JAVA编程：J2EE Enterprise Beans(原文)
先谈谈我对java的一些认识。我选择java，是因为他语法简单，功能强大，从web，到桌面，到嵌入式，无所不能。但当我进一步了解了java后，感叹，java原来也有许多缺点。enterprise|j2eeEnterprisebeansaretheJ2EEcomponentsthatimplementEnterpriseJavaBeans(EJB)technology.EnterprisebeansrunintheEJBcontainer,aruntimeenvironmentwithintheJ2EEserver(seeFigure1-5).Althoughtransparenttotheapplicationdeveloper,theEJBcontainerprovidessystem-levelservicessuchastransactionstoitsenterprisebeans.Theseservicesenableyoutoquicklybuildanddeployenterprisebeans,whichformthecoreoftransactionalJ2EEapplications.WhatIsanEnterpriseBean?WrittenintheJavaprogramminglanguage,anenterprisebeanisaserver-sidecomponentthatencapsulatesthebusinesslogicofanapplication.Thebusinesslogicisthecodethatfulfillsthepurposeoftheapplication.Inaninventorycontrolapplication,forexample,theenterprisebeansmightimplementthebusinesslogicinmethodscalledcheckInventoryLevelandorderProduct.Byinvokingthesemethods,remoteclientscanaccesstheinventoryservicesprovidedbytheapplication.BenefitsofEnterpriseBeans
Forseveralreasons,enterprisebeanssimplifythedevelopmentoflarge,distributedapplications.First,becausetheEJBcontainerprovidessystem-levelservicestoenterprisebeans,thebeandevelopercanconcentrateonsolvingbusinessproblems.TheEJBcontainer--notthebeandeveloper--isresponsibleforsystem-levelservicessuchastransactionmanagementandsecurityauthorization.Second,becausethebeans--andnottheclients--containtheapplicationsbusinesslogic,theclientdevelopercanfocusonthepresentationoftheclient.Theclientdeveloperdoesnothavetocodetheroutinesthatimplementbusinessrulesoraccessdatabases.Asaresult,theclientsarethinner,abenefitthatisparticularlyimportantforclientsthatrunonsmalldevices.Third,becauseenterprisebeansareportablecomponents,theapplicationassemblercanbuildnewapplicationsfromexistingbeans.TheseapplicationscanrunonanycompliantJ2EEserver.WhentoUseEnterpriseBeans
Youshouldconsiderusingenterprisebeansifyourapplicationhasanyofthefollowingrequirements:
[*]Theapplicationmustbescalable.Toaccommodateagrowingnumberofusers,youmayneedtodistributeanapplicationscomponentsacrossmultiplemachines.Notonlycantheenterprisebeansofanapplicationrunondifferentmachines,buttheirlocationwillremaintransparenttotheclients.
[*]Transactionsarerequiredtoensuredataintegrity.Enterprisebeanssupporttransactions,themechanismsthatmanagetheconcurrentaccessofsharedobjects.
[*]Theapplicationwillhaveavarietyofclients.Withjustafewlinesofcode,remoteclientscaneasilylocateenterprisebeans.Theseclientscanbethin,various,andnumerous.
TypesofEnterpriseBeans
Table3-1summarizesthethreedifferenttypesofenterprisebeans.Thefollowingsectionsdiscusseachtypeinmoredetail.<Palign=center>Table3-1SummaryofEnterpriseBeanTypes<Palign=center>EnterpriseBeanType<Palign=center>Purpose<Palign=left>Session<Palign=left>Performsataskforaclient<Palign=left>Entity<Palign=left>Representsabusinessentityobjectthatexistsinpersistentstorage<Palign=left>Message-Driven<Palign=left>ActsasalistenerfortheJavaMessageServiceAPI,processingmessagesasynchronouslyWhatIsaSessionBean?
AsessionbeanrepresentsasingleclientinsidetheJ2EEserver.Toaccessanapplicationthatisdeployedontheserver,theclientinvokesthesessionbeansmethods.Thesessionbeanperformsworkforitsclient,shieldingtheclientfromcomplexitybyexecutingbusinesstasksinsidetheserver.Asitsnamesuggests,asessionbeanissimilartoaninteractivesession.Asessionbeanisnotshared--itmayhavejustoneclient,inthesamewaythataninteractivesessionmayhavejustoneuser.Likeaninteractivesession,asessionbeanisnotpersistent.(Thatis,itsdataisnotsavedtoadatabase.)Whentheclientterminates,itssessionbeanappearstoterminateandisnolongerassociatedwiththeclient.Forcodesamples,seeChapter4.StateManagementModes
Therearetwotypesofsessionbeans:statefulandstateless.StatefulSessionBeans
Thestateofanobjectconsistsofthevaluesofitsinstancevariables.Inastatefulsessionbean,theinstancevariablesrepresentthestateofauniqueclient-beansession.Becausetheclientinteracts("talks")withitsbean,thisstateisoftencalledtheconversationalstate.Thestateisretainedforthedurationoftheclient-beansession.Iftheclientremovesthebeanorterminates,thesessionendsandthestatedisappears.Thistransientnatureofthestateisnotaproblem,however,becausewhentheconversationbetweentheclientandthebeanendsthereisnoneedtoretainthestate.StatelessSessionBeans
Astatelesssessionbeandoesnotmaintainaconversationalstateforaparticularclient.Whenaclientinvokesthemethodofastatelessbean,thebeansinstancevariablesmaycontainastate,butonlyforthedurationoftheinvocation.Whenthemethodisfinished,thestateisnolongerretained.Exceptduringmethodinvocation,allinstancesofastatelessbeanareequivalent,allowingtheEJBcontainertoassignaninstancetoanyclient.Becausestatelesssessionbeanscansupportmultipleclients,theycanofferbetterscalabilityforapplicationsthatrequirelargenumbersofclients.Typically,anapplicationrequiresfewerstatelesssessionbeansthanstatefulsessionbeanstosupportthesamenumberofclients.Attimes,theEJBcontainermaywriteastatefulsessionbeantosecondarystorage.However,statelesssessionbeansareneverwrittentosecondarystorage.Therefore,statelessbeansmayofferbetterperformancethanstatefulbeans.WhentoUseSessionBeans
Ingeneral,youshoulduseasessionbeanifthefollowingcircumstanceshold:
[*]Atanygiventime,onlyoneclienthasaccesstothebeaninstance.
[*]Thestateofthebeanisnotpersistent,existingonlyforashortperiodoftime(perhapsafewhours).
Statefulsessionbeansareappropriateifanyofthefollowingconditionsaretrue:
[*]Thebeansstaterepresentstheinteractionbetweenthebeanandaspecificclient.
[*]Thebeanneedstoholdinformationabouttheclientacrossmethodinvocations.
[*]Thebeanmediatesbetweentheclientandtheothercomponentsoftheapplication,presentingasimplifiedviewtotheclient.
[*]Behindthescenes,thebeanmanagestheworkflowofseveralenterprisebeans.Foranexample,seetheAccountControllerEJBsessionbeaninChapter18.
Toimproveperformance,youmightchooseastatelesssessionbeanifithasanyofthesetraits:
[*]Thebeansstatehasnodataforaspecificclient.
[*]Inasinglemethodinvocation,thebeanperformsagenerictaskforallclients.Forexample,youmightuseastatelesssessionbeantosendane-mailthatconfirmsanonlineorder.
[*]Thebeanfetchesfromadatabaseasetofread-onlydatathatisoftenusedbyclients.Suchabean,forexample,couldretrievethetablerowsthatrepresenttheproductsthatareonsalethismonth.
WhatIsanEntityBean?
Anentitybeanrepresentsabusinessobjectinapersistentstoragemechanism.Someexamplesofbusinessobjectsarecustomers,orders,andproducts.IntheJ2EESDK,thepersistentstoragemechanismisarelationaldatabase.Typically,eachentitybeanhasanunderlyingtableinarelationaldatabase,andeachinstanceofthebeancorrespondstoarowinthattable.Forcodeexamplesofentitybeans,pleaserefertochapters5and6.WhatMakesEntityBeansDifferentfromSessionBeans?
Entitybeansdifferfromsessionbeansinseveralways.Entitybeansarepersistent,allowsharedaccess,haveprimarykeys,andmayparticipateinrelationshipswithotherentitybeans.Persistence
Becausethestateofanentitybeanissavedinastoragemechanism,itispersistent.PersistencemeansthattheentitybeansstateexistsbeyondthelifetimeoftheapplicationortheJ2EEserverprocess.Ifyouveworkedwithdatabases,yourefamiliarwithpersistentdata.Thedatainadatabaseispersistentbecauseitstillexistsevenafteryoushutdownthedatabaseserverortheapplicationsitservices.Therearetwotypesofpersistenceforentitybeans:bean-managedandcontainer-managed.Withbean-managedpersistence,theentitybeancodethatyouwritecontainsthecallsthataccessthedatabase.Ifyourbeanhascontainer-managedpersistence,theEJBcontainerautomaticallygeneratesthenecessarydatabaseaccesscalls.Thecodethatyouwritefortheentitybeandoesnotincludethesecalls.Foradditionalinformation,seethesectionContainer-ManagedPersistence.SharedAccess
Entitybeansmaybesharedbymultipleclients.Becausetheclientsmightwanttochangethesamedata,itsimportantthatentitybeansworkwithintransactions.Typically,theEJBcontainerprovidestransactionmanagement.Inthiscase,youspecifythetransactionattributesinthebeansdeploymentdescriptor.Youdonothavetocodethetransactionboundariesinthebean--thecontainermarkstheboundariesforyou.SeeChapter14formoreinformation.PrimaryKey
Eachentitybeanhasauniqueobjectidentifier.Acustomerentitybean,forexample,mightbeidentifiedbyacustomernumber.Theuniqueidentifier,orprimarykey,enablestheclienttolocateaparticularentitybean.FormoreinformationseethesectionPrimaryKeysforBean-ManagedPersistence.Relationships
Likeatableinarelationaldatabase,anentitybeanmayberelatedtootherentitybeans.Forexample,inacollegeenrollmentapplication,StudentEJBandCourseEJBwouldberelatedbecausestudentsenrollinclasses.Youimplementrelationshipsdifferentlyforentitybeanswithbean-managedpersistenceandthosewithcontainer-managedpersistence.Withbean-managedpersistence,thecodethatyouwriteimplementstherelationships.Butwithcontainer-managedpersistence,theEJBcontainertakescareoftherelationshipsforyou.Forthisreason,relationshipsinentitybeanswithcontainer-managedpersistenceareoftenreferredtoascontainer-managedrelationships.Container-ManagedPersistence
Thetermcontainer-managedpersistencemeansthattheEJBcontainerhandlesalldatabaseaccessrequiredbytheentitybean.Thebeanscodecontainsnodatabaseaccess(SQL)calls.Asaresult,thebeanscodeisnottiedtoaspecificpersistentstoragemechanism(database).Becauseofthisflexibility,evenifyouredeploythesameentitybeanondifferentJ2EEserversthatusedifferentdatabases,youwontneedtomodifyorrecompilethebeanscode.Inshort,yourentitybeansaremoreportable.Inordertogeneratethedataaccesscalls,thecontainerneedsinformationthatyouprovideintheentitybeansabstractschema.AbstractSchema
Partofanentitybeansdeploymentdescriptor,theabstractschemadefinesthebeanspersistentfieldsandrelationships.Thetermabstractdistinguishesthisschemafromthephysicalschemaoftheunderlyingdatastore.Inarelationaldatabase,forexample,thephysicalschemaismadeupofstructuressuchastablesandcolumns.Youspecifythenameofanabstractschemainthedeploymentdescriptor.ThisnameisreferencedbyquerieswrittenintheEnterpriseJavaBeansQueryLanguage("EJBQL").Foranentitybeanwithcontainer-managedpersistence,youmustdefineanEJBQLqueryforeveryfindermethod(exceptfindByPrimaryKey).TheEJBQLquerydeterminesthequerythatisexecutedbytheEJBcontainerwhenthefindermethodisinvoked.TolearnmoreaboutEJBQL,seeChapter8.Youllprobablyfindithelpfultosketchtheabstractschemabeforewritinganycode.Figure3-1representsasimpleabstractschemathatdescribestherelationshipsbetweenthreeentitybeans.Theserelationshipsarediscussedfurtherinthesectionsthatfollow.<P>Figure3-1AHigh-LevelViewofanAbstractSchemaPersistentFields
Thepersistentfieldsofanentitybeanarestoredintheunderlyingdatastore.Collectively,thesefieldsconstitutethestateofthebean.Atruntime,theEJBcontainerautomaticallysynchronizesthisstatewiththedatabase.Duringdeployment,thecontainertypicallymapstheentitybeantoadatabasetableandmapsthepersistentfieldstothetablescolumns.ACustomerEJBentitybean,forexample,mighthavepersistentfieldssuchasfirstName,lastName,phone,andemailAddress.Incontainer-managedpersistence,thesefieldsarevirtual.Youdeclarethemintheabstractschema,butyoudonotcodethemasinstancevariablesintheentitybeanclass.Instead,thepersistentfieldsareidentifiedinthecodebyaccessmethods(gettersandsetters).RelationshipFields
Arelationshipfieldislikeaforeignkeyinadatabasetable--itidentifiesarelatedbean.Likeapersistentfield,arelationshipfieldisvirtualandisdefinedintheenterprisebeanclasswithaccessmethods.Butunlikeapersistentfield,arelationshipfielddoesnotrepresentthebeansstate.RelationshipfieldsarediscussedfurtherinDirectioninContainer-ManagedRelationships.MultiplicityinContainer-ManagedRelationships
Therearefourtypesofmultiplicities:One-to-one:Eachentitybeaninstanceisrelatedtoasingleinstanceofanotherentitybean.Forexample,tomodelaphysicalwarehouseinwhicheachstoragebincontainsasinglewidget,StorageBinEJBandWidgetEJBwouldhaveaone-to-onerelationship.One-to-many:Anentitybeaninstancemayberelatedtomultipleinstancesoftheotherentitybean.Asalesorder,forexample,canhavemultiplelineitems.Intheorderapplication,OrderEJBwouldhaveaone-to-manyrelationshipwithLineItemEJB.Many-to-one:Multipleinstancesofanentitybeanmayberelatedtoasingleinstanceoftheotherentitybean.Thismultiplicityistheoppositeofaone-to-manyrelationship.Intheexamplementionedinthepreviousitem,fromtheperspectiveofLineItemEJBtherelationshiptoOrderEJBismany-to-one.Many-to-many:Theentitybeaninstancesmayberelatedtomultipleinstancesofeachother.Forexample,incollegeeachcoursehasmanystudents,andeverystudentmaytakeseveralcourses.Therefore,inanenrollmentapplication,CourseEJBandStudentEJBwouldhaveamany-to-manyrelationship.DirectioninContainer-ManagedRelationships
Thedirectionofarelationshipmaybeeitherbidirectionalorunidirectional.Inabidirectionalrelationship,eachentitybeanhasarelationshipfieldthatreferstotheotherbean.Throughtherelationshipfield,anentitybeanscodecanaccessitsrelatedobject.Ifanentitybeanhasarelativefield,thenweoftensaythatit"knows"aboutitsrelatedobject.Forexample,ifOrderEJBknowswhatLineItemEJBinstancesithasandifLineItemEJBknowswhatOrderEJBitbelongsto,thentheyhaveabidirectionalrelationship.Inaunidirectionalrelationship,onlyoneentitybeanhasarelationshipfieldthatreferstotheother.Forexample,LineItemEJBwouldhavearelationshipfieldthatidentifiesProductEJB,butProductEJBwouldnothavearelationshipfieldforLineItemEJB.Inotherwords,LineItemEJBknowsaboutProductEJB,butProductEJBdoesntknowwhichLineItemEJBinstancesrefertoit.EJBQLqueriesoftennavigateacrossrelationships.Thedirectionofarelationshipdetermineswhetheraquerycannavigatefromonebeantoanother.Forexample,aquerycannavigatefromLineItemEJBtoProductEJB,butcannotnavigateintheoppositedirection.ForOrderEJBandLineItemEJB,aquerycouldnavigateinbothdirections,sincethesetwobeanshaveabidirectionalrelationship.WhentoUseEntityBeans
Youshouldprobablyuseanentitybeanunderthefollowingconditions:
[*]Thebeanrepresentsabusinessentity,notaprocedure.Forexample,CreditCardEJBwouldbeanentitybean,butCreditCardVerifierEJBwouldbeasessionbean.
[*]Thebeansstatemustbepersistent.IfthebeaninstanceterminatesoriftheJ2EEserverisshutdown,thebeansstatestillexistsinpersistentstorage(adatabase).
WhatIsaMessage-DrivenBean?
Note:ThissectioncontainstextfromTheJavaMessageServiceTutorial.Becausemessage-drivenbeansrelyonJavaMessageService(JMS)technology,tofullyunderstandhowthesebeansworkyoushouldconsultthetutorialatthisURL:http://java.sun.com/products/jms/tutorial/index.htmlAmessage-drivenbeanisanenterprisebeanthatallowsJ2EEapplicationstoprocessmessagesasynchronously.ItactsasaJMSmessagelistener,whichissimilartoaneventlistenerexceptthatitreceivesmessagesinsteadofevents.ThemessagesmaybesentbyanyJ2EEcomponent--anapplicationclient,anotherenterprisebean,oraWebcomponent--orbyaJMSapplicationorsystemthatdoesnotuseJ2EEtechnology.Message-drivenbeanscurrentlyprocessonlyJMSmessages,butinthefuturetheymaybeusedtoprocessotherkindsofmessages.Foracodesample,seeChapter7.WhatMakesMessage-DrivenBeansDifferentfromSessionandEntityBeans?
Themostvisibledifferencebetweenmessage-drivenbeansandsessionandentitybeansisthatclientsdonotaccessmessage-drivenbeansthroughinterfaces.InterfacesaredescribedinthesectionDefiningClientAccesswithInterfaces.Unlikeasessionorentitybean,amessage-drivenbeanhasonlyabeanclass.Inseveralrespects,amessage-drivenbeanresemblesastatelesssessionbean.
[*]Amessage-drivenbeansinstancesretainnodataorconversationalstateforaspecificclient.
[*]Allinstancesofamessage-drivenbeanareequivalent,allowingtheEJBcontainertoassignamessagetoanymessage-drivenbeaninstance.Thecontainercanpooltheseinstancestoallowstreamsofmessagestobeprocessedconcurrently.
[*]Asinglemessage-drivenbeancanprocessmessagesfrommultipleclients.
Theinstancevariablesofthemessage-drivenbeaninstancecancontainsomestateacrossthehandlingofclientmessages--forexample,aJMSAPIconnection,anopendatabaseconnection,oranobjectreferencetoanenterprisebeanobject.Whenamessagearrives,thecontainercallsthemessage-drivenbeansonMessagemethodtoprocessthemessage.TheonMessagemethodnormallycaststhemessagetooneofthefiveJMSmessagetypesandhandlesitinaccordancewiththeapplicationsbusinesslogic.TheonMessagemethodmaycallhelpermethods,oritmayinvokeasessionorentitybeantoprocesstheinformationinthemessageortostoreitinadatabase.Amessagemaybedeliveredtoamessage-drivenbeanwithinatransactioncontext,sothatalloperationswithintheonMessagemethodarepartofasingletransaction.Ifmessageprocessingisrolledback,themessagewillberedelivered.Formoreinformation,seeChapter7.WhentoUseMessage-DrivenBeans
SessionbeansandentitybeansallowyoutosendJMSmessagesandtoreceivethemsynchronously,butnotasynchronously.Toavoidtyingupserverresources,youmayprefernottouseblockingsynchronousreceivesinaserver-sidecomponent.Toreceivemessagesasynchronously,useamessage-drivenbean.TheContentsofanEnterpriseBean
Todevelopanenterprisebean,youmustprovidethefollowingfiles:
[*]Deploymentdescriptor:AnXMLfilethatspecifiesinformationaboutthebeansuchasitspersistencetypeandtransactionattributes.ThedeploytoolutilitycreatesthedeploymentdescriptorwhenyoustepthroughtheNewEnterpriseBeanwizard.
[*]Enterprisebeanclass:Implementsthemethodsdefinedinthefollowinginterfaces.
[*]Interfaces:Theremoteandhomeinterfacesarerequiredforremoteaccess.Forlocalaccess,thelocalandlocalhomeinterfacesarerequired.SeethesectionDefiningClientAccesswithInterfaces.(Pleasenotethattheseinterfacesarenotusedbymessage-drivenbeans.)
[*]Helperclasses:Otherclassesneededbytheenterprisebeanclass,suchasexceptionandutilityclasses.
YoupackagethefilesintheprecedinglistintoanEJBJARfile,themodulethatstorestheenterprisebean.AnEJBJARfileisportableandmaybeusedfordifferentapplications.ToassembleaJ2EEapplication,youpackageoneormoremodules--suchasEJBJARfiles--intoanEARfile,thearchivefilethatholdstheapplication.WhenyoudeploytheEARfilethatcontainsthebeansEJBJARfile,youalsodeploytheenterprisebeanontotheJ2EEserver.NamingConventionsforEnterpriseBeans
Becauseenterprisebeansarecomposedofmultipleparts,itsusefultofollowanamingconventionforyourapplications.Table3-2summarizestheconventionsfortheexamplebeansofthistutorial.<P><P><Palign=center>Table3-2NamingConventionsforEnterpriseBeans<Palign=center>Item<Palign=center>Syntax<Palign=center>Example<Palign=left>Enterprisebeanname(DD)<Palign=left><name>EJB<Palign=left>AccountEJB<Palign=left>EJBJARdisplayname(DD)<Palign=left><name>JAR<Palign=left>AccountJAR<Palign=left>Enterprisebeanclass<Palign=left><name>Bean<Palign=left>AccountBean<Palign=left>Homeinterface<Palign=left><name>Home<Palign=left>AccountHome<Palign=left>Remoteinterface<Palign=left><name><Palign=left>Account<Palign=left>Localhomeinterface<Palign=left>Local<name>Home<Palign=left>LocalAccountHome<Palign=left>Localinterface<Palign=left>Local<name><Palign=left>LocalAccount<Palign=left>Abstractschema(DD)<Palign=left><name><Palign=left>AccountDDmeansthattheitemisanelementinthebeansdeploymentdescriptor.<P>Aboutthisdocument
ThisdocumentisbuiltfromtheHTMLdocumentationsavailableatjava.sun.com.Itisregularlyupdated,whennewversionsoforiginaldocumentationsbecomeavailable.TodownloadupdatesandmanyotherWinHelpandHTMLHelpJavadocumentationsforfree,visitFranckAllimantswebsite:http://www.confluent.fr/javadoc/indexe.html(inEnglish)http://www.confluent.fr/javadoc(inFrench)
没有那个大公司会傻了吧唧用.net开发大型项目，开发了，那等于自己一半的生命线被微软握着呢。而.net不行，限制在window系统，又是捆绑，鄙视微软之！ 你一定会高兴地说，哈哈，原来成为Java高手就这么简单啊!记得Tomjava也曾碰到过一个项目经理，号称Java很简单，只要三个月就可以学会。 多重继承（以接口取代）等特性，增加了垃圾回收器功能用于回收不再被引用的对象所占据的内存空间，使得程序员不用再为内存管理而担忧。在 Java 1.5 版本中，Java 又引入了泛型编程（Generic Programming）、类型安全的枚举、不定长参数和自动装/拆箱等语言特性。 学Java必读的两个开源程序就是Jive和Pet Store.。 Jive是国外一个非常著名的BBS程序，完全开放源码。论坛的设计采用了很多先进的技术，如Cache、用户认证、Filter、XML等，而且论坛完全屏蔽了对数据库的访问，可以很轻易的在不同数据库中移植。论坛还有方便的安装和管理程序，这是我们平时编程时容易忽略的一部份（中国程序员一般只注重编程的技术含量，却完全不考虑用户的感受，这就是我们与国外软件的差距所在）。 关于设计模式的资料，还是向大家推荐banq的网站 http://www.jdon.com/，他把GOF的23种模式以通俗易懂的方式诠释出来，纯Java描述，真是经典中的经典。 Pet Store.（宠物店）是SUN公司为了演示其J2EE编程规范而推出的开放源码的程序，应该很具有权威性，想学J2EE和EJB的朋友不要 错过了。 我大二，Java也只学了一年，觉得还是看thinking in java好，有能力的话看英文原版(中文版翻的不怎么好)，还能提高英文文档阅读能力。 科学超级计算机、移动电话和互联网，同时拥有全球最大的开发者专业社群。 你现在最缺的是实际的工作经验，而不是书本上那些凭空想出来的程序。 自从Sun推出Java以来，就力图使之无所不包，所以Java发展到现在，按应用来分主要分为三大块：J2SE,J2ME和J2EE,这也就是Sun ONE(Open Net Environment)体系。J2SE就是Java2的标准版，主要用于桌面应用软件的编程；J2ME主要应用于嵌入是系统开发，如手机和PDA的编程；J2EE是Java2的企业版，主要用于分布式的网络程序的开发，如电子商务网站和ERP系统。
页:
[1]