Hotspot&hotswap,whoandwhoarebestfreinds(点击这里查看ppt)

java动态替换的ppt。

hotspot&hotswapwhoandwhoarebestfriends
xuanxi@taobao.com

whyweneedhotswap？
主管:你在偷懒？
汇编开发人员：我在编译。
C++开发人员:我在编译打包部署。
java开发人员：我在打包部署。
Jsp开发人员：我在部署。
Python开发人员:………
Ruby开发人员:………

whyweneedhotswap?
线上紧急修改。(方法体的修改、增加和减少方法,字段，类，接口，更改类继承拓扑结构)

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除打包部署外还干嘛？
用IDE(eclipse)调试

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

国家队（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复制一个对象的后会调整指向该对象的pointer
    (MarkSweep::adjust_pointer)

  • UpdatingtheDataStructuresandPointers
    step4:Updateinstancefields.
    allnewfieldsareinitializedtozero.（新建）
    instanceKlass::do_fields_evolution(标记)
    MarkSweep::update_fields(拷贝)

  • Fantastic,but…

  • Hotswap@taobao.com

    HSF_JETTYplugin自动集成hotswap.patch。

    • 你也可以自己打patch。

  • Questiontime

  • Partytime谢谢!xuanxi@taobao.com