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How to Compile Aspects with Real-Time Java

How to Compile Aspects with Real-Time Java. Pengcheng Wu Northeastern University Mar. 14, 2005 FOAL Workshop with AOSD’05. Motivations. Real-time systems tend to have a lot of cross-cutting concerns, e.g., thread scheduling, memory management

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How to Compile Aspects with Real-Time Java

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  1. How to Compile Aspects with Real-Time Java Pengcheng Wu Northeastern University Mar. 14, 2005 FOAL Workshop with AOSD’05

  2. Motivations • Real-time systems tend to have a lot of cross-cutting concerns, e.g., thread scheduling, memory management • AOP is expected to be helpful to modularize those concerns • Real-Time Java Specification (RTSJ) -- an emerging technology to enhance Java with real time features • Current AspectJ compilation approaches do not work well with Real-Time Java Specification’s special memory model.

  3. Background – Real-Time Java Specification (RTSJ) • Java is unsuitable to implement real-time systems • unpredictable garbage collection • unpredictable thread scheduling • RTSJ has been proposed to minimize those unpredictability • Official reference implementation by TimeSys Corp. http://www.timesys.com • Open source implementations also available • We are particularly interested in RTSJ’s special memory management model

  4. Background – RTSJ (cont.) • RTSJ’s Scoped-memory based memory model • Scoped memory areas provide guarantee on object allocation time

  5. Background – RTSJ (cont.) Objects are just like regular Java heap objects Objectsnever reclaimed Threads can enter scopes; objects in a scope are freed altogether when all threads left scope

  6. Background – RTSJ (cont.) T1 × A × × × B × C ×

  7. Background – RTSJ (cont.) To avoid dangling pointers, an object reference rule is checked by RTSJ-compliant JVMs. Programmers are responsible for following the rule. ILLEGAL! A OK × B C

  8. Question: How about using AspectJ with RTSJ? • How about aspect instantiation? It is implicit and beyond control of programmers. • Instance-based aspect instantiation (perthis, pertarget) × • CFLOW-based aspect instantiation (percflow) × • CFLOW pointcuts with variable bindings × • Singleton aspect and reflective access to thisJoinPoint √

  9. Problems of compiling Aspects with RTSJ Program – An Example class Detector implements Runnable { public void run() { Frame frame = receiveFrame(); //gets a frame and stores it // into a table //check if any two planes are //going to collide } } class App extends RealtimeThread { public static void main(String[] args) { MemoryArea mem = ImmortalMemory.instance(); App app = (App) mem.newInstance(App.class); app.start(); } public void run() { ScopedMemory m1 = new LTMemory( ... ); m1.enter(new Runner()); } } class Runner implements Runnable { public void run() { Detector cd = new Detector( ... ); LTMemory m2 = new LTMemory( ... ); while(true) m2.enter(cd); } } cd m1 m2 cd.run()

  10. Problems of compiling Aspects with RTSJ Program – An Example(cont.) • Now we want to aspectize it so that it only does periodical polling. ILLEGAL! aspect PeriodicPoll perthis(p()) { Time lastTimePolled; pointcut p(): execution(* Detector.run(..)); around(): p() { if(hasn’t yet been 2 seconds) getCurrentThread().yield(); //don't do polling else { //update the time lastTimePolled = System.getCurrentTime(); proceed(); //do polling } } } cd m1 PeriodicPoll m2 cd.run() • Aspect instantiation is implicit, programmers cannot avoid the problem.

  11. Problems of compiling Aspects with RTSJ Program – cflow based aspect instantiation Global Stack • Similar problems exist for cflow pointcuts with variable bindings • Singleton aspect instantiation and reflective access to thisJoinPoint should be fine. ILLEGAL! aspect instance for level 2 aspect instance for level 1

  12. Proposed compilation approaches for Aspects + RTSJ • Instance-based Aspect Instantiation • Make instantiation explicit? • Allocate aspect instances in ImmortalMemory or HeapMemory? • Allocate aspect instances in the same scoped memories as the host objects. √ • CFLOW-based aspect instantiation • Make use of the Portal object of a scoped memory area.

  13. Proposed compilation approaches for Aspects + RTSJ (cont.) • CFLOW-based aspect instantiation (cont. ) A portal × portal B portal C • CFLOW-based aspect instantiation looking up – climbing up the memory chain and looking it up in each of the stacks.

  14. Proposed compilation approaches for Aspects + RTSJ (cont.) • CFLOW-based Aspect Instantiation (cont.) • Problem: portal object may be used by user program for threads communication purpose in RTSJ. • Solution: do automatic adaptation during compile time. An aspect may do the job! • CFLOW pointcuts with variable bindings • Similar approach as cflow-based aspect instantiation

  15. Future work • Implementation of an actual compiler • Formal proof that there will be no aspect introduced object reference violations using RTSJ semantics and probably enhanced AspectJ semantics.

  16. Thank you! Questions and Discussions

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