计算机专业文献翻译在 JDK 早期版本中使用 Java 5 的语言特性Word文档格式.docx

计算机专业文献翻译在 JDK 早期版本中使用 Java 5 的语言特性Word文档格式.docx

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Butevennow,whenIaskdeveloperswhichversionoftheJavaplatformtheyareusingindevelopment,typicallyonlyhalfareusingJava5--andtheotherhalfarejealous.TheyareeagertousethelanguagefeaturesaddedinJava5suchasgenericsandannotations,butanumberoffactorsstillpreventmanyfromdoingso.

OnecategoryofdevelopersunabletotakeadvantageofJava5featuresarethosewhodevelopcomponents,libraries,orapplicationframeworks.BecausetheircustomersmaystillbeusingJDK1.4orearlierandclassescompiledwithJava5cannotbeloadedbyJDK1.4orpreviousJVMs,usingJava5languagefeatureswouldlimittheircustomerbasetocompaniesthathavealreadytransitionedtoJava5.

AnothergroupofdevelopersoftenheldbackfromusingJava5arethoseworkingwithJavaEE.ManyshopswillnotuseJava5withJavaEE1.4orearlierforfearthatitwillnotbesupportedbytheirapplicationservervendor.ButitmaybeawhilebeforethoseshopstransitiontoJavaEE5.InadditiontothelagtimebetweentheJavaEE5andJavaSE5specifications,commercialJavaEE5containersarenotnecessarilyavailableassoonastheinkisdryonthespecification,businessesdonotnecessarilyupgradetheirapplicationsserversassoonasthenextversionisavailable,andevenafterupgradingtheirapplicationserver,itmaytakesometimetocertifytheirapplicationsonthenewplatform.

Java5languagefeatureimplementation

ThelanguagefeaturesaddedinJava5--generics,enumerations,annotations,autoboxing,andtheenhancedforloop--requirednochangetotheJVMinstructionset,andarealmostentirelyimplementedinthestaticcompiler（javac）andclasslibraries.Whenthecompilerencounterstheuseofgenerics,itattemptstoverifythattypesafetyispreserved（emittingan"

uncheckedcast"

warningifitcannot）andthenemitsbytecodethatisidenticaltowhatwouldbeproducedfromequivalentnongenericcode,castsandall.Similarly,autoboxingandtheenhancedforlooparesimply"

syntacticsugar"

forequivalent,butmoreverbose,idioms,andenumerationsarecompiledintoordinaryclasses.

Intheory,youcouldtaketheclassfilesproducedbyjavacandloadtheminearlierJVMs,whichwas,infact,theintentionwhenJSR14（theJavaCommunityProcessworkinggroupresponsibleforgenerics）wasconvened.However,otherissues（suchasretentionofannotations）forcedtheclassfileversiontobechangedbetweenJava1.4andJava5,whichpreventscodecompiledforJava5tobeloadedbyearlierJVMs.Further,someofthelanguagefeaturesaddedinJava5havedependenciesontheJava5libraries.Ifyoucompileaclasswithjavac-target1.5andtrytoloaditonanearlierJVM,you'

llgetanUnsupportedClassVersionErrorbecausethe-target1.5optiongeneratesclasseswithaclassfileversionof49,andJDK1.4onlysupportsclassfileversionsthrough48.

Thefor-eachloop

Theenhancedforloop,sometimescalledthefor-eachloop,istranslatedbythecompilerasiftheprogrammerhadsuppliedtheequivalentold-styleforloop.Thefor-eachloopcaniterateovertheelementsofanarrayorofacollection.Listing1showsthesyntaxofiteratingoveracollectionwiththefor-eachloop:

Listing1.Thefor-eachloop

Collection<

Foo>

fooCollection=...

for（Foof:

fooCollection）{

doSomething（f）;

}

Thecompilertranslatesthiscodeintotheequivalentiterator-basedloop,asshowninListing2:

Listing2.Iterator-basedequivalentforListing1

for（Iterator<

iter=f.iterator（）;

f.hasNext（）;

）{

Foof=（Foo）iter.next（）;

Howdoesthecompilerknowthatthesuppliedargumenthasaniterator（）method?

Thearchitectsofthejavaccompilercouldhavebuiltinunderstandingofthecollectionsframework,butthisapproachwouldhavebeenunnecessarilyrestrictive.Instead,anewinterfacewascreated,java.lang.Iterable（seeListing3）,andthecollectionclasseswereretrofittedtoimplementIterable.Thisway,containerclassesthatdonotbuildonthecorecollectionsframeworkcanstilltakeadvantageofthenewfor-eachloop.ButdoingsocreatesadependencyontheJava5classlibrarybecauseIterableisnotpresentintheJDK1.4library.

Listing3.TheIterableinterface

publicinterfaceIterable<

T>

{

Iterator<

iterator（）;

Enumerationsandautoboxing

Justlikethefor-eachloop,enumerationsrequiresupportfromtheclasslibrary.Whenthecompilerencountersanenumeratedtype,itgeneratesaclassthatextendsthelibraryclassjava.lang.Enum.But,justlikeIterable,theEnumclassisnotpresentintheJDK1.4classlibrary.

Similarly,autoboxingreliesonthevalueOf（）methodsbeingaddedtotheprimitivewrapperclasses（suchasInteger）.WhenboxingrequiresconversionfrominttoInteger,ratherthancallingnewInteger（int）,thecompilergeneratesacalltoInteger.valueOf（int）.TheimplementationofthevalueOf（）methodsemploystheflyweightpatterntocachetheIntegerobjectsforcommonlyusedintegervalues（theJava6implementationcachesintegersfrom-128to127）,whichmayimproveperformancebyeliminatingredundantinstantiations.And,justlikeIterableandEnum,thevalueOf（）methodsarenotpresentintheJDK1.4classlibrary.

Varargs

Whenthecompilerencountersamethoddefinedwithavariable-lengthargumentlist,itconvertsitintoamethodthattakesanarrayoftheappropriatecomponenttype;

whenthecompilerencountersacalltoamethodwithavariable-lengthargumentlist,itboxestheargumentsintoanarray.

Annotations

Whenanannotationisdefined,itcanbeannotatedwith@Retention,whichdetermineswhatthecompilerwilldowithclasses,methods,orfieldsthatpossessthatannotation.ThedefinedretentionpoliciesareSOURCE（discardannotationdataatcompilation）,CLASS（recordannotationsintheclassfile）,orRUNTIME（recordannotationsintheclassfileandretainthematruntimesotheycanbeaccessedreflectively）.

Otherlibrarydependencies

PriortoJava5,whenthecompilerencounteredanattempttoconcatenatetwostrings,itusedthehelperclassStringBuffertoperformtheconcatenation.InJava5andlater,itinsteadgeneratescallstothenewStringBuilderclass,whichisnotpresentintheJDK1.4andearlierclasslibraries.

AccessingJava5features

Becauseofdependenciesoflanguagefeaturesonlibrarysupport,eveniftheclassfilesproducedbytheJava5compilercouldbeloadedbyearlierJVMversions,executionwouldstillfailbecauseofclassloadingerrors.However,itshouldbepossibletosolvetheseproblemsbysuitablytransformingthebytecodebecausethesemissingclassesdonotcontainsubstantialnewfunctionality.

JSR14

DuringthedevelopmentoftheJavagenericsspecification（andotherlanguagefeaturesaddedinJava5）,experimentalsupportwasaddedtothejavaccompilertoallowittoconsumeJava5languagefeaturesandgeneratebytecodethatcouldberunonaJava1.4JVM.Whilethesefeaturesarenotsupported（orevendocumented）,theyareusedbyanumberofopensourceprojectstoallowdeveloperstocodeusingJava5languagefeaturesandproduceJARfilesthatcanbeusedonearlierJVMs.And,nowthatjavacisopensource,itispossiblethefeaturesmightbesupportedbyathirdparty.Toactivatethesefeatures,youcaninvokejavacwiththe-source1.5and-targetjsr14options.

TheJSR14targetmodeofjavaccausesthecompilertoemitJDK1.4-compatiblebytecodecorrespondingtoJava5languagefeatures:

∙Genericsandvarargs:

Thecastsinsertedbythecompilerinthepresenceofgenericshavenodependencyontheclasslibrary,andsotheycanexecuteequallywellonapre-5JVM.Similarly,thecodegeneratedbythecompilerinthepresenceofvariable-lengthargumentlistshasnodependencyontheclasslibrary.

∙for-eachloop:

Wheniteratingoveranarray,thecompilergeneratesaninductionvariableandthestandardarrayiterationidiom.WheniteratingoveraCollection,thecompilergeneratesthestandarditerator-basedidiom.Wheniteratingoveranon-CollectionIterable,thecompilerproducesanerror.

∙Autoboxing:

RatherthangeneratingcallstothevalueOf（）methodinthewrapperclass,thecompilergeneratescallstotheconstructorinstead.

∙Stringconcatenation:

TheJSR14targetmodeofjavaccausesthecompilertogeneratecallstoStringBufferinsteadofStringBuilder.

∙Enumerations:

TheJSR14targetmodeofjavachasnospecialsupportforenumerations.CodethatattemptstouseenumerationswillfailwithaNoClassDefFoundErrorlookingforthejava.lang.Enumbaseclass.

UsingtheJSR14targetmodeallowsyoutowritecodethatusesgenerics,autoboxing,andthefor-eachloopinthe"

easy"

cases,whichmaysufficeformanyprojects.Itisconvenient,ifunsupported,andthecompilergeneratesmostlycompatiblebytecodeinasinglepass.

Retroweaver

TherearecertainJava5languagefeaturesnotsupportedbytheJSR14targetmode（suchasIterableandenumerations）.Analternateapproach,takenbyopen-sourceprojectssuchasRetroweaverandRetrotranslator,istogeneratebytecodeusing-ta