Session 4 Stateful Aspects in JAsCo

1
Session 4Stateful Aspects in JAsCo

• Bart Verheecke

2
Aspect-Oriented Programming
Logging in Apache Tomcat

• BAD modularity
• handled by code that is scattered over almost all
  classes

Picture taken from the aspectj.org website
3
Aspect-Oriented Programming
Logging in Apache Tomcat
Logging Aspect
4
AOSD Terminology

• joinpoint point in the programs execution
• e.g. Executing method setX on type Y
• pointcut A set of joinpoints
• e.g. The execution of all methods which name
  start with set on type Y
• advice What to do at a certain pointcut
• e.g. Print a logging string before the program
  continues.
• weaving applying the advice to all joinpoints
  defined in the pointcut declaration

5
Stateful Aspects

• Natural Evolution of joinpoints
• Static
• Dynamic (e.g. cflow)
• Stateful

6
Examples (1)

• Auto-save after five actions

7
Examples (1) AspectJ solution
aspect Autosave private int
actions_since_last_save 0 after()
call( Application.save()) call(
Application.autosave()) actions_since_last_sa
ve0 after() call(??Command.execute()
) actions_since_last_save
if(actions_since_last_savegt4) Application.autosav
e()
8
Examples (2) Safe iterator
9
Examples (3) Timing Aspect
10
Timing Aspect JAsCo Solution

• 1 class DynamicTimer
• 3 private VectorltTimeListenergt listeners new
  VectorltTimeListenergt()
• 4 private long timestampbefore,
  timestampafter
• 6 void addTimeListener(TimeListener aListener)
  
• 7 listeners.add(aListener)
• 8
• 9 void removeTimeListener(TimeListener
  aListener)
• 10 listeners.remove(aListener)
• 11
• 12 void notifyTimeListeners(Method method, long
  time)
• 13 for (TimeListener listener listeners)
• 14 listener.timeStampTaken(method,time)
• 15
• 16
  
• 18 hook TimeStamp
• 19 TimeStamp(timedmethod(..args))
• 20 execution(timedmethod)
• 21
• 22 before

11
Timing Aspect JAsCo solution

• Timing one method
• But how to time a protocol? E.g. time between
  execution of a-b-c on ComponentX.

1 connector TimeConnector 2
DynamicTimer.TimeStamp timer 3 new
DynamicTimer.TimeStamp (void ComponentX.a()) 4
timer.before() 5 timer.after() 6
12
Protocol Timing Aspect?
1 class ProtocolDynamicTimer extends
DynamicTimer 2 3 boolean methodaexecuted,
methodbexecuted false 4
5 hook ProtocolMethodA 7
ProtocolMethodA(methodA(..args)) 8
execution(methodA) 9 10 before 11
timestampbefore System.currentTimeMillis()
12 methodaexecuted true 13 14
isApplicable() return !methodaexecuted 15
16 17 hook ProtocolMethodB 18
ProtocolMethodB(methodB(..args)) 19
execution(methodB) 20 21 before 22
methodbexecuted true 23 24
isApplicable() return methodaexecuted 25
26 27 hook ProtocolMethodC 28
ProtocolMethodC(methodC(..args)) 29
execution (methodC) 30 31 after 32
timestampafter System.currentTimeMillis() 3
3 notifyListeners(method,timestampafter-time
stampbefore) 34 methodaexecuted false
methodbexecuted false 35 36
isApplicable() return methodbexecuted 37
39
connector TimeConnector
DynamicTimer.ProtocolMethodA timera
new DynamicTimer. ProtocolMethodA (void
ComponentX.a()) DynamicTimer.ProtocolMethod
B timerb new DynamicTimer.
ProtocolMethodB (void ComponentX.b())
DynamicTimer.ProtocolMethodC timerc
new DynamicTimer. ProtocolMethodC (void
ComponentX.c())
13
Stateful Aspects Motivation

• Protocol based aspects are rarely supported in
  current AOSD approaches (apart cflow).
• Result protocol checking code is scattered over
  advices, poluting advice code
• Is also not optimal, checks at all possible
  joinpoints while only a subset is required.

14
Stateful Aspects

• Allows specifying regular (DFA based) hook
  triggering conditions in a hooks constructor.
• Advices can be attached to any transition of the
  DFA, or all of them (global).
• Example Realize a more general protocol history
  condition than cflow.

15
Timing Example Revisited

• 1 class ProtocolDynamicTimer extends
  DynamicTimer
• 2
• 3 hook StatefulProtocolTimer
• 4
• 5 long timestamp
• 6
• 7 StatefulProtocolTimer(methodA(..args),metho
  dB(..args),methodC(..args))
• 8 ATrans execution(methodA) gt BTrans
• 9 BTrans execution(methodB) gt CTrans
• 10 CTrans execution(methodC) gt ATrans
• 11
• 12
• 13 before ATrans()
• 14 timestampSystem.currentTimeMillis()
• 15
• 16 after CTrans()
• 17 long resultingtime System.currentTimeMi
  llis()
• 18 notifyListeners(calledmethod,resultingtim
  e-timestamp)
• 19

16
Timing Example Revisited

• static connector TimingConnector
• perthread ProtocolDynamicTimer.StatefulProtoc
  olTimer timer
• new ProtocolDynamicTimer.StatefulProtocolTim
  er(
• void ComponentX.a().,
• void ComponentX.b(),
• void ComponentX.c()
• )

17
Advanced Pointcut Features

• Explicit start transitions
• start gt XTrans
• Starting with two transitions
• start gt XTrans QTrans
• two destination transitions
• XTrans execution(methodA) gt YTrans QTrans
• no destination transition
• QTrans execution(methodB) !cflow(methodC)

18
Advices

• Multiple specific advices possible, one for each
  transition per before/around /after
• Global advice works for all transitions
• isApplicable can also be global or specific

isApplicable() //global condition for all
transitions // returns true if advices should
be executed isApplicable XTrans() //local
condition only relevant for the transition
XTrans // returns true if advices should be
executed for the XTrans transition
19
Strict Protocols

• Per default, stateful aspects are non-strict,
  e.g. protocol a()-d()-b()-c() also matches timing
  aspect.
• When no intermediate transitions are allowed, use
  the strict keyword

strict ATrans execution(methodA) gt BTrans
BTrans execution(methodB) gt CTrans CTrans
execution(methodC) gt ATrans
20
Strict Protocols with Context

• Only strict with respect to a certain set of
  joinpoints. E.g. timing aspect does not accept
  intermediate transitions on ComponentX, on other
  classes is ok though.

StatefulProtocolTimer(methodA(..args),methodB(..ar
gs),methodC(..args), context(..args))
strictexecution(context) ATrans
execution(methodA) gt BTrans BTrans
execution(methodB) gt CTrans CTrans
execution(methodC) gt ATrans
Rejects ComponentX.a() ComponentX.d() Compone
ntX.b() ComponentX.c()
static connector TimingConnector . new
ProtocolDynamicTimer.StatefulProtocolTimer( voi
d ComponentX.a()., void ComponentX.b(),
void ComponentX.c(), void ComponentX.()
)
Accepts ComponentX.a() Y.d() ComponentX.b()
ComponentX.c()
21
Complement of a protocol

• Trigger advices on everything outside the
  protocol COMPLEMENT

StatefulProtocolTimer(methodA(..args),methodB(..ar
gs),methodC(..args)) complement ATrans
execution(methodA) gt BTrans
BTrans execution(methodB) gt CTrans
CTrans execution(methodC) gt ATrans around
complement() throw new SecurityException(this
is not allowed on
thisJoinPoint.getClass())
22
Complement with Context

• Only complement with respect to a certain set of
  joinpoints.

StatefulProtocolTimer(methodA(..args),methodB(..ar
gs),methodC(..args), context(..args))
complementexecution(context) ATrans
execution(methodA) gt BTrans BTrans
execution(methodB) gt CTrans CTrans
execution(methodC) gt ATrans
d() in complement ComponentX.a() ComponentX.d(
) ComponentX.b() ComponentX.c()
static connector TimingConnector . new
ProtocolDynamicTimer.StatefulProtocolTimer( voi
d ComponentX.a()., void ComponentX.b(),
void ComponentX.c(), void ComponentX.()
)
No complement ComponentX.a() Y.d() ComponentX
.b() ComponentX.c()
23
Weaving

• A DFA (Deterministic Finite Automaton) executes
  the stateful aspect at run-time.
• Default weaved at all possible joinpoints
  defined in the pointcut.
• Optionally, jumping aspect in cooperation with
  run-time weaver
• only weaved at next joinpoints in protocol
• overhead of reweaving vs overhead of aspect
  applied at all