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JavaWebStartandApplets

外文作者：

DavidParsons

文献出处:

FoundationalJava，2019,pp503-527（如觉得年份太老，可改为近2年，毕竟很多毕业生都这样做）

英文3584单词，19270字符（字符就是印刷符），中文5622汉字。

（如果字数多了，可自行删减，大多数学校都是要求选取外文的一部分内容进行翻译的。

）

Initsearlydays,JavacametoprominencebecauseofJavaapplets,runninginwebbrowsers.Appletshavehadsomethingofacheckeredhistorysincethen,andmostJavawebtechnologiesbelongintheenterpriseeditionratherthanthestandardeditionofJava.However,appletsarestillwidelyused,andcanbeausefulcomponentofrichInternetapplications.Alsoassociatedwiththeweb,butlessdirectly,isJavaWebStart,whichallowsdesktopapplicationstobedeployedovertheweb.Inthischapter,wewillbeginbylookingatJavaWebStartandthenlookatapplets,whichcannowusethesamedeploymentmechanismasJavaWebStart,makingitveryeasytoswitchbetweenthetwomodesofdeployment.SincebothJavaWebStartandappletsrequireustocreatewebpageswithappropriatelinkstoJavaapplicationsorapplets,wewillbeginwithbriefoverviewsofwebbrowsers,URLs,andHTML.

20.1WebBrowsers,URLs,andHTMLPages

Webbrowsersoftware（e.g.,Explorer,Firefox,Safari,Chrome,etc.）isdesignedtoretrieveinformationfromremotesitesontheWorldWideWeb（WWW）usingUniformResourceLocators（URLs）.MuchoftheinformationismadeavailableintheformofHTML（HyperTextMarkupLanguage）webpages.

20.1.1URLs

AURLisbasicallytheInternetaddressofaresourceonaparticularserver,andtypicallyiswritteninthreeparts:

theprotocol,theserveraddress,andthenameoftheresource（includinganypathinformation）.

20.1.1.1TheHTTPProtocol

Theprotocolprefixforwebpagesis“http:

//”.Thisstandsfor“hypertexttransferprotocol”andmostURLsstartthisway.Indeedthispartisoftenomitted,becauseitisusuallythebrowser’sdefaultprotocol（thoughthereareotherwebprotocolsthatyoumaybefamiliarwithsuchasFTP,theFileTransferProtocol）.

20.1.1.2TheServerAddress

Theserveraddressusuallybeginswith“www”（WorldWideWeb）,followedbythenameofthesiteandits“domain,”whichdefinesitscategoryandmayincludeitscountrycode.Eachoftheseisseparatedbyaperiod（fullstop）.Take,forexample,thefollowingURL:

“introjava”isthenameoftheserversiteand“com”meansacompany.Commonalternativesto“com”are“edu”foracademicinstitutionsand“org”fororganizations.ManyURLsarewithinacountrydomain,suchas“.co.uk”or“.ac.nz”.Ifyouarerunningatestserveronyourlocalmachine,thedomainnamebecomes“localhost.”

20.1.1.3ThePathandResourceName

ThefinalpartofaURLcanincludethelocation（directory）andnameoftheparticularfileatthesite.Forexample,

Thislooksforthefile“getinvolved.html”inthe“foundation”directory.Thefilenamewillgenerallyendin“html”（orsometimesjust“htm”）becauseitwillusuallybeafilewritteninHTML（seebelow）.Ifnofilenameisspecified,“index.html”isoftentheserver’sdefaultfilenamethatitwillsendbacktothebrowser.Ifnoindexfileispresent,thenadirectorylistingmaybedisplayedinstead,oranerrorpage,dependingontheserver’sconfiguration.

20.1.2HTML（HyperTextMarkupLanguage）

WebbrowsersdisplayscreensofinformationwrittenusingHTML（HyperTextMarkupLanguage）.ThebrowsertakesthebasicinformationstoredintheHTMLfileandformatsitappropriatelyusingtagsembeddedintothetextofthefile.AllHTMLfilesbeginwithan<

html>

tagandendwith<

/html>

.MostHTMLtagsareterminatedlikethis,usingaforwardslashfollowedbythetagname.ThisisaverysimpleHTMLfile,showingadditionaltagsforthepageheader（<

head>

whichcanincludeatitle<

title>

）,themainbodyofthetext（<

body>

）,andsometext.Headertags（suchas<

h1>

）canbeusedtoformatthesizeoftext.The<

p>

tagisusedtospecifyparagraphs.The“DOCTYPE”atthetopthefilespecifiestheversionofHTMLthatisbeingused.ThisexampleusesHTML5（differentDOCTYPEShavedifferentexpectationsabouthowthedocumentshouldbestructured）.

Figure20.1showswhatthispagelookslikewhenloadedintoabrowserasalocalfile.Simplefilescaneasilybewrittenbyhand,butformorecomplexpagesitisbettertouseoneofthemanyavailableHTMLeditingtools.SomesimpleHTMLpageswillbeneededtolaunchtheapplicationsandappletsdescribedinthischapter.

20.2JavaWebStart

JavaWebStartisagenericRichInternetApplication（RIA）launchmechanismthatcanbeusedtodeploybothdesktopapplicationsandapplets.Itprovidesawaytoeasilydeployapplicationsviatheweb,andprovidesanalternativetosomeothertypesofwebdeployment,suchassomeserversidewebapplications,thoughitprimarilyeasesthedeploymentofdesktopapplications.AJavaWebStartapplicationcanbeself-contained,becauseitcanautomaticallydownloadandinstallaJRE.ProgrammerscanspecifywhichJREversionagivenprogramneedsinordertoexecuteandensurethatitisavailablefortheapplicationtorun.Althoughitisaweb-baseddeploymentmechanism,noInternetconnectionisrequiredtoexecutetheapplicationsoncetheyhavebeendownloaded.

ThecorecomponentsofaJavaWebStartdeploymentare:

theapplicationtobedeployed,aJavaNetworkLaunchProtocol（JNLP）file,andaweb（HTML）pagetohosttheapplicationlauncher.TheJavaapplicationneedstobearchivedinaJARfilewithanidentifiable“main”classthatservesastheprogramentrypoint.AswesawinChap.14,aJARprovidesasingleunitofdeployment,andcanspecifyamainclassinitsmanifest.Havingthe“main”classidentifiedinthemanifestisnotessentialforJavaWebStartsincewecanalsospecifythemainclassintheJNLPfile,butitisusefulnottorelyentirelyontheJNLPfiletospecifyamainclass.

TheJNLPfileisanXMLfilethatdescribeshowtheapplicationshouldbelaunched.Itcontainsdeployment-relatedinformationsuchasthenameoftheJARfile,themainclassintheJAR,andtheminimumversionofJavarequiredtoruntheapplication.ThewebpagecaneithercontainasimplehyperlinktotheJNLPfileoritcanuseaJavaScriptbuttontolaunchtheapplication.Normally,wewouldalsoneedtosetupawebapplicationsothatthefilescanbedownloadedoverthewebfromaserver.However,fortestingpurposesitispossibletodeployaJavaWebStartapplicationusingalocalHTMLfile,whichwewilldoforthefirstexample,whichdeploystheDistanceViewerapplicationfromChap.18.

20.2.1CreatingaJARinEclipse

ThefirststepistocreateaJARfilecontainingtherequiredclasses.ThiscouldofcoursebebuiltusinganAntscript,whichwewilldolaterinthischapter,butwewilldescribethefirststepinthisexampleusingtheEclipseexportfacility.Figure20.2showsthekeystepsinthedialogsthatexportfilesfromEclipse.Thefirststepistoselect“export”fromthe“file”menu,whichwillopenthe“Export”dialogshowninFig.20.1.Inthisdialog,withinthe“Java”folderyoucanselect“JARFile”asthetypeofexport.Clickingthe“Next”buttontakesyoutothe“JARExport”dialog.Itisessentialheretochooseallofthefilesthatneedtobeinthearchive.Forthe“DistanceViewer”application,youneedtoexportnotonlytheDistanceViewerclassitself（theview）,buttheFlightDistancesclassaswell（themodel）.ThelowerpartofthedialogletsyouselecttheexportfolderandthenameoftheJARfile.Clicking“Next”takesyoutothefinalstageofthedialogwhereyoucanselectoneoftheclassesintheJARastheclassusedastheapplicationentrypoint.Clicking“Finish”createstheJARfile.

20.2.2JavaNetworkLaunchProtocol（JNLP）

TheJavaNetworkLaunchProtocol（JNLP）enablesadesktopapplicationtobelaunchedfromresourceshostedonawebserver.JavaWebStartrequiresaJNLPfilethatconfiguresthevariousaspectsoftheprotocol.

ThiscanbecreatedasatextfileinEclipse,witha“jnlp”extension.ItneedstobeexplicitlyopenedinatexteditororEclipsemayattempttorunitdirectly（right-clickonthefileandchoose“openwith”,thenselect“texteditor”）.ThefollowingexamplecontainsthebasicinformationrequiredinaJNLPfile.The“information”elementrequires“title”and“vendor”information.The“resources”elementspecifiestheminimumversionofJavarequiredtoruntheapplication（withtheURLoftheautomaticdownloadsitefortheJavaruntime）,andthenameoftheJARfile.Finallythe“applicationdesc”elementspecifiesthenameofthemainclasstoberunfromtheJAR.ThisparticularJNLPfilehasbeenwrittenusingtherequiredelementsfromJavaWebStartversion1;

hence,the“spec”attributeissetto“1.0+”（thisisactuallythedefault）.

Notethatthe“codebase”and“href”attributesareemptyinthisexample–theyarenotrequiredwhentestinglocalfiles.

Hyperlinksdonotparticularlystandoutonawebpage.AnotherwayoflaunchingaJavaWebStartapplicationistouseaJavaScriptbuttontolaunchtheapplication.ThisfragmentofJavaScriptusesthe“deployJava.createWebStartLaunchButton”methodthatisavailableoverthewebfromthe“deployJava.js”file.Thecodethatprocessesitusesthe“href”propertyoftheJavaScript“location”objecttolocatetheJNLPfilerelativetotheURLofthewebpage.

20.2.3DeployingtoaServer

SofarwehaveonlytestedourWebStartapplicationsusinglocalfiles,butofcourseWebStartapplicationsneedtobedeployedtoaserverinordertobedownloadedovertheInternet.Inthissection,wewillcoverthebasicsofdeployingaWebStartapplicationtotheTomcat