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外文原文CHAPTER3INTRODUCTIONTOSESSIONBEANSINTHISCHAPTER,WELLTAKEOURFIRSTLOOKATENTERPRISEBEANDEVELOPMENTCONCEPTSTHISCHAPTERCOVERSTHEFOLLOWINGTOPICS1WHATANENTERPRISEBEANCOMPONENTISCOMPOSEDOF,INCLUDINGTHEENTERPRISEBEANCLASS,THEREMOTEINTERFACE,THEEJBOBJECT,THEHOMEINTERFACE,THEHOMEOBJECT,THEDEPLOYMENTDESCRIPTOR,THEMANIFEST,ANDTHEEJBJARFILE2THECHARACTERISTICSOFSESSIONBEANSWELLSEEWHATMAKESSESSIONBEANSUNIQUE,ANDWELLINTRODUCETHEDIFFERENCESBETWEENSTATEFULANDSTATELESSSESSIONBEANS3THERULESFORWRITINGSESSIONBEANCLASSES4HOWTOWRITECLIENTCODETOCALLSESSIONBEANSTHISCHAPTERLAYSTHENECESSARYCONCEPTUALFRAMEWORKFORYOUTOBEGINEJBPROGRAMMINGWELLSEECOMPLETECODEEXAMPLESINCHAPTERS4,5,AND631WHATCONSTITUTESANENTERPRISEBEANENTERPRISEBEANSAREDISTRIBUTED,DEPLOYABLESERVERSIDECOMPONENTSTHATCANBEASSEMBLEDINTOLARGERAPPLICATIONSENTERPRISEBEANSCANBEPARTITIONEDACROSSMULTIPLETIERS,CANBETRANSACTIONAL,CANBEMULTIUSERSECURE,ANDCANBEDEPLOYEDINANYEJBCOMPATIBLECONTAINER/SERVERPRODUCTENTERPRISEBEANSCURRENTLYHAVETWOFLAVORSSESSIONBEANSANDENTITYBEANSSESSIONBEANSREPRESENTABUSINESSPROCESS,WHEREASENTITYBEANSREPRESENTPERMANENTBUSINESSDATASUNMICROSYSTEMSMAYINTRODUCEOTHERBEANTYPESINTHEFUTUREASWELLTHISSECTIONEXAMINESEXACTLYWHATCONSTITUTESANENTERPRISEBEANASWEWILLSEE,ANENTERPRISEBEANCOMPONENTISNOTASINGLEMONOLOTHICFILEANUMBEROFFILESWORKTOGETHERTOMAKEUPANENTERPRISEBEAN311THEENTERPRISEBEANCLASSINPARTI,WELEARNEDTHATTHEENTERPRISEJAVABEANSSPECIFICATIONDEFINESTHECONTRACTSBETWEENTHEDIFFERENTPARTIESINVOLVEDINADEPLOYMENTINORDERFORABEANTOWORKINANYCONTAINER,ANDTOWORKWITHANYCLIENTOFTHATBEAN,THEBEANMUSTADHERETOAWELLDEFINEDINTERFACEINEJB,YOUPROVIDEYOURENTERPRISEBEANCOMPONENTIMPLEMENTATIONINANENTERPRISEBEANCLASSTHISISSIMPLYAJAVACLASSTHATCONFORMSTOAWELLDEFINEDINTERFACEANDOBEYSCERTAINRULESANENTERPRISEBEANCLASSCONTAINSIMPLEMENTATIONDETAILSOFYOURCOMPONENTANDALTHOUGHTHEREARENOHARDANDFASTRULESINEJB,ASESSIONBEANIMPLEMENTATIONWILLBEVERYDIFFERENTFROMANENTITYBEANIMPLEMENTATIONFORSESSIONBEANS,ANENTERPRISEBEANCLASSTYPICALLYCONTAINSBUSINESSPROCESSRELATEDLOGIC,SUCHASLOGICTOCOMPUTEPRICES,TRANSFERFUNDSBETWEENBANKACCOUNTS,ORPERFORMORDERENTRYFORENTITYBEANS,ANENTERPRISEBEANCLASSTYPICALLYCONTAINSDATARELATEDLOGIC,SUCHASLOGICTOCHANGETHENAMEOFACUSTOMER,REDUCETHEBALANCEOFABANKACCOUNT,ORMODIFYAPURCHASEORDERTHEEJBSPECIFICATIONDEFINESAFEWSTANDARDINTERFACESTHATYOURBEANCLASSCANIMPLEMENTTHESEINTERFACESFORCEYOURBEANCLASSTOEXPOSECERTAINMETHODSTHATALLBEANSMUSTPROVIDE,ASDEFINEDBYTHEEJBCOMPONENTMODELTHECONTAINERCALLSTHESEREQUIREDMETHODSTOMANAGEYOURBEANANDALERTYOURBEANTOSIGNIFICANTEVENTSTHEMOSTBASICINTERFACETHATALLBEANCLASSESMUSTIMPLEMENTBOTHSESSIONANDENTITYISTHEJAVAXEJBENTERPRISEBEANINTERFACE,SHOWNINSOURCE31THISINTERFACESERVESASAMARKERINTERFACEIMPLEMENTINGTHISINTERFACEINDICATESTHATYOURCLASSISINDEEDANENTERPRISEBEANCLASSTHEINTERESTINGASPECTOFJAVAXEJBENTERPRISEBEANISTHATITEXTENDSJAVAIOSERIALIZABLETHISMEANSTHATALLENTERPRISEBEANSCANBECONVERTEDTOABITBLOBANDSHAREALLTHEPROPERTIESOFSERIALIZABLEOBJECTSDESCRIBEDINAPPENDIXATHISWILLBECOMEIMPORTANTLATER,SOKEEPITINMINDBOTHSESSIONBEANSANDENTITYBEANSHAVEMORESPECIFICINTERFACESTHATEXTENDTHEJAVAXEJBENTERPRISEBEANINTERFACEALLSESSIONBEANSMUSTIMPLEMENTJAVAXEJBSESSIONBEAN,WHILEALLENTITYBEANSMUSTIMPLEMENTJAVAXEJBENTITYBEANWELLSEETHEDETAILSOFTHESEINTERFACESABITLATERFORNOW,KNOWTHATYOURENTERPRISEBEANCLASSNEVERNEEDSTOIMPLEMENTTHEJAVAXEJBENTERPRISEBEANINTERFACEDIRECTLYRATHER,YOURBEANCLASSIMPLEMENTSTHEINTERFACECORRESPONDINGTOITSBEANTYPE312THEEJBOBJECTWHENACLIENTWANTSTOUSEANINSTANCEOFANENTERPRISEBEANCLASS,THECLIENTNEVERINVOKESTHEMETHODDIRECTLYONANACTUALBEANINSTANCERATHER,THEINVOCATIONISINTERCEPTEDBYTHEEJBCONTAINERANDTHENDELEGATEDTOTHEBEANINSTANCETHISHAPPENSFORMANYREASONSYOURENTERPRISEBEANCLASSCANTBECALLEDACROSSTHENETWORKDIRECTLYBECAUSEANENTERPRISEBEANCLASSISNOTNETWORKENABLEDYOUREJBCONTAINERHANDLESNETWORKINGFORYOUBYWRAPPINGYOURBEANINANETWORKENABLEDOBJECTTHENETWORKENABLEDOBJECTRECEIVESCALLSFROMCLIENTSANDDELEGATESTHESECALLSTOINSTANCESOFYOURBEANCLASSTHISSAVESYOUFROMHAVINGTOWORRYABOUTNETWORKINGISSUESTHECONTAINERPROVIDESNETWORKINGASAFREESERVICETOYOUBYINTERCEPTINGREQUESTS,THEEJBCONTAINERCANAUTOMATICALLYPERFORMSOMENECESSARYMANAGEMENTTHISINCLUDESTRANSACTIONLOGIC,SECURITYLOGIC,BEANINSTANCEPOOLINGLOGIC,ANDANYOTHERLOGICTHATTHECONTAINERMAYREQUIRETHEEJBCONTAINERCANTRACKWHICHMETHODSAREINVOKED,DISPLAYAREALTIMEUSAGEGRAPHONASYSTEMADMINISTRATORSUSERINTERFACE,GATHERDATAFORINTELLIGENTLOADBALANCING,ANDMORETHEREISNOREQUIREMENTTHATANEJBCONTAINERPERFORMTHESETASKSBUTBECAUSETHEEJBCONTAINERINTERCEPTSALLMETHODCALLS,THEREISANOPPORTUNITYFORCONTAINERSTOPERFORMTHEMTHUS,THEEJBCONTAINERISACTINGASALAYEROFINDIRECTIONBETWEENTHECLIENTCODEANDTHEBEANTHISLAYEROFINDIRECTIONMANIFESTSITSELFASASINGLENETWORKAWAREOBJECT,CALLEDTHEEJBOBJECTTHEEJBOBJECTISASURROGATEOBJECTTHATKNOWSABOUTNETWORKING,TRANSACTIONS,SECURITY,ANDMOREITISANINTELLIGENTOBJECTTHATKNOWSHOWTOPERFORMINTERMEDIATELOGICTHATTHEEJBCONTAINERREQUIRESBEFOREAMETHODCALLISSERVICEDBYABEANCLASSINSTANCEANEJBOBJECTACTSASGLUEBETWEENTHECLIENTANDTHEBEAN,ANDITEXPOSESEVERYBUSINESSMETHODTHATTHEBEANITSELFEXPO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READTHIS,THERESHOULDBEANUMBEROFTOOLSAVAILABLETOAUTOGENERATEEJBJARFILES,SUCHASJAVAIDESYOUCANALSOGENERATETHESEFILESYOURSELFWELLSHOWYOUHOWINCHAPTER4ONCEYOUVEMADEYOUREJBJARFILE,YOURENTERPRISEBEANISCOMPLETE,ANDITISADEPLOYABLEUNITWITHINANAPPLICATIONSERVERWHENTHEYAREDEPLOYEDPERHAPSAFTERBEINGPURCHASED,THETOOLSTHATEJBCONTAINERVENDORSSUPPLYARERESPONSIBLEFORDECOMPRESSINGANDREADINGANDEXTRACTINGTHEINFORMATIONCONTAINEDWITHINTHEEJBJARFILEFROMTHERE,THEDEPLOYERHASTOPERFORMVENDORSPECIFICTASKS,SUCHASGENERATINGEJBOBJECTS,GENERATINGHOMEOBJECTS,IMPORTINGYOURBEANINTOTHECONTAINER,ANDTUNINGTHEBEANSUPPORTFOREJBJARFILESISASTANDARD,REQUIREDFEATURETHATALLEJBTOOLSSUPPORT中文翻译第三章了解会话BEAN在这一章，我们先介绍一下ENTERPRISEBEAN的开发概念。本章有以下概念1ENTERPRISEBEAN组件的内容，包括实现组件，远程接口，EJB对象，本地接口，本地对象，部署描述文件，清单和EJBJAR文件。2会话BEAN的特性。我们将了解什么使会话BEAN成为独一无二的，我们还会介绍有状态会话BEAN和无状态会话BEAN的不同。3写会话BEAN的规则4怎样写的客户端代码去调用会话BEAN31什么组成了ENTERPRISEBEANENTERPRISEBEAN是分散部署在服务端的组件，这种组件能够装载到大型应用程序中。ENTERPRISEBEAN是一种可以被分配到多行，相互作用，多用户安全执行，部署到任何EJB容器/服务产品中。ENTERPRISEBEAN一般有两种会话BEAN和实体BEAN。会话BEAN是一种商业程序，然而实体BEAN是一种永久的商业数据。SUN公司将来也会提供其他类型的BEAN。这部分确切地检测ENTERPRISEBEAN的组成。我们将会看到，ENTERPRISEBEAN不是一个单独的整体文件大量的文件在一起协调工作才组成了ENTERPRISEBEAN311实现组件在第一部分，我们学习了ENTERPRISEJAVABEAN规格定义了开发中不同部分中的合约。为了让BEAN能够在任何容器中运行，在任何客户端下运行，BEAN必须继承定义好的接口。在EJB中，你要在实现组件中实现你的ENTERPRISEBEAN这仅仅是一个遵循定义好的接口和听从某些规则的JAVA类。一个ENTERPRISEBEAN包含了你组件中的实现的细节。尽管EJB中没有必须遵守的规则，会话BEAN的实现和实体BEAN的实现还是非常不同的。因为会话BEAN和ENTERPRISEBEAN包含了典型的商业程序逻辑，就像计算价格的逻辑，转移不同银行账户中的资金的逻辑。对于实体BEAN，ENTERPRISEBEAN包含了与数据有关的逻辑，像更改客户名字的逻辑，减少银行账户差额的逻辑，或修改购物订单的逻辑。EJB规格定义了一些你的BEAN类可以实现的接口标准。这些接口使你的类不得不暴露一些可以供所有类都可以继承的接口，就像定义EJB组件模式那样。容器调用这些要求的方法管理你的类并留意你类中有意义的方法。所有类必须实现的基本接口是JAVAXEJBENTERPRISEBEAN接口。这个接口服务就像制造商接口；实现这个接口说明你的类确实是一个ENTERPRISEBEAN类。这个JAVAXEJVENTERPRISEBEAN的有趣的地方是从JAVAIOSERIALIZABLE中继承来的。这意味着所有的ENTERPRISEBEAN都能被转化成块，并分享所有序列化对象的部分。这在以后很重要，所以不要忘记。会话BEAN和实体BEAN都有从JAVAXEJBENTERPRISEBEAN接口继承来的特殊接口。所有的会话BEAN都必须实现JAVAX,EJB,SESSIONBEAN，同样的，所有的实体BEAN都必须实现JAVAXEJBENTITYBEAN。我们稍后将会了解这些接口的详细内容。至此，要知道你的ENTERPRISEBEAN类根本不需要直接实现JAVAXEJBENTERPRISEBEAN接口，只要知道你的BEAN类实现了与它相同类型BEAN的接口就可以了。312EJB对象当一个客户想要使用ENTERPRISEBEAN类的实例时，这个客户不需要直接调用真实的BEAN接口。只要这个调用被EJB容器截断，它就会被这个BEAN实例所代表。这种情况有很多原因你的ENTERPRISEBEAN类不能被网络直接呼叫，因为ENTERPRISEBEAN类不是网络化的类。你的EJB容器为你控制网络，通过一个可以网络化的对象包裹你的BEAN这种方法。这个网络化的对象从客户那里接受呼叫并把这些呼叫转换成你BEAN类中的对象。这使你不用再担心网络问题（这种容器把网络作为一种免费服务提供给你）。通过截断请求，EJB容器可以自动产生某些管理手段。包括处理逻辑问题，安全逻辑，BEAN实例联合逻辑，以及任何其他的容器有可能要求的逻辑。EJB容器能追踪方法和调用，在系统管理员接口上显示真实的时间习惯图表，为智能平衡载入收集数据等等。EJB容器形成这些任务并不需要特别请求。但是因为EJB容器截断了所有的方法呼叫，容器就有机会产生这些任务。因此，EJB容器在客户端代码和BEAN之间扮演者间接律师的角色。这个间接的律师本身是一个网络化的对象，叫做EJB对象。这个EJB对象是网络，装载，安全等对象的替代品。这种智能对象知道怎样形成EJB容器的间接逻辑。EJB对象在客户端和BEAN之间起着粘合剂的作用，他暴露出每一个BEAN本身所暴露的商业方法。EJB对象可以代表所有的客户端请求并传递给BEAN。你应该把EJB对象考虑成容器的一个物理组成部分，所有EJB对象内部都有容器特别代码。因为每一个BEAN的EJB对象都是不同的，你的容器厂商提供了可以为你的EJB对象自动产生类文件的粘贴代码工具。1远程接口就像我们以前提到的那样，BEAN客户端调用EJB对象上的方法，而不是BEAN本身。为了形成这些方法，EJB对象必须克隆每一个你的BEAN本身暴露的商业方法。但是这个工具是怎样产生知道该克隆哪一个方法的EJB对象呢答案是在一个BEAN提供者所写的特别接口中。这个接口复制了与之对应BEAN的所有暴露的商业方法。这个接口叫做远程接口。远程接口必须遵守EJB规范中所定义的特别规则。例如，所有的远程接口必须起源于