How to Compile Aspects with RealTime Java

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How to Compile Aspects with Real-Time Java

• Pengcheng Wu
• Northeastern University
• Mar. 14, 2005
• FOAL Workshop with AOSD05

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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 Specifications
  special memory model.

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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 RTSJs special
  memory management model

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Background RTSJ (cont.)

• RTSJs Scoped-memory based memory model
• Scoped memory areas provide guarantee on object
  allocation time

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Background RTSJ (cont.)
Objects are just like regular Java heap objects
Objects never reclaimed
Threads can enter scopes objects in a scope are
freed altogether when all threads left scope
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Background RTSJ (cont.)
T1

A




B

C

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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
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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 v

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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()
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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(hasnt
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.

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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
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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. v
• CFLOW-based aspect instantiation
• Make use of the Portal object of a scoped memory
  area.

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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.

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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

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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.

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• Thank you!
• Questions and Discussions