java多线程机制

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ThreadsObjective1)Writecodetodefine，instantiateandstartnewthreadsusingbothjava.Iang.Threadandjava.lang.Runnable•Javaisfundamentallymulti-threaded.•Everythreadcorrespondstoaninstanceofjava.Iang.Threadclassorasub-class.•Athreadbecomeseligibletorun，whenitsstart()methodiscalled.Threadschedulerco-ordinatesbetweenthethreadsandallowsthemtorun.•Whenathreadbeginsexecution,theschedulercallsitsrunmethod.Signatureofrunmethod-publicvoidrun()•Whenathreadreturnsfromitsrunmethod(orstopmethodiscalled-deprecatedin1.2),itsdead.Itcannotberestarted，butitsmethodscanbecalled,(it’sjustanobjectnomoreinarunningstate)•Ifstartiscalledagainonadeadthread,IllegalThreadStateExceptionisthrown.•Whenathreadisinrunningstate，itmaymoveoutofthatstateforvariousreasons.Whenitbecomeseligibleforexecutionagain，threadschedulerallowsittorun.•Therearetwowaystoimplementthreads.1.ExtendThreadclasspublicclassThreadextendsObjectimplementsRunnableThread()Thread(Runnabletarget)Thread(Runnabletarge,Stringname)Thread(Stringname)Thread(ThreadGroupgroup，Runnabletarget)Thread(ThreadGroupgroup，Runnabletarget，Stringname)Thread(ThreadGroupgroup，Stringname)•Createanewclass，extendingtheThreadclass.•Provideapublicvoidrunmethod，otherwiseemptyruninThreadclasswillbeexecuted.•Createaninstanceofthenewclass.•Callstartmethodontheinstance(don’tcallrun-itwillbeexecutedonthesamethread)2.ImplementRunnableinterfacepublicinterfaceRunnableonlyonemethod:publicvoidrun•CreateanewclassimplementingtheRunnableinterface.•Provideapublicvoidrunmethod. •Createaninstanceofthisclass. •CreateaThread，passingtheinstanceasatarget-newThread(object)•TargetshouldimplementRunnable，Threadclassimplementsit，soitcanbeatargetitself.•CallthestartmethodontheThread.•JVMcreatesoneuserthreadforrunningaprogram.Thisthreadiscalledmainthread.Themainmethodoftheclassiscalledfromthemainthread.Itdieswhenthemainmethodends.Ifotheruserthreadshavebeenspawnedfromthemainthread,programkeepsrunningevenifmainthreaddies.Basicallyaprogramrunsuntilalltheuserthreads(non-daemonthreads)aredead.•AthreadcanbedesignatedasadaemonthreadbycallingsetDaemon(boolean)method.Thismethodshouldbecalledbeforethethreadisstarted，otherwiseIllegalThreadStateExceptionwillbethrown.•Athreadspawnedbyadaemonthreadisadaemonthread.•Threadshavepriorities.ThreadclasshaveconstantsMAX_PRIORITY(10)，MIN_PRIORITY(1)，NORM_PRIORITY(5)•Anewlycreatedthreadgetsitspriorityfromthecreatingthread.Normallyit’llbeNORM_PRIORITY.•getPriorityandsetPriorityarethemethodstodealwithpriorityofthreads.•JavaleavestheimplementationofthreadschedulingtoJVMdevelopers.Twotypesofschedulingcanbedone.1.Pre-emptiveScheduling.Waysforathreadtoleaverunningstate-•Itcanceasetobereadytoexecute(bycallingablockingi/omethod)•Itcangetpre-emptedbyahigh-prioritythread，whichbecomesreadytoexecute.•Itcanexplicitlycallathread-schedulingmethodsuchaswaitorsuspend.•SolarisJVM’sarepre-emptive.•WindowsJVM’swerepre-emptiveuntilJava1.0.22.Time-slicedorRoundRobinScheduling•Athreadisonlyallowedtoexecuteforacertainamountoftime.Afterthat，ithastocontendfortheCPU(virtualCPU，JVM)timewithotherthreads.•Thispreventsahigh-prioritythreadmono-policingtheCPU.•Thedrawbackwiththisschedulingis-itcreatesanon-deterministicsystem-atanypointintime,youcannottellwhichthreadisrunningandhowlongitmaycontinuetorun.•MactinoshJVM’s•WindowsJVM’safterJava1.0.2OfthetwomethodsofcreatinganewthreadtheuseofRunnableisprobablymorecommon.TheothermethodforcreatingathreadistocreateaclassthatisdescendedfromThread.Thisiseasytodobutitmeansyoucannotinheritfromanyotherclass, Objective2)Recognizeconditionsthatmightpreventathreadfromexecuting.Differentstatesofathread:1.Yielding•Yieldisapublicstaticvoidmethod.Operatesoncurrentthread.Forstaticmethod，callThread.yield()isok,don’tneedcallt.yield()，wheretisinstanceofThread,butcompilerwillpassit.•Movesthethreadfromrunningtoreadystate.•Iftherearenothreadsinreadystate,theyieldedthreadmaycontinueexecution,otherwiseitmayhavetocompetewiththeotherthreadstorun.•Runthethreadsthataredoingtime-consumingoperationswithalowpriorityandcallyieldperiodicallyfromthosethreadstoavoidthosethreadslockinguptheCPU.2.Sleeping•Sleepisalsoapublicstaticvoidmethod.•Sleepsforacertainamountoftime,(passingtimewithoutdoinganythingandw/ousingCPU)•Twooverloadedversions-onewithmilliseconds,onewithmillisecondsandnanoseconds.•ThrowsanInteiTuptedException.(mustbecaught)•Afterthetimeexpires，thesleepingthreadgoestoreadystate.Itmaynotexecuteimmediatelyafterthetimeexpires.Ifthereareotherthreadsinreadystate，itmayhavetocompetewiththosethreadstorun.Thecorrectstatementisthesleepingthreadwouldexecutesometimeafterthespecifiedtimeperiodhaselapsed.•Ifinterruptmethodisinvokedonasleepingthread,thethreadmovestoreadystate.Thenexttimeitbeginsrunning，itexecutestheInterruptedExceptionhandler.3.Suspending•Suspendandresumeareinstancemethodsandaredeprecatedin1.2•Athreadthatreceivesasuspendcall，goestosuspendedstateandstaysthereuntilitreceivesaresumecallonit-•Athreadcansuspendititself，oranotherthreadcansuspendit.•But，athreadcanberesumedonlybyanotherthread.•Callingresumeonathreadthatisnotsuspendedhasnoeffect.•Compilerwon’twarnyouifsuspendandresumearesuccessivestatements，althoughthethreadmaynotbeabletoberestarted.4.Blocking•MethodsthatareperformingI/Ohavetowaitforsomeoccurrenceintheoutsideworldtohappenbeforetheycanproceed.Thisbehaviorisblocking.•IfamethodneedstowaitanindeterminableamountoftimeuntilsomeI/Otakesplace，thenthethreadshouldgraciouslystepoutoftheCPU.AllJavaI/Omethodsbehavethisway.•Athreadcanalsobecomeblocked，ifitfailedtoacquirethelockofamonitor. 1.Waiting•wait，notifyandnotifyAllmethodsarenotcalledonThread，they’recalledonObject.Becausetheobjectistheonewhichcontrolsthethreadsinthiscase.Itasksthethreadstowaitandthennotifieswhenitsstatechanges.It’scalledamonitor.•Waitputsanexecutingthreadintowaitingstate.(tothemonitor’swaitingpool)•Notifymovesonethreadinthemonitor’swaitingpooltoreadystate.Wecannotcontrolwhichthreadisbeingnotified.notifyAllisrecommended.•NotifyAllmovesallthreadsinthemonitor’swaitingpooltoready.•Thesemethodscanonlybecalledfromsynchronizedcode,oranIllegalMonitorStateExceptionwillbethrown.Inotherwords,onlythethreadsthatobtainedtheobject’slockcancallthesemethods.Thesleepmethodisstaticandpausesexecutionforasetnumberofmilliseconds.Thereisaversionthatissupposedtopauseforasetnumberofnanoseconds,HereisanexampleofputtingaThreadtosleep,notehowthesleepmethodthrowsInterruptedException.publicclassTSleepextendsThread{publicstaticvoidmain(Stringargv[]){TSleept=newTSleep();t•start();}publicvoidrun(){try{while(true){this.sleep(1000);System•out•printIn(’’loopingwhile’’)；}}catch(工nterruptedExceptionie){}}}Objective3)WritecodeusingsynchronizedwaitnotifyandnotifyAlltoprotectagainstconcurrentaccessproblemsandtocommunicatebetweenthreads.DefinetheinteractionbetweenthreadsandbetweenthreadsandobjectlockswhenexecutingsynchronizedwaitnotifyornotifyAll.Locks,MonitorsandSynchronization•Everyobjecthasalock(foreverysynchronizedcodeblock).Atanymoment,thislockiscontrolledbyatmostonethread.•Athreadthatwantstoexecuteanobject’ssynchronizedcodemustacquirethelockoftheobject.Ifitcannotacquirethelock,thethreadgoesintoblockedstateandcomestoreadyonlywhentheobject’slockisavailable. •Whenathread，whichownsalock，finishesexecutingthesynchronizedcode，itgivesupthelock.•Monitorisanobjectthatcanblockandrevivethreads,anobjectthatcontrolsclientthreads.Askstheclientthreadstowaitandnotifiesthemwhenthetimeisrighttocontinue,basedonitsstate.InstrictJavaterminology,anyobjectthathassomesynchronizedcodeisamonitor.•2waystosynchronize:1.Synchronizetheentiremethod•Declarethemethodtobesynchronized-verycommonpractice.•Threadshouldobtaintheobject’slock.2.Synchronizepartofthemethod•Havetopassanarbitraryobjectwhichlockistobeobtainedtoexecutethesynchronizedcodeblock(partofamethod).Synchronized(target){statements}•Wecanspecify“this”inplaceobject，toobtainverybrieflocking—notverycommon•Iftargetisnull，thentheNullPointerExceptionisthrown.•wait-pointstoremember■callingthreadgivesupCPU■callingthreadgivesupthelock■callingthreadgoestomonitor’swaitingpool■waitalsohasaversionwithtimeoutinmilliseconds.Usethisifyou’renotsurewhenthecurrentthreadwillgetnotified,thisavoidsthethreadbeingstuckinwaitstateforever.•notify-pointstoremember■onethreadgetsmovedoutofmonitor'swaitingpooltoreadystate■notifyAllmovesallthethreadstoreadystate■Threadgetstoexecutemustre-acquirethelockofthemonitorbeforeitcanproceed.•Notethedifferencesbetweenblockedandwaiting.BlockedWaitingThreadiswaitingtogetalockonthemonitor.(orwaitingforablockingi/omethod)Threadhasbeenaskedtowait,(bymeansofwaitmethod)Causedbythethreadtriedtoexecutesomesynchronizedcode,(orablockingi/omethod)Thethreadalreadyacquiredthelockandexecutedsomesynchronizedcodebeforecomingacrossawaitcall.Canmovetoreadyonlywhenthelockisavailable.(orthei/ooperationiscomplete)Canmovetoreadyonlywhenitgetsnotified(bymeansofnotifyornotifyAll)•Pointsforcomplexmodels:1.Alwayscheckmonitor’sstateinawhileloop，ratherthaninanifstatement.2.AlwayscallnotifyAll，insteadofnotify. •waitandsleepmustbeenclosedinatry/catchforInterruptedException.•Asinglethreadcanobtainmultiplelocksonmultipleobjects(oronthesameobject)•Athreadowningthelockofanobjectcancallothersynchronousmethodsonthesameobject(thisisanotherlock)Otherthreadscan’tdothat.Theyshouldwaittogetthelock.•Non-synchronousmethodscanbecalledatanytimebyanythread.•Synchronousmethodsarere-entrant.Sotheycanbecalledrecursively.•Synchronizedmethodscanbeoverridedtobenon-synchronous.synchronizedbehavioraffectsonlytheoriginalclass.•Locksoninner/outerobjectsareindependent.Gettingalockonouterobjectdoesn’tmeangettingthelockonaninnerobjectaswell，thatlockshouldbeobtainedseparately.•LocksonstaticsynchronizedmethodcalledClasswidelock.Theclasswidelockandtheinstancelockbeingindependentofeachother.•waitandnotifyshouldbecalledfromsynchronizedcode.Thisensuresthatwhilecallingthesemethodsthethreadalwayshasthelockontheobject.Ifyouhavewait/notifyinnon-synchronizedcodecompilerwon’tcatchthis.Atruntime,ifthethreaddoesn’thavethelockwhilecallingthesemethods,anIllegalMonitorStateExceptionisthrown.•Deadlockscanoccureasily,e.g，ThreadAlockedObjectAandwaitingtogetalockonObjectB，butThreadBlockedObjectBandwaitingtogetalockonObjectA.They’llbeinthisstateforever.•It’stheprogrammer’sresponsibilitytoavoidthedeadlock.Alwaysgetthelocksinthesameorder.•While‘suspended’，thethreadkeepsthelocksitobtained-sosuspendisdeprecatedin1.2•Useofstopisalsodeprecated,insteaduseaflaginrunmethod.Compilerwon’twarnyou，ifyouhavestatementsafteracalltostop，eventhoughtheyarenotreachable.Atypicalexampleofusingthewait/notifyprotocoltoallowcommunicationbetweenThreadsappearstoinvolveapparentlyendlessloopssuchas//producingcodewhile(true){try{wait();}catch(InterruptedExceptione){}}//someproducingactiongoesherenotifyAll();Conditionsthatmightpreventathreadfromexecuting:♦ThethreadisnotthehighestprioritythreadandsocannotgetCPUtime.♦Thethreadiswaitingonaconditionbecausesomeoneinvokedwait()forthethread. ♦ThethreadhasexplicitlyyieldedcontrolbyinvokingyieldOtoallowanotherthreadofthesameprioritytorun.♦Thethreadhasbeenputtosleepusingthesleep()method♦SomeonehassuspendedthethreadusingthesuspendOmethod,(deprecatedinJava2)♦ItisblockedforfileI/O♦ThereismorethanonethreadwiththesamehighestpriorityandJVMisswitchingbetweenthesethreads,atthemoment,thethreadinquestionisawaitingCPUtime.