JAVA网页编程之An Overview of RMI Applications

你通过从书的数量和开发周期及运行速度来证明：net和ruby要比java简单。application
AnOverviewofRMIApplications

RMIapplicationsareoftencomprisedoftwoseparateprograms:aserverandaclient.Atypicalserverapplicationcreatessomeremoteobjects,makesreferencestothemaccessible,andwaitsforclientstoinvokemethodsontheseremoteobjects.Atypicalclientapplicationgetsaremotereferencetooneormoreremoteobjectsintheserverandtheninvokesmethodsonthem.RMIprovidesthemechanismbywhichtheserverandtheclientcommunicateandpassinformationbackandforth.Suchanapplicationissometimesreferredtoasadistributedobjectapplication.

Distributedobjectapplicationsneedto

Locateremoteobjects:Applicationscanuseoneoftwomechanismstoobtainreferencestoremoteobjects.AnapplicationcanregisteritsremoteobjectswithRMIssimplenamingfacility,thermiregistry,ortheapplicationcanpassandreturnremoteobjectreferencesaspartofitsnormaloperation.

Communicatewithremoteobjects:DetailsofcommunicationbetweenremoteobjectsarehandledbyRMI;totheprogrammer,remotecommunicationlookslikeastandardJavamethodinvocation.

Loadclassbytecodesforobjectsthatarepassedaround:BecauseRMIallowsacallertopassobjectstoremoteobjects,RMIprovidesthenecessarymechanismsforloadinganobjectscode,aswellasfortransmittingitsdata.

ThefollowingillustrationdepictsanRMIdistributedapplicationthatusestheregistrytoobtainareferencetoaremoteobject.Theservercallstheregistrytoassociate(orbind)anamewitharemoteobject.Theclientlooksuptheremoteobjectbyitsnameintheserversregistryandtheninvokesamethodonit.TheillustrationalsoshowsthattheRMIsystemusesanexistingWebservertoloadclassbytecodes,fromservertoclientandfromclienttoserver,forobjectswhenneeded.





Thislessoncontainsthefollowingsections:

AdvantagesofDynamicCodeLoading

RemoteInterfaces,Objects,andMethods

CreatingDistributedApplicationsUsingRMI

BuildingaGenericComputeEngine

AdvantagesofDynamicCodeLoading

AdvantagesofDynamicCodeLoading

OneofthecentralanduniquefeaturesofRMIisitsabilitytodownloadthebytecodes(orsimplycode)ofanobjectsclassiftheclassisnotdefinedinthereceiversvirtualmachine.Thetypesandthebehaviorofanobject,previouslyavailableonlyinasinglevirtualmachine,canbetransmittedtoanother,possiblyremote,virtualmachine.RMIpassesobjectsbytheirtruetype,sothebehaviorofthoseobjectsisnotchangedwhentheyaresenttoanothervirtualmachine.Thisallowsnewtypestobeintroducedintoaremotevirtualmachine,thusextendingthebehaviorofanapplicationdynamically.ThecomputeengineexampleinthischapterusesRMIscapabilitytointroducenewbehaviortoadistributedprogram.

RemoteInterfaces,Objects,andMethods

Likeanyotherapplication,adistributedapplicationbuiltusingJavaRMIismadeupofinterfacesandclasses.Theinterfacesdefinemethods,andtheclassesimplementthemethodsdefinedintheinterfacesand,perhaps,defineadditionalmethodsaswell.Inadistributedapplicationsomeoftheimplementationsareassumedtoresideindifferentvirtualmachines.Objectsthathavemethodsthatcanbecalledacrossvirtualmachinesareremoteobjects.

Anobjectbecomesremotebyimplementingaremoteinterface,whichhasthefollowingcharacteristics.

Aremoteinterfaceextendstheinterfacejava.rmi.Remote.

Eachmethodoftheinterfacedeclaresjava.rmi.RemoteExceptioninitsthrowsclause,inadditiontoanyapplication-specificexceptions.

RMItreatsaremoteobjectdifferentlyfromanonremoteobjectwhentheobjectispassedfromonevirtualmachinetoanother.Ratherthanmakingacopyoftheimplementationobjectinthereceivingvirtualmachine,RMIpassesaremotestubforaremoteobject.Thestubactsasthelocalrepresentative,orproxy,fortheremoteobjectandbasicallyis,tothecaller,theremotereference.Thecallerinvokesamethodonthelocalstub,whichisresponsibleforcarryingoutthemethodcallontheremoteobject.

Astubforaremoteobjectimplementsthesamesetofremoteinterfacesthattheremoteobjectimplements.Thisallowsastubtobecasttoanyoftheinterfacesthattheremoteobjectimplements.However,thisalsomeansthatonlythosemethodsdefinedinaremoteinterfaceareavailabletobecalledinthereceivingvirtualmachine.

CreatingDistributedApplicationsUsingRMI

WhenyouuseRMItodevelopadistributedapplication,youfollowthesegeneralsteps.

Designandimplementthecomponentsofyourdistributedapplication.

Compilesourcesandgeneratestubs.

Makeclassesnetworkaccessible.

Starttheapplication.

DesignandImplementtheApplicationComponents

First,decideonyourapplicationarchitectureanddeterminewhichcomponentsarelocalobjectsandwhichonesshouldberemotelyaccessible.Thisstepincludes:

Definingtheremoteinterfaces:Aremoteinterfacespecifiesthemethodsthatcanbeinvokedremotelybyaclient.Clientsprogramtoremoteinterfaces,nottotheimplementationclassesofthoseinterfaces.Partofthedesignofsuchinterfacesisthedeterminationofanylocalobjectsthatwillbeusedasparametersandreturnvaluesforthesemethods;ifanyoftheseinterfacesorclassesdonotyetexist,youneedtodefinethemaswell.

Implementingtheremoteobjects:Remoteobjectsmustimplementoneormoreremoteinterfaces.Theremoteobjectclassmayincludeimplementationsofotherinterfaces(eitherlocalorremote)andothermethods(whichareavailableonlylocally).Ifanylocalclassesaretobeusedasparametersorreturnvaluestoanyofthesemethods,theymustbeimplementedaswell.

Implementingtheclients:Clientsthatuseremoteobjectscanbeimplementedatanytimeaftertheremoteinterfacesaredefined,includingaftertheremoteobjectshavebeendeployed.

CompileSourcesandGenerateStubs

Thisisatwo-stepprocess.Inthefirststepyouusethejavaccompilertocompilethesourcefiles,whichcontaintheimplementationoftheremoteinterfacesandimplementations,theserverclasses,andtheclientclasses.Inthesecondstepyouusethermiccompilertocreatestubsfortheremoteobjects.RMIusesaremoteobjectsstubclassasaproxyinclientssothatclientscancommunicatewithaparticularremoteobject.

MakeClassesNetworkAccessible

Inthisstepyoumakeeverything--theclassfilesassociatedwiththeremoteinterfaces,stubs,andotherclassesthatneedtobedownloadedtoclients--accessibleviaaWebserver.

StarttheApplication

StartingtheapplicationincludesrunningtheRMIremoteobjectregistry,theserver,andtheclient.

Therestofthislessonwalksthroughthestepstocreateacomputeengine.

BuildingaGenericComputeEngine

Thistrailfocusesonasimpleyetpowerfuldistributedapplicationcalledacomputeengine.Thecomputeengine,aremoteobjectintheserver,takestasksfromclients,runsthem,andreturnsanyresults.Thetasksarerunonthemachinewheretheserverisrunning.Thissortofdistributedapplicationcouldallowanumberofclientmachinestomakeuseofaparticularlypowerfulmachineoronethathasspecializedhardware.

Thenovelaspectofthecomputeengineisthatthetasksitrunsdonotneedtobedefinedwhenthecomputeengineiswritten.Newkindsoftaskscanbecreatedatanytimeandthengiventothecomputeenginetoberun.Allthatisrequiredofataskisthatitsclassimplementaparticularinterface.Suchataskcanbesubmittedtothecomputeengineandrun,eveniftheclassthatdefinesthattaskwaswrittenlongafterthecomputeenginewaswrittenandstarted.ThecodeneededtoaccomplishthetaskcanbedownloadedbytheRMIsystemtothecomputeengine,andthentheenginerunsthetask,usingtheresourcesonthemachineonwhichthecomputeengineisrunning.

TheabilitytoperformarbitrarytasksisenabledbythedynamicnatureoftheJavaplatform,whichisextendedtothenetworkbyRMI.RMIdynamicallyloadsthetaskcodeintothecomputeenginesJavavirtualmachineandrunsthetaskwithoutpriorknowledgeoftheclassthatimplementsthetask.Anapplicationlikethis,whichhastheabilitytodownloadcodedynamically,isoftencalledabehavior-basedapplication.Suchapplicationsusuallyrequirefullagent-enabledinfrastructures.WithRMIsuchapplicationsarepartofthebasicmechanismsfordistributedcomputingontheJavaplatform



不得不提一下的是：.net是看到java红，而开发出来的工具。

是一种将安全性(Security)列为第一优先考虑的语言

Jive的资料在很多网站上都有，大家可以找来研究一下。相信你读完代码后，会有脱胎换骨的感觉。遗憾的是Jive从2.5以后就不再无条件的开放源代码，同时有licence限制。不过幸好还有中国一流的Java程序员关注它，外国人不开源了，中国人就不能开源吗？这里向大家推荐一个汉化的Jive版本—J道。Jive(J道版)是由中国Java界大名 鼎鼎的banq在Jive 2.1版本基础上改编而成, 全中文，增加了一些实用功能，如贴图，用户头像和用户资料查询等，而且有一个开发团队在不断升级。你可以访问banq的网站

是一种使网页(Web Page)产生生动活泼画面的语言

Java语言支持Internet应用的开发，在基本的Java应用编程接口中有一个网络应用编程接口（java net），它提供了用于网络应用编程的类库，包括URL、URLConnection、Socket、ServerSocket等。Java的RMI（远程方法激活）机制也是开发分布式应用的重要手段。

你快去找一份Java的编程工作来做吧（如果是在校学生可以去做兼职啊），在实践中提高自己，那才是最快的。不过你得祈祷在公司里碰到一个高手，而且他 还愿意不厌其烦地教你，这样好象有点难哦！还有一个办法就是读开放源码的程序了。我们知道开放源码大都出自高手，他们设计合理，考虑周到，再加上有广大的程序员参与，代码的价值自然是字字珠叽，铿锵有力（对不起，偶最近《金装四大才子》看多了）。

不过，每次的执行编译后的字节码需要消耗一定的时间，这同时也在一定程度上降低了 Java 程序的运行效率。

Sun公司看见Oak在互联网上应用的前景，于是改造了Oak，于1995年5月以Java的名称正式发布。Java伴随着互联网的迅猛发展而发展，逐渐成为重要的网络编程语言。

吧，现在很流行的Structs就是它的一种实现方式，不过Structs用起来实在是很繁，我们只要学习其精髓即可，我们完全可以设计自己的MVC结构。然后你再研究一下软件Refactoring (重构)和极限XP编程，相信你又会上一个台阶。 做完这些，你不如整理一下你的Java代码，把那些经典的程序和常见的应用整理出来，再精心打造一番，提高其重用性和可扩展性。你再找几个志同道合的朋友成立一个工作室吧

是一种突破用户端机器环境和CPU

仓酷云

《JAVA语言程序设计》或《JAVA从入门到精通》这两本书开始学，等你编程有感觉的时候也可以回看一下。《JAVA读书笔记》这本书，因为讲的代码很多，也很容易看懂，涉及到面也到位。是你学习技术巩固的好书，学完后就看看《JAVA编程思想》这本书，找找一个自己写的代码跟书上的代码有什么不一样。

所以现在应用最广泛又最好学的就是J2EE了。 J2EE又包括许多组件，如Jsp,Servlet,JavaBean,EJB,JDBC,JavaMail等。要学习起来可不是一两天的事。那么又该如何学习J2EE呢？当然Java语法得先看一看的，I/O包，Util包，Lang包你都熟悉了吗？然后再从JSP学起。

Java 不同于一般的编译执行计算机语言和解释执行计算机语言。它首先将源代码编译成二进制字节码（bytecode），然后依赖各种不同平台上的虚拟机来解释执行字节码。从而实现了“一次编译、到处执行”的跨平台特性。

《JAVA语言程序设计》或《JAVA从入门到精通》这两本书开始学，等你编程有感觉的时候也可以回看一下。《JAVA读书笔记》这本书，因为讲的代码很多，也很容易看懂，涉及到面也到位。是你学习技术巩固的好书，学完后就看看《JAVA编程思想》这本书，找找一个自己写的代码跟书上的代码有什么不一样。

Java 编程语言的风格十分接近C、C++语言。

不过，每次的执行编译后的字节码需要消耗一定的时间，这同时也在一定程度上降低了 Java 程序的运行效率。

让你能够真正掌握接口或抽象类的应用，从而在原来的Java语言基础上跃进一步，更重要的是，设计模式反复向你强调一个宗旨：要让你的程序尽可能的可重用。

你就该学一学Servlet了。Servlet就是服务器端小程序，他负责生成发送给客户端的HTML文件。JSP在执行时，也是先转换成Servlet再运行的。虽说JSP理论上可以完全取代Servlet，这也是SUN推出JSP的本意，可是Servlet用来控制流程跳转还是挺方便的，也令程序更清晰。接下来你应该学习一下Javabean了，可能你早就看不管JSP在HTML中嵌Java代码的混乱方式了，这种方式跟ASP又有什么区别呢？

吧，现在很流行的Structs就是它的一种实现方式，不过Structs用起来实在是很繁，我们只要学习其精髓即可，我们完全可以设计自己的MVC结构。然后你再研究一下软件Refactoring (重构)和极限XP编程，相信你又会上一个台阶。 做完这些，你不如整理一下你的Java代码，把那些经典的程序和常见的应用整理出来，再精心打造一番，提高其重用性和可扩展性。你再找几个志同道合的朋友成立一个工作室吧