Transcript Slides, powerpoint

How to Compile Aspects
with Real-Time Java
Pengcheng Wu
Northeastern University
Mar. 14, 2005
FOAL Workshop with AOSD’05
Motivations
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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.
Background – Real-Time Java
Specification (RTSJ)
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Java is unsuitable to implement real-time systems
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unpredictable garbage collection
unpredictable thread scheduling
RTSJ has been proposed to minimize those
unpredictability
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Official reference implementation by TimeSys Corp.
http://www.timesys.com
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Open source implementations also available
We are particularly interested in RTSJ’s special
memory management model
Background – RTSJ (cont.)
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RTSJ’s Scoped-memory based memory
model
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Scoped memory areas provide guarantee on
object allocation time
Background – RTSJ (cont.)
Objects never
reclaimed
Threads can enter
scopes; objects in a
scope are freed
altogether when all
threads left scope
Objects are just like
regular Java heap
objects
Background – RTSJ (cont.)
T1
×
×
×
A
×
×
B
C
×
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
Question: How about using AspectJ with
RTSJ?
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How about aspect instantiation? It is implicit and
beyond control of programmers.
Instance-based aspect instantiation (perthis,
pertarget) ×
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CFLOW-based aspect instantiation (percflow) ×
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CFLOW pointcuts with variable bindings ×
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Singleton aspect and reflective access to
thisJoinPoint
√
Problems of compiling Aspects with RTSJ
Program – An Example
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);
}
}
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
}
}
cd
cd.run()
m2
m1
Problems of compiling Aspects with RTSJ
Program – An Example(cont.)
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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
PeriodicPoll
m2
m1
cd.run()
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Aspect instantiation
is implicit,
programmers cannot
avoid the problem.
Problems of compiling Aspects with RTSJ
Program – cflow based aspect
instantiation
 Similar problems exist for cflow pointcuts
Global
Stack
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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
Proposed compilation approaches for
Aspects + RTSJ
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Instance-based Aspect Instantiation
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Make instantiation explicit?
Allocate aspect instances in ImmortalMemory or
HeapMemory?
Allocate aspect instances in the same scoped
memories as the host objects. √
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CFLOW-based aspect instantiation
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Make use of the Portal object of a scoped memory
area.
Proposed compilation approaches for
Aspects + RTSJ (cont.)
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CFLOW-based aspect instantiation (cont. )
A
portal
portal
B
×
portal
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C
CFLOW-based aspect instantiation looking up –
climbing up the memory chain and looking it up
in each of the stacks.
Proposed compilation approaches for
Aspects + RTSJ (cont.)
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CFLOW-based Aspect Instantiation (cont.)
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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
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Similar approach as cflow-based aspect
instantiation
Future work
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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.
Thank you!
Questions and Discussions