Hotspot&hotswap,whoandwhoarebestfreinds(點擊這里查看ppt)

java動態替換的ppt。

hotspot&hotswapwhoandwhoarebestfriends
xuanxi@taobao.com

whyweneedhotswap？
主管:你在偷懶？
匯編開發人員：我在編譯。
C++開發人員:我在編譯打包部署。
java開發人員：我在打包部署。
Jsp開發人員：我在部署。
Python開發人員:………
Ruby開發人員:………

whyweneedhotswap?
線上緊急修改。(方法體的修改、增加和減少方法,字段，類，接口，更改類繼承拓撲結構)

JVM1.2以后支持了hotswap功能

JVMTMToolInterface

typedefstruct { jclassklass; jintclass_byte_count; const unsigned char* class_bytes; } jvmtiClassDefinition; jvmtiError RedefineClasses(jvmtiEnv* env, jintclass_count, const jvmtiClassDefinition* class_definitions)

JVM1.2以后支持了hotswap功能

JavaTMVirtualMachineDebugInterfaceReference

typedefstruct { jclassclazz; /* Class to be redefined */ jintclass_byte_count; /* number of bytes defining class (below) */ jbyte *class_bytes; /* bytes defining class */ /* (in Class File Format of JVM spec) */ } JVMDI_class_definition;jvmdiError RedefineClasses(jintclassCount, JVMDI_class_definition *classDefs)

Javadev除打包部署外還干嘛？
用IDE(eclipse)調試

Soundsgood,what’stheproblem?
Unfortunately,thisredefinitionislimitedonlytochangingmethodbodies—itcannoteitheraddmethodsorfieldsorotherwisechangeanythingelse,exceptforthemethodbodies.

So,Herewego!
Discardingclassloaders
java.lang.instrument
HackingchangesintotheJVMitselffeelsafragileapproach

Discardingclassloaders

Discardingclassloaderscanhelp,but
Notparticularlygranular
Instancestateislost
Canrunintomemoryproblems
Demo(discarding.classloaders.hotswap)byken.wug@gmail.com

java.lang.instrument
aJRubyobjectisnotmuchmorethanaMapfrommethodnamestotheirimplementationsandfromfieldnamestotheirvalues.
byte[]transform(ClassLoaderloader,
StringclassName,
Class<?>classBeingRedefined,
ProtectionDomainprotectionDomain,
byte[]classfileBuffer)
throwsIllegalClassFormatException

java.lang.instrument
java.lang.instrument.ClassFileTransformer.Thisclasscandotwothings:
replacemethodbodiesofexistingclasses(butnothingelse,itisessentiallythesameashotswap).
modifyclassesbeforetheygetloadedintothejavavirtualmachine
classClass1
{protectedstaticintfield;
}

java.lang.instrument

classClass1{protectedstaticintfield;publicstaticObject__REDEFINED_STATIC_METHOD_(intmethodNo,Object[]parameters){if(methodNo==0){returnfield;}if(methodNo==1){Integerp0=(Integer)parameters[0];intunboxedP0=p0.intValue();field=unboxedP0;returnnull;}returnnull;}}

java.lang.instrument
publicclassProxy1{publicvoidsetField(intparam){Object[]params=newObject[1];params[0]=newInteger(param);Class1.__REDEFINED_STATIC_METHOD_(1,params);}}

Soundswell,but…
Perfomance.subjecttoindirection&Pemmemory
JavaSDKclasses.Thenativecode
Compatibility.A.class.getMethods()

fakereplace
GoogleCodeproject
Usualstructurechangessupported
Methods,fields,constructors
Annotations
Reflectioninstrumentation
Integratessupportdirectlyforsomeframeworks

Fakereplacedemo

Jrebel（notfree）
onemasterclass+severalanonymous
Solveproblem
directionandlightweight
AvoidsinstrumentingtheJava
TweakstheresultsoftheReflectionAPI,sothatwecancorrectlyincludetheadded/removedmembersintheseresults

JrebelDemo

Soundsperfect

國家隊（nationteam）
Hotswap.patch

hotspotandhotswap
Arbitrarychangespossible(includingchangestosubtyperelationships)
Noperformancepenaltybeforeorafterthechange
Nointroducedindirections
Continuedexecutionofoldactivemethods
Onlysimple,comprehensiblestrategiesforfieldmatchingormethodtransitions
ChangepossibleatanypointwhentheVMissuspended
Gracefulhandlingofaccessestodeletedfieldsorcallsofdeletedmethods

HotswapStatus

TransformerMethods

class A { int x; intdoubleX; static void $staticTransformer() //class transformer { System.out.println(“Class A has a new version”); } void $transformer() { doubleX = 2 * x; } //instance’s }

Hotswap.patchdemo

• Hotswap.patchAlgorithm

  LoadingtheNewClasses

• UpdatingtheDataStructuresandPointers

• LoadingtheNewClasses
  step1:Findallaffectedclasses.
  (VM_RedefineClasses::FindAffectedKlassesClosure)
  RedefiningDandB
  Findallaffectedclasses

• LoadingtheNewClasses
  step2:Sorttheclassestopologically.
  (VM_RedefineClasses::TopologicalClassSorting)
  Topologicalorder.

• UpdatingtheDataStructuresandPointers
  step1:Flushdependentcode.
  Currentlyallcompiledmethodsaredeoptimized
  (Classes::flush_dependent_code)

• UpdatingtheDataStructuresandPointers
  step2:Updateconstantpoolcacheentries.
  (VM_RedefineClasses::adjust_cpool_cache)

• UpdatingtheDataStructuresandPointers
  step3:Swappointers.
  利用gc的特性,gc復制一個對象的后會調整指向該對象的pointer
  (MarkSweep::adjust_pointer)

• UpdatingtheDataStructuresandPointers
  step4:Updateinstancefields.
  allnewfieldsareinitializedtozero.（新建）
  instanceKlass::do_fields_evolution(標記)
  MarkSweep::update_fields(拷貝)

• Fantastic,but…

• Hotswap@taobao.com

  HSF_JETTYplugin自動集成hotswap.patch。

  • 你也可以自己打patch。

• Questiontime

• Partytime謝謝!xuanxi@taobao.com

轉載于:https://blog.51cto.com/aliapp/1327693

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