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)

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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。

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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）。

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仓酷云

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。

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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。