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Java ME - 04 - Timer, Tasks and Threads

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Most mobile applications will have to use asynchronous processing — for running the game loop or for connecting your MIDlet to the Internet. This module is a repetition of the basic thread concepts that you will need for your mobile applications. It also contains details of how to use the Timer to generate regular call-backs. In the first challenge, you will write a small alarm clock based on the high level user interface. The second challenge is about drawing a ball that bounces around on the screen using the low level graphics API.

Contents:

* Timer
* Scheduling
* Threads
* Runnable vs. Thread

Veröffentlicht in: Technologie
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Java ME - 04 - Timer, Tasks and Threads

  1. 1. Java™ Platform, Micro Edition<br />Part 4 – Timer, Tasks and Threads<br />v3.0 – 02 April 2009<br />1<br />Andreas Jakl, 2009<br />
  2. 2. Disclaimer<br />These slides are provided free of charge at http://www.symbianresources.com and are used during Java ME courses at the University of Applied Sciences in Hagenberg, Austria at the Mobile Computing department ( http://www.fh-ooe.at/mc )<br />Respecting the copyright laws, you are allowed to use them:<br />for your own, personal, non-commercial use<br />in the academic environment<br />In all other cases (e.g. for commercial training), please contact andreas.jakl@fh-hagenberg.at<br />The correctness of the contents of these materials cannot be guaranteed. Andreas Jakl is not liable for incorrect information or damage that may arise from using the materials.<br />This document contains copyright materials which are proprietary to Sun or various mobile device manufacturers, including Nokia, SonyEricsson and Motorola. Sun, Sun Microsystems, the Sun Logo and the Java™ Platform, Micro Edition are trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and other countries. <br />Andreas Jakl, 2009<br />2<br />
  3. 3. Contents<br />Timer<br />Threads<br />Andreas Jakl, 2009<br />3<br />
  4. 4. Timer<br />Execute tasks at a specified time<br />Andreas Jakl, 2009<br />4<br />
  5. 5. Timer<br />Asynchronous execution of a task <br />after a specified amount of time (nonrecurring) or …<br />in certain intervals (periodic)<br />Two classes are involved:<br />Andreas Jakl, 2009<br />5<br />Timer<br />Scheduling when a task will occur<br />TimerTask<br />Performing a task<br />
  6. 6. Nonrecurring Events<br />Andreas Jakl, 2009<br />6<br />Start timer<br />void schedule(TimerTask task, long delay)<br />… 1500 ms …<br />TimerTask.run()<br />void schedule(TimerTask task, Date time)<br />16.11.2007, 10:27:31<br />TimerTask.run()<br />Time<br />
  7. 7. Applications<br />After x milliseconds<br />Delayed start of an asynchronous processe.g. MIDletstartup: give VM time to draw the splash screen before loading and initializing the game<br />Countdown<br />At a specified point in time<br />Alarm clock<br />Andreas Jakl, 2009<br />7<br />
  8. 8. Periodic Tasks<br />Andreas Jakl, 2009<br />8<br />voidscheduleAtFixedRate([…], longperiod)<br />300 ms<br />300 ms<br />300 ms<br />300 ms<br />300 ms<br />delay<br />Time<br />delay<br />task<br />task<br />task<br />task<br />task<br />t2<br />Thread<br />350 ms<br />300 ms<br />300 ms<br />300 ms<br />voidschedule([…], longperiod)<br />delay<br />delay<br />300 ms<br />300 ms<br />300 ms<br />300 ms<br />Time<br />task<br />task<br />task<br />task<br />task<br />t2<br />Thread<br />300 ms<br />350 ms<br />250 ms<br />300 ms<br />
  9. 9. Fixed Delay<br />If consistency is more important than accuracy<br />Execution is delayed (e.g. by garbage collection):<br />Only one execution is delayed<br />After this, the constant interval is kept up again<br />Suitable for:<br />Animations: shouldn’t suddenly move faster<br />Unsuitable for:<br />Clock: Inaccuracies would accumulate<br />Andreas Jakl, 2009<br />9<br />voidschedule([…], longperiod)<br />delay<br />delay<br />300 ms<br />300 ms<br />300 ms<br />300 ms<br />Time<br />task<br />task<br />task<br />task<br />task<br />t2<br />Thread<br />
  10. 10. Fixed Rate<br />When accuracy is more important<br />Execution is delayed (e.g. by garbage collection):<br />Two or more subsequent executions are scheduled at shorter intervals to “catch up”<br />None of the events will be dropped<br />(Un)suitable:<br />Inverse to fixed delay<br />Andreas Jakl, 2009<br />10<br />voidscheduleAtFixedRate([…], longperiod)<br />300 ms<br />300 ms<br />300 ms<br />300 ms<br />300 ms<br />delay<br />Time<br />delay<br />task<br />task<br />task<br />task<br />task<br />t2<br />Thread<br />
  11. 11. Combination<br />Andreas Jakl, 2009<br />11<br />When to start first task?<br />Execute 1st task after x ms<br />Execute 1st task at specified time<br />1: One Time<br />2: Fixed Delay<br />3: Fixed Rate<br />4: One Time<br />5: Fixed Delay<br />6: Fixed Rate<br />1: schedule(TimerTask task, long delay)<br />2: schedule(TimerTask task, long delay, long period)<br />3: scheduleAtFixedRate(TimerTask task, long delay, long period)<br />4: schedule(TimerTask task, Date time)<br />5: schedule(TimerTask task, Date firstTime, long period)<br />6: scheduleAtFixedRate(TimerTask task, Date firstTime, long period)<br />
  12. 12. TimerTask<br />Create your own class derived from TimerTask<br />Pass an instance of it to the Timer-object<br />Calls run() of your TimerTask-class at the specified time<br /> implement the abstract run()-method!<br />Andreas Jakl, 2009<br />12<br />
  13. 13. Example<br />Andreas Jakl, 2009<br />13<br />// Allocate a timer<br />Timertm = newTimer();<br />// Schedule thetimerwithappropriateschedulingoption, eg. in 1000 milliseconds<br />tm.schedule(newTodoTask(), 1000);<br />[…]<br />private classTodoTaskextendsTimerTask<br />{<br />public final voidrun()<br /> {<br /> // Do something …<br /> }<br />}<br />
  14. 14. Threads<br />Control an asynchronoustask<br />Andreas Jakl, 2009<br />14<br />
  15. 15. Why Threads?<br />Examples:<br />Game loop, has to be executed all the time, as often as possible<br />To make animations as smooth as possible.<br />A fixed delay timer would result in a fixed amount of frames per second and not utilize better hardware<br />Animated progress bar while loading or processing<br />Connect through HTTP / Sockets<br />Connection process can be stopped by JVM to display a security warning<br />Without threads: app. couldn’t process key press anymore, would lock<br />Andreas Jakl, 2009<br />15<br />
  16. 16. Possibilities<br />Andreas Jakl, 2009<br />16<br />1. Derive your own object from Thread,implement run()<br />2. Extra (2nd) object implements the Runnable-interface, start through Thread-object.<br />Thread<br />Thread<br />Interface<br />Runnable<br />run()<br />Interface<br />Runnable<br />run()<br />MyThread t = new MyThread();t.start()<br />DoSomething doIt = new DoSomething();Thread t = new Thread( doIt );t.start();<br />
  17. 17. Example – Thread-Object<br />Andreas Jakl, 2009<br />17<br />DoAnotherThingdoIt = new DoAnotherThing();<br />doIt.start();<br />// …<br />public class DoAnotherThingextends Thread {<br />publicvoid run() {<br />// Here is where you do something<br /> // Executed within its own thread!<br /> }<br />}<br />
  18. 18. Example – Runnable-Object<br />Andreas Jakl, 2009<br />18<br />DoSomethingdoIt = newDoSomething();Thread t = new Thread( doIt );t.start();<br />publicclassDoSomethingimplementsRunnable{ private booleanquit = false;<br />publicvoidrun() {while( !quit ) {// do something, e.g. processthe<br /> // nextframe in a game } } publicvoidquit() {quit = true; }}<br />
  19. 19. Differences: Runnable / Thread?<br />Almost identical.<br />Advantages of the Runnable-variant (interface):<br />Can be implemented by existing class (no multiple inheritance in Java (extend Thread)!)<br />Therefore, saves an additional class<br />Andreas Jakl, 2009<br />19<br />
  20. 20. Structure<br />run()-method can be used for single action<br />Once run() method is left, thread stops and is cleaned up<br />Commonly, run()repeats an action until some condition is satisfied (e.g., in a game loop)<br />Implement infinite loop in run()<br />To exit the loop and stop the thread:<br />Repeatedly query boolean status variable<br />If set to true through any function  break loop and therefore leave run()-method, thread is stopped<br />Andreas Jakl, 2009<br />20<br />
  21. 21. Additional Possibilities<br />For more complex threading tasks, use:<br />Synchronisation, Wait, … <br />More information:<br />http://developers.sun.com/techtopics/mobility/midp/articles/threading2/<br />Andreas Jakl, 2009<br />21<br />
  22. 22. … let’s move on to the challenges!<br />Interactive<br />Andreas Jakl, 2009<br />22<br />

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