DIV CSS 佈局教程網

 DIV+CSS佈局教程網 >> 網頁腳本 >> JavaScript入門知識 >> AJAX入門 >> AJAX詳解 >> AsynchronousJavaScriptandXML(AJAX)withJava2EnterpriseEdition
AsynchronousJavaScriptandXML(AJAX)withJava2EnterpriseEdition
編輯:AJAX詳解      日期:2016/12/30 17:51:12
AnyonewhohasusedFlickr,GMail,GoogleSuggest,orGoogleMapswillrealizethatanewbreedofdynamicwebapplicationsareemerging.Theseapplicationslookandactverysimilartotraditionaldesktopapplicationswithoutrelyingonpluginsorbrowserspecificfeatures.WebapplicationshavetraditionallybeenasetofHtmlpagesthatmustbereloadedtochangeanyportionofthecontent.Overthepastfewyears,technologIEssuchasJavaScriptandCascadingStyleSheet(CSS)havematuredtothepointwheretheycanbeusedeffectivelytocreateverydynamicwebapplicationsthatwillworkonallofthemajorbrowsers.Thisdocumentandthesolutionsassociatedwithitwilldetailseveraltechniquesthatyoucanusetodaytoenableyourwebapplicationstobemorerichandinteractivelikedesktopapplications.

  IntroducingAsynchronousJavaScriptandXML(AJax)

  UsingJavascriptanHTMLpagecanasynchronouslymakecallstotheserverfromwhichitwasloadedandfetchXMLdocuments.TheXMLdocumentsmaythenbeusedbyJavascripttoupdateormodifytheDocumentObjectModel(DOM)oftheHtmlpage.ThetermAsynchronousJavaScriptandXML(AJax)hasemergedrecentlytodescribethisinteractionmodel.

  AJaxisnotnew.ThesetechniqueshavebeenavailabletodeveloperstargetingInternetExplorer(IE)ontheWindowsplatformformanyyears.Upuntilrecentlythetechnologywasreferredtoaswebremotingorremotescripting.Webdevelopershavealsousedacombinationofplugins,Javaapplets,andhiddenframestoemulatethisinteractionmodelforsometime.WhathaschangedrecentlyisthattheinclusionofsupportfortheXMLHttpRequestobjecthasbecameubiquitousinthemainstreambrowsersacrossallplatforms.TherealmagicistheresultoftheJavascriptXMLHttpRequestobject.WhilethisobjectisnotspecifIEdintheformalJavascriptspecification,itissupportedbyallofthemainstreambrowserstoday.WhiletherearesubtledifferenceswiththeJavaScriptandCSSsupportamongcurrentgenerationbrowserssuchasFirefox,IEandSafari,theyaremanageable.Ifyouarerequiredtosupportolderbrowsers,AJaxmaynotbetheanswerforyou.

  WhatmakesAJaxbasedclientsuniqueisthattheclIEntcontainspagespecificcontrollogicembeddedasJavascript.ThepageinteractswiththeJavaScriptbasedoneventssuchasthedocumentbeingloaded,amouseclick,focuschanges,orevenatimer.AJAXinteractionsallowfOraclearseparationofpresentationlogicfromthedata.AnHTMLpagecanpullinbitesizepIEcesofdataasneededversusreloadingthewholepageeverytimeachangeneedstobedisplayed.AJaxwillrequireadifferentserver-sidearchitecturetosupportthisinteractionmodel.Traditionally,server-sidewebapplicationshavefocusedongeneratingHTMLdocumentsforeveryclienteventresultinginacalltotheserverandtheclientswouldrefreshandre-renderthecompleteHTMLpageforeachresponse.RichwebapplicationsfocusonaclIEntfetchinganHtmldocumentwhichactsasatemplateorcontainerforinjectingcontentintobasedonclIEnteventsusingXMLdataretrIEvedfromaserver-sidecomponent.

  SomeusecasesforAJaxinteractionsare:

  RealtimeFormDataValidation:Formdatasuchasuserids,serialnumbers,postalcodes,orevenspecialcouponcodesthatrequireserver-sidevalidationcanbevalidatedinaformbeforetheusersubmitsaform.

  Auto-Completion:ASPecificportionofformdatasuchasanemailaddress,name,orcitynamemaybeauto-completedastheusertypes.

  MasterDetailsOperations:BasedonaclIEnteventanHtmlpagecanfetchmoredetailedinformationondatasuchasaproductlistingthatenablestheclienttovIEwtheindividualproductinformationwithoutrefreshingthepage.

  SophisticatedUserInterfaceControls:Controlssuchastreecontrols,menus,andprogressbarsmaybeprovidedthatdonotrequirepagerefreshes.

  RefreshingDataonthePage:Htmlpagesmaypolldatafromaserverforuptodatedatasuchasscores,stockquotes,weather,orapplicationspecficdata.

  Server-sideNotifications:AnHtmlpagemaysimulateaserver-sidepushbypollingtheserverforeventnotificationswhichmaynotifytheclientwithamessage,refreshpagedata,orredirecttheclIEnttoanotherpage.

  Thislistisnotallinclusive,however,itdoesshowthatAJaxinteractionsallowwebapplicationstodomuchmorethantheyhavetraditionallydoneinthepast.Whilemanyofthesebenefitsarenoteworthytherearesomedrawbackstothisapproachaswell:

  Complexity:Server-sidedeveloperswillneedtounderstandthatpresentationlogicwillberequiredintheHTMLclIEntpagesaswellasintheserver-sidelogictogeneratetheXMLcontentneededbytheclIEntHtmlpages.JavaScriptskillsarerequiredfortheHtmlpagedevelopers.OvertimeitwillbecomeeasIErtocreateAJaxenabledapplicationsasnewframeworksarecreatedandexistingframeworksevolvetosupporttheinteractionmodel.

  StandardizationoftheXMLHttpRequestObject:TheXMLHttpRequestisnotyetapartoftheJavaScriptspecificationwhichmeansthatthebehaviormayvarydependingontheclIEnt.

  JavascriptImplementations:AJaxinteractionsdependheavilyonJavaScriptwhichhassubtledifferencesdependingontheclIEnt(seeQuirksMode.orgformoredetailsonbrowserspecificdifferences.

  Debugging:AJaxapplicationsarealsodifficulttodebugbecausetheprocessinglogicisbothembeddedintheclIEntandontheserver.

  ViewableSource:TheclIEntsideJavaScriptmaybeviewedbytheclientbysimplyselecting"VIEwSource"formanAJaxenabledHtmlpage.ApoorlydesignedAJaxbasedapplicationcouldopenitselfuptohackersorplagarism.

  FrameworksandpatternsforAJAXtechnologyarelikelytoemergeasdevelopersgainmoreexperIEncedwrittingapplicationsthatusetheAJAXinteractionmodel.Itisstillearlytofocusona"onesizefitsall"frameworkforAJAXinteractions.ThisdocumentandtheassociatedsolutionsfocusonhowAJaxinteractionscanbesupportedtodaybyexistingJava2EnterpriseEdition(J2EE)technologIEssuchservlets,JavaServerPages,JavaServerFacesandtheJavaStandardTagLibrarIEs(JSTL).

  TheAnatomyofanAJaxinteraction

  NowthatwehavediscussedwhatAJAXisandwhatsomehigherlevelissuesare,let'sputallthepIEcestogetherandshowanAJaxenabledJ2EEapplication.

  Let'sconsideranexample,awebapplicationcontainsastaticHTMLpageoranHTMLpagegeneratedinJSPtechnologycontainsanHtmlformthatrequiresserver-sidelogictovalidatetoformdatawithoutrefreshingthepage.Aserver-sidewebcomponent(servlet)namedValidateServletwillprovidethevalidationlogic.ThegraphicbelowdescribesthedetailsoftheAJaxinteractionthatwillprovidethevalidationlogic.

  TheitemsbelowrepresentthesetupsofanAJaxinteractionastheyappearinthegraphicabove.

  AclIEnteventoccurs.

  AnXMLHttpRequestobjectiscreatedandinitialized.

  TheXMLHttpRequestobjectmakesacall.

  TherequestisprocessedbytheValidationServlet.

  TheValidationServletreturnsanXMLDocumentcontainingtheresult.

  TheXMLHttpRequestobjectcallsthecallback()functionandprocessestheresult.

  TheHtmlDOMisupdated.

  NowletlookateachstepoftheAJaxinteractioninmoredetail.

  1.AclIEnteventoccurs.

  JavaScriptfunctionsarecalledastheresultofanevent.Inthecasethefunctionvalidate()maybemappedtoaonkeyupeventonalinkorformcomponent.

 

  size="20"

  id="userid"

  name="id"

  onkeyup="validate();">

  Theformelementabovewillcallthevalidate()eachtimeakeyispressedintheformfIEld.

  2.AXMLHttpRequestobjectiscreatedandinitialized.

  AnXMLHttpRequestobjectisinitializedandconfigured.

  varreq;

  functionvalidate(){

  varidField=document.getElementById("idFIEld");

  varurl="validate?id="+escape(idFIEld.value);

  if(window.XMLHttpRequest){

  req=newXMLHttpRequest();

  }elseif(window.ActiveXObject){

  req=newActiveXObject("Microsoft.XMLHTTP");

  }

  req.open("GET",url,true);

  req.onreadystatechange=callback;

  req.send(null);

  }

  Thevalidate()functioninitializesaXMLHttpRequestobjectopenmethodrequiresthreeargumentswhicharewithaurlStringofGETorPOSTrepresetingtheHTTPmethodtobeused,aStringforthetargetURL,andabooleanindicatingwhetherornotthecallwillbemadeasynchronously.Ifaninteractionissetasasynchronous(true)acallbackfuctionmustbespecfIEd.Thecallbackfunctionforthisinteractionissetwiththestatementreq.onreadystatechange=callback;.Seethesectiontitled"TheXMLHttpRequestobjectcallsthecallback()functionandprocessestheresult"formoredetails.

  3.TheXMLHttpRequestobjectmakesacall.

  Whenthestatmentreq.send(null);isreachedthecallwillbemade.InthecaSEOfaHTTPgetthiscontentmaybenullorleftblank.WhenthethisfunctioniscalledontheXMLHttpRequestobjectthecalltotheURLthatwassetduringtheinitializationoftheobjectiscalled.InthecaSEOfthisexamplethedatathatisposted(id)isincludedasaURLparameter.

  UseanHTTPGETwhentherequestisidempotentmeaningthattwoduplicaterequestswillreturnthesameresults.WhenusingHTTPGETmethodthelengthofURLincludingescapedURLparametersislimitedbysomebrowsersandbyserver-sidewebcontainers.TheHTTPPOSTmethodshouldbeusedwhensendingdatatotheserverthatwilleffecttheserver-sideapplicationstate.AHTTPPOSTrequiresaContent-TypeheadertobesetontheXMLHttpRequestobjectbyusingthefollowingstatement:

  req.setRequestHeader("Content-Type","application/x-www-form-urlencoded");

  req.send("id="+escape(idTextFIEld.value));

  WhensendingformvaluesfromJavascriptyoushouldtakeintoconsiderationtheencodingofthefIEldvalues.JavaScriptincludesanescape()functionwhichshouldbeusedtoensurelocalizedcontentisencodedproperlyandthatspecialcharachtersareescapedcorrectly.

  4.TherequestisprocessedbytheValidationServlet.

  AservletmappedtotheURI"validate"checksiftheuseridisintheuserdatabSEOrnot.

  AservletprocessesaXMLHttpRequestjustasitwouldanyotherHTTPrequest.Theexamplebelowshowaserverextractingtheidparameterfromtherequestandvalidatingwhetherornottheparameterhasbeentaken.

  publicclassValidationServletextendsHttpServlet{

  privateServletContextcontext;

  privateHashMapusers=newHashMap();

  publicvoidinit(ServletConfigconfig)throwsServletException{

  this.context=config.getServletContext();

  users.put("greg","accountdata");

  users.put("duke","accountdata");

  }

  publicvoiddoGet(HttpServletRequestrequest,HttpServletResponseresponse)

  throwsIOException,ServletException{

  StringtargetId=request.getParameter("id");

  if((targetId!=null)&&!users.containsKey(targetId.trim())){

  response.setContentType("text/XML");

  response.setHeader("Cache-Control","no-cache");

  response.getWriter().write("valid");

  }else{

  response.setContentType("text/XML");

  response.setHeader("Cache-Control","no-cache");

  response.getWriter().write("invalid");

  }

  }

  }

  InthisexampleasimpleHashMapisusedtocontaintheusers.InthecaSEOfthisexampleletusassumetheuserid"duke"wassubmitted.

  5.TheValidationServletreturnsanXMLDocument.

  Theuserid"duke"ispresentinthelistofuseridsintheusersHashMap.TheValidationServletwillwriteaXMLdocumenttotheresponsecontainingan"message"elementwiththevalueof"invalid".MorecomplexusecasesmayrequireDOM,XSLT,orotherAPIstogeneratetheresponse.

  response.setContentType("text/XML");

  response.setHeader("Cache-Control","no-cache");

  response.getWriter().write("invalid");

  TwothingsthatadeveloperneedstobeawareofisthattheContent-Typeissetto"text/xml"andtheCache-Controlneedstobesetto"no-cache".TheXMLHttpRequestobjectwillonlyprocessrequeststhatareoftheContent-Typeof"text/XML"andtheCache-Controlbeingsetto"no-cache"willkeepbrowsersfromlocallycachingresponsesforcaseswhereduplicaterequestsforthesameURL(includingURLparameters)mayreturndifferentresponses.

  6.TheXMLHttpRequestobjectcallsthecallback()functionandprocessestheresult.

  TheXMLHttpRequestobjectwasconfiguredtocallthecallback()functionwhentherearechangestothereadyStateoftheXMLHttpRequestobject.LetusassumethecalltotheValidationServletwasmadeandthereadyStateis"4"signifyingtheXMLHttpRequestcalliscomplete.TheHTTPstatuscodeof"200"signifIEsasuccessfulHTTPinteraction.

  functioncallback(){

  if(req.readyState==4){

  if(req.status==200){

  //updatetheHtmlDOMbasedonwhetherornotmessageisvalid

  }

  }

  }

  Browsersmaintainanobjectrepresentationofthedocumentsbeingdisplayed(referredtoasthedocumentobjectmodel(DOM)).JavascriptinanHtmlpagehasAccesstotheDOMandAPIsareavailablethatallowJavaScripttomodifytheDOMafterthepagehasloaded.

  Followingasuccessfulrequest,JavaScriptcodemaymodifytheDOMoftheHTMLpage.TheobjectrepresentationoftheXMLdocumentthatwasretrIEvedfromtheValidationServletisavailabletoJavascriptcodeusingthereq.responseXMLwherereqisanXMLHttpRequestobject.TheDOMAPIsprovideameansforJavascripttonavigatethecontentfromthatdocumentandusethatcontenttomodifytheDOMoftheHtmlpage.AstringrepresentationofthedocumentthatwasretrIEvedbycallingreq.responseText.NowletuslookatthehowtousetheDOMAPIsinJavaScriptbylookingatthefollowingXMLdocumentreturnedfromtheValidateServlet.

 

  valid

 

  TheexampleaboveisasimpleXMLfragmentthatcontainsthesenderofthemessageelementwhichissimplythestring"valid"or"invalid".Amoreadvancedsamplemaycontainmorethanonemessageandvalidnamesthatmightbepresentedtotheuser.

  functionparseMessage(){

  varmessage=req.responseXML.getElementsByTagName("message")[0];

  setMessage(message.childNodes[0].nodeValue);

  }

  TheparseMessages()functionwillprocessXMLdocumentretrIEvedfromtheValidationServlet.ThisfunctionwillcallthesetMessage()withvalueofthemessageelementtoupdatetheHtmlDOM.

  7.TheHtmlDOMisupdated.

  JavascriptcangainareferencetoanyelementintheHTMLDOMusinganumberofAPIs.Therecommendedwaytogainareferencetoaelementistocalldocument.getElementById("userIdMessage")where"userIdMessage"istheidattributeofanelementappearingintheHtmldocument.WithareferencetotheelementJavaScriptmaynowbeusedtomaybemodifyattributesoftheelement,modifythestylepropertIEsofelement,oraddorremove,modifychildelements.

  OnecommonmeanstochangethebodycontentofanelementistosettheinnerHtmlpropertyontheelementascanbeseeninthefollowingexample.

 

  TheportionsoftheHTMLpagethatwereaffectedarere-renderedimmediatelyfollowingthesettingoftheinnerHTML.IftheinnerHtmlpropertycontainselementssuchas

XML學習教程| jQuery入門知識| AJAX入門| Dreamweaver教程| Fireworks入門知識| SEO技巧| SEO優化集錦|
Copyright © DIV+CSS佈局教程網 All Rights Reserved