works for “sleep”

  17. Alternative III (i/o) public void run () { try { while (true) { byte [] buffer = inputStream.readBytes(4000); ... do something ... } } catch (IOException e) { .. clean up ... } }

  18. To Stop the Thread inputStream.close();

  19. Requirement • Thread is waiting for I/O in its inner loop

  20. Three ways to stop • Rapid inner loop • use a loop exit variable • Wait state • call interrupt() • use the interrupted exception • Waiting on I/O • close the I/O stream • use the IOException

  21. Periodic Action • Examples • Check the mail server every 10 minutes • Animate something on the screen • Autosave • Need • a thread that sleeps for a specified period • then runs • possibly repeats

  22. java.util.Timer • Timer • schedule (TimerTask, delay) • schedule (TimerTask, delay, period) • TimerTask • implements Runnable

  23. java.util.Timer Lifecycle • Timer created • Task scheduled • wait setup in Timer thread • Time arrives • TimerTask run method called • (not a separate thread) • Timer canceled • timer.cancel()

  24. javax.swing.Timer • Timer • Timer (delay, ActionListener) • ActionListener • actionPerformed (ActionEvent)

  25. javax.swing.Timer Lifecycle • ActionListener created • Timer created • Timer started • timer.start() • wait setup (in Timer thread) • Time arrives • actionEvent created and inserted in event queue • ActionListener handles event • Timer canceled • timer.stop

  26. Progress Monitoring

  27. Example • Add progress monitor to a process • What needs to happen? • ProgressMonitor dialog must open • Monitor must be updated • Cancel/Completion must be handled

  28. ProgressMonitor class • ProgressMonitor(Component parentComponent, Object message, String note, int min, int max) • setProgress • tell the dialog to change progress indication

  29. Example

  30. Note • Updating must happen in EH thread • Swing timer ensures this • If updating from another thread • must place updates into the EH thread

  31. Interaction between threads and UI • Swing components are not thread-safe • methods not synchronized • example • Solution • only modify components from with the EH thread • after the component has been “realized”

  32. Accessing the EH Thread • EventQueue.invokeLater (Runnable) • What happens • event inserted into event queue • when it comes to top • run method of object is called • Also • invokeAndWait(Runnable) • not as useful

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