JAVA网站制作之Remote scripting using a servlet
j2EE和asp比较，其实也没什么比的，原因和我上面说那些比较差不了多少，也是稳定性，安全性，J2EE比asp高，速度上比不过asp，asp也是延续着它的拖拽控件的方法，提高速度。servletErikHatcher(erik@hatcher.net)President,eHatcherSolutions,Inc.01Feb2001
TheusersofWebapplicationshavesufferedadramaticshiftinexperiencesfromtheworldofdesktopapplications.ManyWebapplicationsdonotatallmimictheusability,interactivity,anddynamicnaturethatisavailableintypicalstandaloneorclient-serverdesktopapplicationsbecauseoftheconstraintsthatHTMLandHTTPimpose.Here,ErikHatcherexplainshowremotescriptingcanbeusedtoenhancetheinteractivityanddynamicnatureofaWebapplicationexperience.
OneofthemajordrawbackstoWebapplicationsisthattheuserexperienceistypicallyinferiortothatofdesktopapplications.MostcurrentWebapplicationslackininteractivitybecauseoncethebrowserreceivesaresponsetoitsURLrequestitisonitsown,failingtocommunicatebacktotheserveruntilahyperlinkisclickedoraformissubmitted.TechniquessuchasusingJavaScriptandDHTMLcanbeusedtomakethebrowserfeelmorelikeadesktopapplication;fanciertechniqueslikeusingFlash,ActiveX,andJavaappletscanalsoaccomplishthisend.
Butevenwiththesenewertechniquesandtechnologies,theclientisstillmostlyonitsownafteritreceivesthecontentfromtheWebserver.ThetechniquedescribedinthisarticleoffersasolutionthatallowsthebrowserandWebservertocommunicatebehindthescenes.ThebrowsercaninvokeremoteJavaservletmethodsthatenabletheuserexperiencetoresemblethatofadesktopapplication,suchaspopulatingadrop-downboxdynamicallybasedontheselectionofarelateddrop-downbox(thatis,category/subcategory),orpollingtheserverformessagesandrefreshingthedisplaydynamicallywithcontinuouslyupdatedcontent.
ClientsideTherearetwopopularwaysofaccomplishingremotemethodinvocationfromaWebbrowser:Microsoftremotescripting(MSRS)andBrentAshleysJavaScriptremotescripting(JSRS).Thegoalofbothisthesame:toinvokeremotemethodsandreturntheresultstothebrowser.BothmethodswereoriginallydesignedtocommunicatewithremotemethodsdefinedinMicrosoftsActiveServerPages.ThisarticleprovidesawayforbothofthesemethodstocommunicatewithaJavaservletontheserverside.Letsgetintothearchitecturedetailsofeachofthesetechniques.
RedmondscriptingMicrosoftsremotescriptingispartoftheVisualInterDevdevelopmentenvironment.Itconsistsofthreepieces:aninvisibleJavaapplet,client-sideJavaScript,andserver-sideJavaScriptrunninginActiveServerPages.TheJavaapplethandlesthecommunicationswiththeserver.Theclient-sideJavaScriptcommunicateswiththeapplet.Theserver-sideJavaScriptdealswithtakingtherequestedparametersanddispatchingthemtothespecifiedserver-sidemethod.ThecommunicationsaresimplyHTTPGETrequestsandresponses,withthedetailsofthemethodcallsentasqueryparameterstotheserver-sidescript.ThereismuchmoretobesaidaboutMicrosoftsremotescriptingarchitecture,butthatisbeyondthescopeofthisarticle(seeResources).
Figure1showsthearchitectureofMicrosoftremotescriptingusingaservletontheserver-side.
StepsinvolvedininvokingaremotemethodusingMicrosoftremotescripting:
ThebrowserexecutesaJavaScriptcalltoRSExecute(thisresidesinanincludedJavaScriptframeworkprovidedbyMicrosoftVisualInterDev).TheremotescriptingappletusesHTTPGETtoaccessaspecialservletURLontheserver,completewithmethodnameandparameters.TheservletreturnsitsXML-likeresponseandtheappletreceivesit.TheresponseisinterpretedbytheremotescriptingJavaScriptandreturnedtothecallingcode.Acallobjectisreturned,andtheactualreturnvalueisthe.return_valuepropertyifthemethodisreturnedsuccessfully.
Figure1.MicrosoftRemoteScriptingArchitecture
JSRSArchitectureBrentAshleysJavaScriptremotescriptingaccomplishesthesamegoalusinganiftyDHTMLtrickofinjectingahidden<IFRAME>or<LAYER>(dependingonthebrowsertype)foreachconcurrentremotescriptingcallmade.ThehiddenpieceisnavigatedtotheremotescriptingURLusingHTTPGET.TheresultreturnedfromtheserverisHTMLwithanonLoadJavaScriptcalltothemainwindowcallbackfunction.
Figure2showsthearchitectureofJavaScriptremotescriptingusingaservletontheserver-side.
StepsinvolvedininvokingaremotemethodusingJavaScriptremotescripting:
ThebrowserexecutesaJavaScriptcalltojsrsExecute(thisresidesintheexternaljsrsClient.jsfile).CodeinjsrsClient.jscreatesan<IFRAME>or<LAYER>(orre-usesanexistingone)andnavigatesittoaURLwiththeappropriateparameters.TheservletreturnsitsHTMLresponseandtheclientreceivesit.The<BODY>onLoadofthereturnedHTMLinvokesthespecifiedcallbackwiththereturnedvalue.
Note:TheexampleinthisarticleshowsMicrosoftsremotescriptingbeingusedsynchronously,butitcouldalsobeusedasynchronouslywithacallbackliketheJSRSexample.However,JSRSisnotcapableofmakingsynchronousmethodinvocations.
Figure2.BrentAshleysJavaScriptremotescriptingarchitecture
LetsseesomecodeTheservletdescribedbelowhasbeendesignedtosupportbothMSRSandJSRS.Thisflexibilitywillbedemonstratedbyallowingtheclienttotogglebetweenusingeithermethod.AsingleHTMLpageiscreatedwithboththeMSRSpieces(JavaScriptandapplet)andtheJSRS(asingleexternalJavaScript)piece.Usingthecategory/subcategoryidea,thegoalistohaveacategoryselectionwhichthendetermineswhichsubcategoryselectionsareavailable.HereistheHTML<BODY>:
Listing1.Categoryselection
<BODYonLoad="javascript:categoryChanged()"><FORMname="form1"><TABLE><TR><TH>RemoteScriptingType:</TH><TD><inputtype="radio"name="clientType"value="MSRS">MSRS<br/><inputtype="radio"name="clientType"value="JSRS"CHECKED>JSRS</TD></TR><TR><TH>Category:</TH><TD><SELECTname="category"onChange="javascript:categoryChanged()"><OPTIONvalue="0"SELECTED>Category0</OPTION><OPTIONvalue="1">Category1</OPTION><OPTIONvalue="2">Category2</OPTION><OPTIONvalue="3">ErrorTest</OPTION></SELECT></TD></TR><TR><TH>Subcategory:</TH><TD><SELECTname="subcategory"><!--Needaplaceholderuntilitcangetloaded--><OPTIONvalue="-1"SELECTED>---------------------</OPTION></SELECT></TD></TR></TABLE></FORM></BODY>
EverythingabouttheHTMLisstraightforward.NotethatcategoryChangediscalledwhenthedocumentisloadedandwhenthecategoryfieldischangedbytheuser.ThecategoryChangedmethodisdefinedas:
Listing2.categoryChangedmethod
functioncategoryChanged(){if(document.form1.clientType.checked){//MSRSvarco=RSExecute("/servlet/RSExample","getSubcategories",document.form1.category.options.value);if(co.status!=0){return;}varsubcatstr=co.return_value;populateDropDown(subcatstr);}else{//JSRSjsrsExecute("/servlet/RSExample",populateDropDown,"getSubcategories",document.form1.category.options.value);}}
RemotescriptingnowcomesintoplaybycallingtheremotemethodgetSubcategories.RefertotheMicrosoftremotescriptingdocumentationfordetailsoftheremotescripting"callobject"returnedfromRSExecute(seeResources).ThecalltojsrsExecutespecifiesthatpopulateDropDownwillbecalledwhentheasynchronouscallcompletes.RefertoResourcesforthedetailsofpopulateDropDown.
Ontheserverside,theservletisdefinedasinListing3.
RSExampleservlethasastaticpublicmethodnamed,notcoincidentally,getSubcategories.Thisimplementationis,ofcourse,simplyaproof-of-concept,butitcouldeasilybeextendedtodoadatabaselookupofthesubcategoriesrelatingtoacategory.NotethatthegetSubcategoriesfunctioncanonlydealwithacatidof0,1,or2.Thesamplecodeincludesademonstrationofinvokingthismethodwithaninvalidvalueof3("ErrorTest"inthecategorydrop-down).Usingremotescriptingisusefulwhenthecategory/subcategorycombinationsaretoonumeroustosendtothebrowserasJavaScriptobjects.Anynumberofmethodscouldbedefinedinthisservletandcalledsimilarlyfromtheclient.
Thesubcategoriesarereturnedtotheclientintheformat"index,value;index,value...."Ideally,thiskindofinformationshouldbepassedusingXML,butinordertomakethisschemebeascross-browserfriendlyaspossible,XMLisnottherightchoice.IftheapplicationenvironmentisconfinedtoInternetExplorer5browsers,thenXMLwouldbeaveryelegantwaytopassinformationwithremotescripting.
Inordertomaketheservletarchitectureasextensibleaspossible,anabstractclassthatextendsHttpServletwaswrittentodispatchthemethodinvocationgenericallyandpackagethereturnvalueinawaytheremotescriptingclient-sidecodeisbuilttohandle.TheHTTPGETmadebyMSRSandJSRSlookslikethecodeinListing4.
BecausetheURLsareuniqueenoughtoallowtheservlettodistinguishbetweenthetwodifferentclient-sidetechniques,asingleabstractservletclasswascreatedthatdynamicallyhandlesbothmethods.Toshowabitofthedetailsofhowthetwoclient-sideremotescriptingmethodsworkinternally;Listing5showstheformatthattheMicrosoftclient-sideremotescriptingpieceisexpectingastheresponsetothatrequest.
AtfirstglanceitwouldappearthatXMLisbeingusedbehindthescenes,butnotethatthevalueoftheTYPEattributeoftheRETURN_VALUEelementisnotdouble-quotedandthereforedoesnotfollowtheXMLspecification.Welljustshakeourheadsatthatapparentoversightandmoveon.TheVERSIONattributeoftheMETHODelementisignoredbytheclient-sidecode,butitwasleftinjusttokeepthingsconsistentwiththereturnvaluesfromMicrosoftsserver-sideimplementationinASP.ThevalueoftheRETURN_VALUEelementisURL-encoded.Thisvalueisunescapedintheclient-sideframeworkautomatically(notespacesreplacedby"%20"ratherthan"+,"asthatishowtheJavaScriptunescapefunctionrequires).Theclient-sideJavaScriptjustusestextfunctionstofindthenecessaryinformationintheresponseandbuildsthe"callobject"accordingly,whichisthenreturnedfromRSExecutetothecallingcode.
TheservletsimilarlybuildstheresponsetoJSRScallsintheformatthatitexpects.ThereturnvalueisshowninListing6.
TheheartofthemethodinvocationoccursinthedoGetmethodofRemoteScriptingServlet(asubclassofHttpServlet).HereistherelevantpartofdoGet:
Listing7.doGet
Stringmethod;intpcount=0;callbackName=request.getParameter("C");if(callbackName!=null){//clientisJSRS-itpassesa"C"parameterclientType=JSRS;method=request.getParameter("F");//JSRSdoesnttellushowmanyparameters,socountthemwhile(request.getParameter("P"+pcount)!=null)pcount++;}else{clientType=MSRS;method=request.getParameter("_method");pcount=Integer.parseInt(request.getParameter("pcount"));}//...somecodeomitted,refertothefullcodeincluded//findandinvoketheappropriatestaticmethodintheconcreteclassClassc=this.getClass();Methodm=c.getMethod(method,paramSpec);returnValue=(String)m.invoke(null,params);
RefertotheincludedcodeforthefullRemoteScriptingServletclass(seeListing8).Firsttheclienttypeisdetermined,keyedoffthe"C"parameterthatispresentforJSRS,butnotforMSRS.ThentheideaistogetareferencetotheClassoftheservletbeinginvoked(RSExample,inthiscase),getareferencetotheMethodbeinginvoked(getSubcategories)basedonthenumberofparameters,andtheninvokethatmethodwiththeappropriateparameters.Theparameterstothedispatchedmethodsmustmatchinnumberwiththecallfromtheclient,andmustallbeoftypeString(internallytothatmethod,theStringscanbeconvertedtoothertypesifnecessary).Dispatchingtostaticmethodswaschosenarbitrarily,butitcouldeasilydispatchtoinstancemethodsratherthanclassmethodsbyspecifying"this"asthefirstparametertoinvoke.Theentirecodeaboveiswrappedinsideatry/catchblock,andanyexceptionthrownwillbegracefullysentbacktotheclient.MSRSerrorsarereturnedbysettingtheTYPEattributeofRETURN_VALUEtoERROR,andspecifyingtheescapederrortextasthevalueofRETURN_VALUE.JSRSerrorsarereturnedasHTMLthatexecutesjsrsErrorintheonLoad.
Nowwhat?Thereareallkindsofinterestingthingsthatcanbeaccomplishedusingremotescripting.TakingadvantageoftheasynchronousfeatureofremotescriptingandJavaScriptwindowtimers,amessagingsystemcouldbebuilttoallowabrowsertopolltheserverformessages,content,orothertypesofupdateinformation.
IssuesBecauseofthetechnologiesusedinremotescripting,therearelimitationsonthenumberofbrowsersitwillworkwith.TouseMSRS,JavaScriptmustcommunicatewiththehiddenapplettoperformtheremotecall.TheJavaVirtualMachine(JVM)isonly"scriptable"(suchthatJavaScriptandJavaappletscancommunicate)onalimitednumberofbrowsers,meaningonlyNetscapeandInternetExploreronWin32(andpossiblyonotherplatforms)cansupportMSRS.BecauseallparametersareencodedintheURLforanHTTPGETrequest,thereisalimitationtothesizeoftheparameters.AmodifiedappletexiststhatdoesHTTPPOSTratherthanGETtoeliminatethislimitation.BrentAshleyssitehasthePOSTversionoftheappletavailablefordownload(seeResources.
Scalabilityissuesariseifremotescriptingisusedforcontinuouspolling.Dependingonanapplicationsneeds,thefrequencyofrequestscanbereducedorevenmadedynamicsuchthatitpollslessfrequentlyifitreceivesfewermessagesandincreasesthefrequencywhenlargersetsofmessagesarereceived.SinceHTTPGETisusedforthemethodcall,thesizeofthedatasenttothemethodisrestricted.Theresponsetoaremotemethodhasnoexplicitsizerestrictionsonsize;however,iflargeamountsofdataperrequestarebeingpassedtoorfromtheserver,thismessagingarchitectureisnotappropriate.
Thesecurityoftheremotescriptingmethodsshouldbeconsidered.BysimplyopeningtheappropriateURLalongwithitsnecessaryparametersinaWebbrowser,theremotemethodisbeingcalledanditsresultsarebeingsentback.Itmaybenecessarytoassurethattheuserisloggedintotheapplicationbeforereturninganyresults.Therearesecurityrestrictionsthatdonotallowabrowsertousecross-hostscripting,sothatthehostforthemainHTMLpagemustbethesameasthehostfortheremotescriptingservlet.
FirewallsarenotanissueforcommunicationsbecauseHTTPisused,althoughinsomeenvironmentsJavaappletsareblockedbyfirewallsthatwouldpreventMicrosoftsremotescriptingfromworking.HTTPSwillalsoworkfinewithbothclient-sidemethods.
ConclusionRemotescriptingisagreattechniqueforcreatingamoredesktop-likefeelforWebbrowser-basedapplications.TheRemoteScriptingServletbaseclassprovidedwiththisarticleopensthedoorforserver-sideJavaapplicationstousetrickspreviouslyonlyavailabletoActiveServerPageapplications.
Resources
ReadMicrosoftsdocumentationonremotescripting.DownloadBrentAshleysJSRS.GetthedetailsofpopulateDropDownDownloadwiththisexamplecode(includingtheRemoteScriptingServletclass).LearnthebeautyofJavasreflectionAPI.ApachesTomcatwasusedtotestthisservlet.
AbouttheauthorErikisthePresidentofeHatcherSolutions,Inc.HealsorecentlyjoinedPromoFuelastheirHighOctaneSiteArchitect.BeingactivelyinvolvedinsoftwaredeveloperorganizationsinTucson,AZ,hehaspresentedonWindowsNTsecurityanalysisattheBackOfficeAdministratorsConferenceandonXMLfortheTucsonDeveloperSeries.Youcanreachhimaterik@hatcher.net.
而学习JAVA我觉得最应该避免的就是：只学习，不思考，只记忆，不实践！ 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 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。 http://www.jdon.com/去下载，或到同济技术论坛的服务器ftp://nro.shtdu.edu.cn去下，安装上有什么问题，可以到论坛上去提问。 是一种将安全性(Security)列为第一优先考虑的语言 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？ 是一种由美国SUN计算机公司(Sun Microsystems, Inc.)所研究而成的语言 当然你也可以参加一些开源项目，一方面可以提高自己，另一方面也是为中国软件事业做贡献嘛！开发者在互联网上用CVS合作开发，用QQ,MSN,E-mail讨论联系，天南海北的程序员分散在各地却同时开发同一个软件，是不是很有意思呢？ 那么我书也看了，程序也做了，别人问我的问题我都能解决了，是不是就成为高手了呢？当然没那么简单，这只是万里长征走完了第一步。不信？那你出去接一个项目，你知道怎么下手吗，你知道怎么设计吗，你知道怎么组织人员进行开发吗？你现在脑子里除了一些散乱的代码之外，可能再没有别的东西了吧！ 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 Java 编程语言的风格十分接近C、C++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。 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。 是一种使网页(Web Page)产生生动活泼画面的语言 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。 是一种简化的C++语言 是一种安全的语言，具有阻绝计算机病毒传输的功能 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 如果要向java web方向发展也要吧看看《Java web从入门到精通》学完再到《Struts2.0入门到精通》这样你差不多就把代码给学完了。有兴趣可以看一些设计模块和框架的包等等。 有时间再研究一下MVC结构（把Model-View-Control分离开的设计思想） 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。
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