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Gots ta move dat data...... and not trash your threads...

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Presentation on theme: "Gots ta move dat data...... and not trash your threads..."— Presentation transcript:

1 Gots ta move dat data and not trash your threads...

2 Administrivia No noose is good noose...

3 A peek at history Last time: Finished up the WOTL exercise Multi-processing and security: race conditions Today: Stupid Eclipse tricks Communicating with the Swing thread Timer threads

4 Client-side communications Issue: Data enters client through network listener thread GUI being processed on Swing/AWT event queue Need to transfer data between them Need to handle synchronization...

5 Classic MVC design Network Listener thread Model (GridWorld2d + stuff) GUI data structures (JLabel, etc.) data (server update) set(x,y,data) notify(x,y) get(x,y)

6 Classic MVC design Network Listener thread Model (GridWorld2d + stuff) GUI data structures (JLabel, etc.) data (server update) set(x,y,data) notify(x,y) get(x,y) SWING/AWT event proc thread GUI event (click, etc.) Danger Will Robinson! Multiple access! Data corruption! Here be Monsters!

7 What you want Network Listener thread Model (GridWorld2d + stuff) GUI data structures (JLabel, etc.) data (server update) set(x,y,data) notify(x,y) get(x,y) SWING/AWT event proc thread A miracle occurs GUI event (click, etc.) Deferred ! WTF?

8 How to get there... Need some way for listener thread to: Store the incoming data temporarily Notify the event thread: “Hey! There’s some new data! Come deal with it!” Requires: Synchronized access to temp data store Rapid turnaround in listener thread

9 How to get there... Clever, clever SWING designers thought of this... javax.swing.SwingUtilities.invokeLater() Takes a Runnable Event thread executes Runnable.run() “when it’s convenient” After rest of outstanding AWT events have cleared

10 How to get there... Clever, clever SWING designers thought of this... javax.swing.SwingUtilities.invokeLater() Immediately returns control to calling thread (network listener) Executes Runnable.run() once Does not create a new thread

11 Network listener code public void listenToNetwork(Socket s) { while (!toStop) { Message data= // read from network synchronizedBuffer.add(data); SwingUtilities.invokeLater(new _msgHandler()); }

12 Network listener code public void listenToNetwork(Socket s) { while (!toStop) { Message data= // read from network synchronizedBuffer.add(data); SwingUtilities.invokeLater(new _msgHandler()); } private static class _msgHandler implements Runnable { public void run() { Message m=synchronizedBuffer.remove(); while (m!=null) { m.execute(model); m=synchronizedBuffer.remove(); } } } }

13 Alternately... public void listenToNetwork(Socket s) { while (!toStop) { Message data= // read from network SwingUtilities.invokeLater(new _msgHandler(data)); } private static class _msgHandler implements Runnable { public _msgHandler(Message m) { _data=m; } public void run() { m.execute(model); } private final Message _data; }

14 A final note The example chat client you have does not do this May be a bug Or the author may know something I don’t This is my best understanding from SWING docs Caveat emptor!

15 Timer threads

16 Single thread solution public void doYourThing(long period) { assert period>0; while (!_toStop) { long last=System.currentTimeMillis(); // do inner loop stuff long delta=System.currentTimeMillis()-last; try { Thread.sleep(period-delta); } catch (InterruptedException ex) { // do something with this }

17 Analysis of single-thread Tricky to handle “roll-over” What if “inner loop stuff” takes more than one period?

18 Analysis of single-thread System.currentTimeMillis() has some overhead Requires a trap to kernel space Small beans if period is long

19 Analysis of single-thread Hard to align w/ other threads Multiple timers running simultaneously Interface to GUI etc...

20 Multi-thread approaches In general, want a multi-thread model “timer thread”: only job is to keep time beat “worker threads”: respond to timer, do something at each beat Can have many worker threads for one timer Possibilities: java.util.Timer javax.swing.Timer roll your own...

21 Timers behind the scenes Need 2 things: Time base: way to delay for p time units Communication: way to make some other thread do something

22 Timers behind the scenes Time base: Thread.sleep(p) Object.wait(p) Essentially: Go to sleep now; wake up in p ms Both subject to InterruptedException Have to run in try/catch block

23 Timers behind the scenes wait requires that you “own the object’s monitor” Technically, you wait on a specific object Have to synchronize w.r.t. that object before you can call wait() sleep() does this implicitly for you

24 Thread communication Timer thread can now while (!_toStop) { try { this.wait(period); } catch (InterruptedException ex) {} } But what does it do next? How does it tell other threads to do something?

25 Thread communication Opposite of wait() is notify()/notifyAll() Picks one/all threads that are waiting on an object Breaks them out of any wait() they’re in They go back into normal thread schedule

26 Thread communication Like wait(), call notify() on a specific object Again, have to “own monitor for object” before can call notify() on it Essentially the “observable/observer” design pattern for multi-threaded apps

27 Thread communication public Object sharedObject=new Object(); // in “worker” thread while (!done) { synchronized (sharedObject) { try { sharedObject.wait(); // indefinite wait } catch (InterruptedException ex) { // do something } // do main stuff of loop }

28 Thread communication // in “timer” thread while (!_toStop) { synchronized (this) { try { this.wait(period); } catch (InterruptedException ex) { // do something with exception } synchronized (sharedObject) { sharedObject.notifyAll(); }


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