Region Pointcut for AspectJ Shumpei Akai Shigeru Chiba Muga Nishizawa

Background: Synchronization Granularity of synchronization alters performance – Fine-grained: Increase concurrency – Coarse-grained: Less overhead Which granularity is better on a given machine?

Motivating example: Javassist A bug report of synchronization for Javassist [ – Adding synchronization to fix the bug. – coarse-grained is chosen in this case. Separate synchronization code as an aspect – enable choosing appropriate granularity. public class ProxyFactory { public Class createClass() { if (thisClass == null) { ClassLoader cl = getClassLoader(); synchronized (proxyCache) { if (useCache){createClass2(cl);} else {createClass3(cl);} }} return thisClass;} private void createClass2(ClassLoader cl) { CacheKey key = new CacheKey(…); synchronized (proxyCache) { HashMap cacheForTheLoader =…; if (cacheForTheLoader == null) { cacheForTheLoader = new HashMap(); proxyCache.put(cl, cacheForTheLoader); cacheForTheLoader.put(key, key); }else {...}} synchronized (key) { Class c = isValidEntry(key); if (c == null) { createClass3(cl); key.proxyClass = new WeakReference(…); }else{thisClass = c;} }}} Coarse-grained candidate Fine-grained

Separating synchronization with AOP New pointcuts are needed – AspectJ provides execution, call, get, set pointcuts They pick out Join “points”. – But synchronization is applied to a “range” of code. Join points are not suitable Need to pick out regions (or pieces of code)

Region pointcut Regions are treated like join points – Use pattern matching – Static analysis and code transformation Regions are determined by static analysis. Extension to AspectJ

region[pc1;pc2;pc3;…] A new pointcut for a region – Specify a sequence of join points – Find a matching path public void foo() { … obj.criticalSectionBegin();//from here if(…){ a(); } … obj.criticalSectionEnd();//to here …} void around(): region[ call(* *. criticalSectionBegin()); call(* *.a()); call(* *. criticalSectionEnd()); ] { synchronized(someObject){ proceed(); }

New pointcut: all() all(pc1, pc2, pc3, …) – Matches a sequence including the join points all the pointcuts pc1, pc2, pc3, … match in some order. – The order of pointcuts does not matter. because the order of join points may be changed by refactoring

Example of all() region[ all( call(* *.a()), call(* *.b()), call(* *.c()) ) ] { a(); … b(); … c(); } { b(); … c(); … a(); } { c(); … b(); c(); … a(); } matches

A region may conflict with a control structure If a region intersects a control structure – we cannot weave an around advice public void foo() {... for(int i=0;i<100;i++){ bar(); beginCriticalSection(); // begin... } endCriticalSection(); // end... } void around(): region[ call(* *. criticalSectionBegin()); call(* *. criticalSectionEnd()) ]{ /* do something*/ }

Region adjustment Solves the conflict – Expand a selected region: to fit a control structure to include the first and the last selected join points – A minimum region satisfying the criteria above is selected public void foo() {... for(int i=0;i<100;i++){ bar(); beginCriticalSection(); // begin... } endCriticalSection(); // end... }

Context passing Synchronization needs a lock object – Contexts must be available in an advice body args(), target() are available in a pattern – Specified values must be accessible at the beginning of the region

Example of context passing … List list =... ;... for(int index = 0; i < 100; i++){ beginCriticalSection();... Object o=list.get(index);... endCriticalSection(); }... pointcut pc(List l,int i) : region[ call(* *.beginCriticalSection()); call(* List+.get(int)) && target(l) && args(i) ; call(* *.endCriticalSection()); ];

Implementation We implemented by extending abc – abc: The AspectBench Compiler for AspectJ Need information, where blocks, control structures and statements start/end – Add new instruction into Jimple

public void toBeAdvised(int x){ a(); b(x); c(); } original method Around advice In abc, some join points (e.g. execution) are split into static methods to weave an around advice – Local variables are passed through arguments – We adopt this mechanism This causes a few problems for the region pointcut public void toBeAdvised(int x){ advice(x); } public static void advice(int x){ beforeJoinPoint(); shadow(x); afterJoinPoint(); } public static shadow(int x){ a(); b(x); c(); } woven method advice original join point

Support an around advice : assignment (problem) Assignment does not change the value of the local variable – Local variables are replicated. public void toBeAdvised(int x){ String s="initial string"; a(); s="string was replaced"; b(); System.out.println(s); // what is the value of s? } void around(): region[call(* *.a());call(* *.b())] { proceed(); }

Support an around advice : assignment (solution) Share an object between the region and the original method – Each field contains a value of a local variable public void toBeAdvised(int x){ String s="initial string"; $localStorage = new LocalStorage(); $localStorage.s=s; s=$localStorage.s; a(); s="string was replaced"; b(); $localStorage.s=s; s=$localStorage.s; System.out.println(s); }

Support an around advice : jumps (problem) Jumps (break or continue) to outside of region are unavailable public void includeJump(){ labelOfFor: for(;;){ a(); if(isFinished()){ break; //goto label0; } b(); } //label0: } void around(): region[call(* *.a());call(* *.b())] { proceed(); }

Support an around advice : jumps (solution) Transform the bytecode – Share the ID for each jump target – Jump to the tail of the region – After region, check the ID and jump public void includeJump(){ labelOfFor: for(;;){ a(); if(isFinished()){ $i=0; goto endLabel; } b(); endLabel: switch($i){ case 0: goto label0; } label0: }

Applying to Javassist (coarse-grained) Fixed Javassist’s synchronization problem using region pointcut static WeakHashMap proxyCache; public Class createClass() { if (thisClass == null) { ClassLoader cl = …; if (useCache) createClass2(cl); else createClass3(cl); } return thisClass;} void around(): region[ get(static boolean *.useCache); call(* *.createClass2(..)); ] { synchronized(ProxyFactory.class) { proceed(); } }

Applying to javassist (fine-grained) void around(): region[ call(* WeakHashMap.get(..)); call(* WeakHashMap.put(..)); ]{ synchronized( ProxyFactory.proxyCache){ proceed(); }} static WeakHashMap proxyCache; private void createClass2(ClassLoader cl) { CacheKey key =…; HashMap cacheForTheLoader = (HashMap)proxyCache.get(cl); if (cacheForTheLoader == null) { proxyCache.put(...); }else{...}

Related work Tracematch [Allan et al. ’05] – History-based pointcut – Dynamic pattern matching with execution history LoopsAJ [Harbulot et al. ’06] – Picking out loops

Conclusion Region pointcut: – Picks out regions as join points – We extended abc Solved some design issues to weave an around advice Work in progress – Formalize the pattern matching semantics using hedge automata – Case study