精品文档504关于计算机专业JAVA语言类有关的外文文献翻译成品Nashorn的Java API中英文双语对照Word文件下载.docx

精品文档504关于计算机专业JAVA语言类有关的外文文献翻译成品Nashorn的Java API中英文双语对照Word文件下载.docx

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Inthischapter,youwilllearn:

WhatJavaAPIsforNashornare

HowtoinstantiatetheNashornenginedirectly

HowtopassoptionstotheNashornengineinJavacodeandonthecommand-line

HowtoshareglobalsamongscriptcontextsinaNashornengine

Howtoadd,update,delete,andreadpropertiesofscriptobjectsinJavacode

HowtocreatescriptobjectsandinvoketheirmethodsinJavacode

HowtoinvokescriptfunctionsfromJavacode

HowtoconvertscriptdatestoJavadates

WhatAreJavaAPIsforNashorn?

ItisstraightforwardtouseJavaclassesinNashornscripts.SometimesyoumaywanttouseNashornobjectsinJavacode.YoumaypassNashornobjectstoJavacodeorJavacodemayevaluateaNashornscriptandretrievethereferenceofNashornobjects.WhenNashornobjectscrosstheboundary（script-to-Java）,theyneedtoberepresentedasobjectsofaJavaclassandyoushouldbeabletousethemlikeanyotherJavaobjects.

IfyourapplicationusesonlyNashorn,totakefulladvantageoftheNashornengineyoumaywanttouseoptionsandextensionsavailableinNashorn.YouwillneedtoinstantiatetheNashornengineinJavacode,usingtheclassesthatarespecifictotheNashornengine,notusingtheclassesintheJavaScriptingAPI.TheJavaAPIsforNashornprovideJavaclassesandinterfacesthatletyoudealwiththeNashornscriptingengineandNashornobjectsdirectlyinJavacode.Figure12-1depictsaclassdiagramofthoseclassesandinterfacesthatyouaresupposedtouseintheclientcodewhenyouaredealingwiththeNashornengine.Theyareinthenjdk.nashorn.api.scriptingpackage.

NotethattheNashornscriptingengineinternallyusesalotofotherclassesinotherpackages.However,youarenotsupposedtousetheminyourapplicationdirectly,exceptforclassesfromthejdk.nashorn.api.scriptingpackage.Thewebpageathttps:

//wiki.openjdk.java.net/display/Nashorn/Nashorn+jsr223+engine+notescontainsthelinksforthedocumentationforthejdk.nashorn.api.scriptingpackage.

IwilldiscusssomeNashorn-specificJavaclassesandinterfacesindetailinsubsequentsections.Table12-1liststheclassesandinterfacesinthejdk.nashorn.api.scriptingpackagewiththeirdescriptions.

InstantiatingtheNashornEngine

Inpreviouschapters,youhavebeeninstantiatingtheNashornengineinJavacodeusingthestandardJavaScriptingAPI.TheNashornengineprovidesseveralcustomfeatures.Totakeadvantageofthosefeatures,youwillneedtoinstantiatetheNashornenginedirectly.First,youwillneedtocreateanobjectoftheNashornScriptEngineFactoryclass,andthencalloneoftheoverloadedversionsofthegetScriptEngine（）methodtocreateaNashornengine:

Bydefault,theNashornenginefactorycreatesaNashornenginewiththe--dump-on-erroroptionenabled.IwillshowyouhowtosetotheroptionsfortheNashornengineinthenextexample.Becauseofthe--no-javaoption,youcannotuseanyJavaclassesinsidethescriptsexecutedbythisengine.The–strictoptionwillforcetheenginetoexecuteallscriptsinstrictmode.ThefollowingsnippetofcodecreatesaNashornengineusingaclassfilter,whichisaninstanceoftheClassFilterinterface,torestricttheusageofanyclassesfromthecom.jdojopackageanditssubpackages.NoticethattheClassFilterinterfacewasaddedinJDK8u40;

itisnotavailableinJDK8.

ClassFilterisafunctionalinterface.ItsmethodispassedthefullyqualifiednameoftheJavaclassthattheNashornscriptistryingtouse.Ifthemethodreturnstrue,theclasscanbeusedinthescript;

otherwise,theclasscannotbeusedinthescript.

ThefollowingisalistoftheoverloadedversionsofthegetScriptEngine（）methodoftheNashornScriptEngineFactoryclass:

SharingtheEngineGlobals

Bydefault,theNashornenginemaintainstheglobalobjectsperscriptcontext.Inthisdiscussion,theterm“globals”meanstheglobalvariablesanddeclarationsstoredintheglobalscopeoftheNashornscript,whichyourefertobythisinthetop-levelscripts.Donotmixuptheglobalscopebindingsinthescriptcontextwiththeglobalsinthescript.Thescriptglobalsaresearchedfirstwhenyoureferenceanyvariablesorcreatevariablesinyourscript.Ifyoureferenceavariableinascriptthatisnotfoundinthescriptglobals,theenginewillsearchforitintheglobalscopebindingsofthescriptcontext.Forexample,thescriptobjectsObject,Math,String,andsoonarepartofthescriptglobals.ConsidertheprogramanditsoutputinListing12-1.

Theprogramperformsthefollowingsteps:

1.CreatesaNashornenginewithdefaultoptions.

2.Executesascriptusingtheeval（）methodthatusesthedefaultscriptcontextoftheengine.Thescriptcreatesaglobalvariablenamedmsgthatisstoredinthescriptglobals.Youcanrefertothevariableusingitssimplenamemsgorthis.msgintheglobalscope.

3.Executesascriptusingtheeval（）methodthatusesthedefaultscriptcontextoftheenginethatprintsthevalueofthemsgvariable.Thefirstlineintheoutputconfirmsthatthecorrectvalueofthemsgvariableisprinted.

4.Executesascriptusingtheeval（）methodthatusesanewscriptcontext.Thescripttriestoprintthevalueoftheglobalvariablemsg.Thesecondlineintheoutput,byprintingundefined,confirmsthatthevariablenamedmsgdoesnotexistinthescriptglobals.

ThisisthedefaultbehavioroftheNashornengine.Itcreatesafreshcopyoftheglobalsperscriptcontext.Ifyouwanttousetheglobalsofthedefaultcontext（ifyouwanttoshareglobals）,youcandosobycopyingtheenginescopeBindingsofthedefaultcontexttoyournewscriptcontext.Listing12-2usesthisapproachtosharethescriptglobalsbetweentwoscriptcontexts.

The–-global-per-engineoptionfortheNashornengineaccomplishesthesamethingsasinthepreviousexample.Itsharesthescriptglobalsamongallscriptcontexts.Listing12-3showshowtosetthisoptionfortheengine.Theoutputconfirmsthattheengineisusingonlyonecopyofglobalstoexecuteallscripts.

UsingScriptObjectsinJavaCode

WhenobjectsandvaluesfromNashorncrossthescript-JavaboundarytoenterJavacode,theyneedtoberepresentedasJavaobjectsandvalues.Table12-2liststhemappingofclassesbetweenscriptobjectsandtheircorrespondingJavaobjects.

Notethatyouarenotsupposedtousetheclassesandinterfacesinthejdk.nashorn.internalpackageanditssubpackages.Theyarelistedinthetableonlyforinformationalpurposes.Inthistable,themappingfortheNashornNumbertypeisshownasjava.lang.Numbertype.AlthoughNashorntriestopassjava.lang.Integerandjava.lang.

Doubleobjectsfornumericvaluewheneverpossible,itisnotreliabletodependonsuchspecializedJavatypes.Youshouldexpectajava.lang.NumberinstancewhenaNashornNumberispassedfromascripttoJavacode.Whenevernecessary,valuesofNumberandBooleantypesinNashornareconvertedtothecorrespondingJavaprimitivetypes,suchasint,double,boolean,andsoon.

InNashorn,ascriptobjectisrepresentedasaninstanceofthejdk.nashorn.internal.runtime.ScriptObjectclass.WhenthescriptobjectispassedtoJavacode,itiswrappedinaScriptObjectMirrorobject.BeforeJDK8u40,ifyoupassedascriptobjecttoaJavamethodthatdeclareditsparameterasjava.lang.Objecttype,aScriptObjectwaspassed,notaScriptObjectMirror.JDK8u40haschangedthisbehaviortopassascriptobjecttoJavacodeasaScriptObjectMirroreverytime.Ifyouhaddeclaredthemethod’sparameterasJSObjectorScriptObjectMirrorinJava,NashornalwayspassedthescriptobjecttoJavaasaScriptObjectMirror.

TheScriptObjectMirrorclassimplementstheJSObjectandBindingsinterfaces.Theclassaddsmoremethodstodealwithscriptobjects.YoucanstorethereferenceofascriptobjectinJavacodeinanyofthethreetypesofvariables:

JSObject,Bindings,orScriptObjectMirror.Whattypeyouusedependsonyourneeds.UsingtheScriptObjectMirrortypegivesyoumoreflexibilityandaccesstoallfeaturesofthescriptobject.Table12-3containsthelistofmethodsdeclaredintheJSObjectinterface.

IwillshowhowtousethesemethodsandsomemethodsoftheScriptObjectMirrorclassinthesubsequentsection.TheJSObjectinterfacecanbeimplementedbyanyJavaclass.Objectsofsuchaclasscanbeusedjustasiftheyarescriptobjectsandcanusethesyntaxobj.func（）,obj[prop],deleteobj.prop,andsoontoworkwiththeirmethodsandpropertiesinscripts.

UsingPropertiesofScriptObjects

TheScriptObjectMirrorclassimplementsJSObjectandBindingsinterfaces.Youcanusemethodsofbothinterfacestoaccessproperties.YoucanusethegetMember（）andgetSlot（）methodsofJSObjecttoreadthenamedandindexedpropertiesofthescriptobject.Youcanusetheget（）methodofBindingstogetthevalueofaproperty.YoucanusethesetMember（）andsetSlot（）methodsofJSObjecttoaddandupdateaproperty.Theput（）methodofBindingsletsyoudothesame.

YoucanusethehasMember（）andhasSlot（）methodsofJSObjecttocheckwhetheranamedpropertyandanindexedpropertyexist.Atthesametime,youcanusethecontainsKey（）methodoftheBindingsinterfacetocheckwhetherapropertyexists.YoucanthinkoftheJSObjectandBindingsinterfacesimplementedbytheScriptObjectMirrorclassprovidingtwoviewsofthesamescriptobject—theformerasasimpleobjectandthelatterasamap.

Listing12-4