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PrinciplesofSo3wareConstruc9on: Objects,Design,andConcurrencyPart42:ConcurrencyIntroduc9ontoconcurrencyJoshBlochCharlieGarrodDaryaMelicher

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Administrivia

•  Homework5teamsign-updeadlinetonight•  Teamsizes,presenta9onslots,…

–  MidtermexaminclassThursday(November1st)•  Reviewsessiontoday7-9p.m.PorterHall100

–  NextrequiredreadingdueTuesday•  JavaConcurrencyinPrac9ce,Sec9ons11.3and11.4

–  Homework5frameworksdiscussion

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Today

•  Somepuzzlers•  APIdesignconclusions•  Introduc9ontoconcurrency

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1.“TimeforaChange” (2002)

Ifyoupay$2.00foragasketthatcosts$1.10,howmuchchangedoyouget? publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(2.00-1.10);}}

From An Evening Of Puzzlers by Josh Bloch

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Whatdoesitprint?

(a) 0.9 (b) 0.90 (c) It varies (d) None of the above

publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(2.00-1.10);}}

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(a)0.9(b)0.90(c)Itvaries(d)Noneoftheabove:0.8999999999999999 Decimalvaluescan'tberepresentedexactlyby float or double

Whatdoesitprint?

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Anotherlook

publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(2.00-1.10);}}

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Howdoyoufixit?

//Youcouldfixitthisway...importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(newBigDecimal("2.00").subtract(newBigDecimal("1.10")));}}

//...oryoucouldfixitthiswaypublicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(200-110);}}

Prints0.90

Prints90

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Themoral

•  Avoid float and doublewhereexactanswersarerequired–  Forexample,whendealingwithmoney

•  UseBigDecimal,int,orlonginstead

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2.“AChangeisGonnaCome”

Ifyoupay$2.00foragasketthatcosts$1.10,howmuchchangedoyouget? importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal(2.00);BigDecimalcost=newBigDecimal(1.10);System.out.println(payment.subtract(cost));}}

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Whatdoesitprint?

importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal(2.00);BigDecimalcost=newBigDecimal(1.10);System.out.println(payment.subtract(cost));}}

(a)0.9(b)0.90(c)0.8999999999999999(d)None of the above

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(a)0.9(b)0.90(c)0.8999999999999999(d)Noneoftheabove:0.899999999999999911182158029987476766109466552734375

WeusedthewrongBigDecimalconstructor

Whatdoesitprint?

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Anotherlook

importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal(2.00);BigDecimalcost=newBigDecimal(1.10);System.out.println(payment.subtract(cost));}}

Thespecsays: publicBigDecimal(doubleval)TranslatesadoubleintoaBigDecimalwhichistheexactdecimalrepresenta9onofthedouble'sbinaryfloa9ng-pointvalue.

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Howdoyoufixit?

importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal("2.00");BigDecimalcost=newBigDecimal("1.10");System.out.println(payment.subtract(cost));}}

Prints0.90

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The moral

•  Use newBigDecimal(String), not newBigDecimal(double)

•  BigDecimal.valueOf(double) is better, but not perfect –  Use it for non-constant values.

•  For API designers –  Make it easy to do the commonly correct thing –  Make it hard to misuse –  Make it possible to do exotic things

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KeyconceptsfromlastThursday

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Keydesignprinciple:Informa9onhiding

•  "Whenindoubt,leaveitout."

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Minimizemutability

•  Classesshouldbeimmutableunlessthere'sagoodreasontodootherwise–  Advantages:simple,thread-safe,reusable

•  Seejava.lang.String–  Disadvantage:separateobjectforeachvalue

•  Mutableobjectsrequirecarefulmanagementofvisibilityandsideeffects–  e.g.Component.getSize()returnsamutableDimension

•  Documentmutability–  Carefullydescribestatespace

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Failfast

•  Reporterrorsassoonastheyaredetectable–  Checkprecondi9onsatthebeginningofeachmethod–  Avoiddynamictypecasts,run-9metype-checking

//APropertiesinstancemapsStringstoStringspublicclassPropertiesextendsHashTable{publicObjectput(Objectkey,Objectvalue);//ThrowsClassCastExceptionifthisinstance//containsanykeysorvaluesthatarenotStringspublicvoidsave(OutputStreamout,Stringcomments);}

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Subtle9esofinforma9onhiding

•  Preventsubtleleaksofimplementa9ondetails–  Documenta9on–  Lackofdocumenta9on–  Implementa9on-specificreturntypes–  Implementa9on-specificexcep9ons–  Outputformats–  implementsSerializable

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Avoidbehaviorthatdemandsspecialprocessing

•  Donotreturnnulltoindicateanemptyvalue–  e.g.,UseanemptyCollectionorarrayinstead

•  Donotreturnnulltoindicateanerror–  Useanexcep9oninstead

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Throwexcep9onsonlyforexcep9onalbehavior

•  Donotforceclienttouseexcep9onsforcontrolflow:privatebyte[]a=newbyte[CHUNK_SIZE];voidprocessBuffer(ByteBufferbuffer){try{while(true){buffer.get(a);…}}catch(BufferUnderflowExceptione){intremaining=buffer.remaining();buffer.get(a,0,remaining);…}}

•  Conversely,don’tfailsilently:ThreadGroup.enumerate(Thread[]list)

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Context:Theexcep9onhierarchyinJava

Throwable

Exception

RuntimeException IOException

EOFException

FileNotFoundException

NullPointerException

IndexOutOfBoundsException

ClassNotFoundException… …

. . .

Object

unchecked checked

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Avoidcheckedexcep9ons,ifpossible

•  Overuseofcheckedexcep9onscausesboilerplatecode:try{

Foof=(Foo)g.clone();}catch(CloneNotSupportedExceptione){//Thisexceptioncan'thappenifFooisCloneablethrownewAssertionError(e);}

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Don'tletyouroutputbecomeyourdefactoAPI

•  Documentthefactthatoutputformatsmayevolveinthefuture•  Provideprogramma9caccesstoalldataavailableinstringform

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Don'tletyouroutputbecomeyourdefactoAPI

•  Documentthefactthatoutputformatsmayevolveinthefuture•  Provideprogramma9caccesstoalldataavailableinstringform

publicclassThrowable{publicvoidprintStackTrace(PrintStreams);publicStackTraceElement[]getStackTrace();//since1.4}publicfinalclassStackTraceElement{publicStringgetFileName();publicintgetLineNumber();publicStringgetClassName();publicStringgetMethodName();publicbooleanisNativeMethod();}

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APIdesignsummary

•  Acceptthefactthatyou,andothers,willmakemistakes–  UseyourAPIasyoudesignit–  Getfeedbackfromothers–  Hideinforma9ontogiveyourselfmaximumflexibilitylater–  Designforinaien9ve,hurriedusers–  Documentreligiously

•  Ittakesalotofworktomakesomethingthatappearsobvious

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Semesteroverview

•  Introduc9ontoJavaandO-O•  Introduc9ontodesign

–  Designgoals,principles,paierns•  Designingclasses

–  Designforchange–  Designforreuse

•  Designing(sub)systems–  Designforrobustness–  Designforchange(cont.)

•  Designcasestudies•  Designforlarge-scalereuse•  Explicitconcurrency

•  Crosscukngtopics:–  Moderndevelopmenttools:

IDEs,versioncontrol,buildautoma9on,con9nuousintegra9on,sta9canalysis

–  Modelingandspecifica9on,formalandinformal

–  Func9onalcorrectness:Tes9ng,sta9canalysis,verifica9on

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Concurrency,mo9va9onandprimi9ves

•  Thebackstory–  Mo9va9on,goals,problems,…

•  Concurrencyprimi9vesinJava•  Comingsoon(nottoday):

–  Higher-levelabstrac9onsforconcurrency–  Programstructureforconcurrency–  Frameworksforconcurrentcomputa9on

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PowerrequirementsofaCPU

•  Approx.:Capacitance*Voltage2*Frequency•  Toincreaseperformance:

–  Moretransistors,thinnerwires•  Morepowerleakage:increaseV

–  IncreaseclockfrequencyF•  Changeelectricalstatefaster:increaseV

•  Dennardscaling:Astransistorsgetsmaller,powerdensityisapproximatelyconstant…–  …un9learly2000s

•  Heatoutputispropor9onaltopowerinput

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Oneop9on:fixthesymptom

•  Dissipatetheheat

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Oneop9on:fixthesymptom

•  Beier:Dissipatetheheatwithliquidnitrogen–  OverclockingbyTom'sHardware's5GHzproject

http://www.tomshardware.com/reviews/5-ghz-project,731-8.html

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Processorcharacteris9csover9me

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Concurrencythenandnow

•  Inthepast,mul9-threadingjustaconvenientabstrac9on–  GUIdesign:eventdispatchthread–  Serverdesign:isolateeachclient'swork–  Workflowdesign:isolateproducersandconsumers

•  Now:requiredforscalabilityandperformance

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Weareallconcurrentprogrammers

•  Javaisinherentlymul9threaded•  Tou9lizemodernprocessors,wemustwritemul9threadedcode•  Goodnews:alotofitiswriienforyou

–  Excellentlibrariesexist(java.util.concurrent)•  Badnews:yous9llmustunderstandfundamentals

–  …touselibrarieseffec9vely–  …todebugprogramsthatmakeuseofthem

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Aside:Concurrencyvs.parallelism,visualized

•  Concurrencywithoutparallelism:

•  Concurrencywithparallelism:

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BasicconcurrencyinJava

•  Aninterfacerepresen9ngataskpublicinterfaceRunnable{voidrun();}

•  AclasstoexecuteataskinathreadpublicclassThread{publicThread(Runnabletask);publicvoidstart();publicvoidjoin();…}

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Example:Money-grab(1)

publicclassBankAccount{privatelongbalance;publicBankAccount(longbalance){this.balance=balance;}staticvoidtransferFrom(BankAccountsource,BankAccountdest,longamount){source.balance-=amount;dest.balance+=amount;}publiclongbalance(){returnbalance;}}

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Example:Money-grab(2)publicstaticvoidmain(String[]args)throwsInterruptedException

{BankAccountbugs=newBankAccount(100);BankAccountdaffy=newBankAccount(100);ThreadbugsThread=newThread(()->{for(inti=0;i<1_000_000;i++)transferFrom(daffy,bugs,100);});ThreaddaffyThread=newThread(()->{for(inti=0;i<1_000_000;i++)transferFrom(bugs,daffy,100);});bugsThread.start();daffyThread.start();bugsThread.join();daffyThread.join();System.out.println(bugs.balance()+daffy.balance());}

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Whatwentwrong?

•  Daffy&Bugsthreadshadaracecondi.onforshareddata–  Transfersdidnothappeninsequence

•  Readsandwritesinterleavedrandomly–  Randomresultsensued

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Thechallengeofconcurrencycontrol

•  Notenoughconcurrencycontrol:safetyfailure–  Incorrectcomputa9on

•  Toomuchconcurrencycontrol:livenessfailure–  Possiblynocomputa9onatall(deadlockorlivelock)

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Sharedmutablestaterequiresconcurrencycontrol

•  Threebasicchoices:1.  Don'tmutate:shareonlyimmutablestate2.  Don'tshare:isolatemutablestateinindividualthreads3.  Ifyoumustsharemutablestate:limitconcurrencytoachievesafety

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Aneasyfix:

publicclassBankAccount{privatelongbalance;publicBankAccount(longbalance){this.balance=balance;}staticsynchronizedvoidtransferFrom(BankAccountsource,BankAccountdest,longamount){source.balance-=amount;dest.balance+=amount;}publicsynchronizedlongbalance(){returnbalance;}}

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ConcurrencycontrolwithJava'sintrinsiclocks

•  synchronized(foo){…}–  Synchronizesen9reblockonobjectfoo;cannotforgettounlock–  Intrinsiclocksareexclusive:Onethreadata9meholdsthelock–  Intrinsiclocksarereentrant:Athreadcanrepeatedlygetsamelock

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ConcurrencycontrolwithJava'sintrinsiclocks

•  synchronized(foo){…}–  Synchronizesen9reblockonobjectfoo;cannotforgettounlock–  Intrinsiclocksareexclusive:Onethreadata9meholdsthelock–  Intrinsiclocksarereentrant:Athreadcanrepeatedlygetsamelock

•  synchronizedonaninstancemethod–  Equivalenttosynchronized(this){…}foren9remethod

•  synchronizedonasta9cmethodinclassFoo–  Equivalenttosynchronized(Foo.class){…}foren9remethod

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Summary

•  Likeitornot,you’reaconcurrentprogrammer•  Ideally,avoidsharedmutablestate

–  Ifyoucan’tavoidit,synchronizeproperly