翻译文献-第三章

外文原文CHAPTER3PARSINGXMLWITHTWOSOLIDCHAPTERSOFINTRODUCTIONBEHINDUS,WEAREREADYTOCODEBYNOWYOUHAVESEENTHENUMEROUSACRONYMSTHATMAKEUPTHEWORLDOFXML,YOUHAVEDELVEDINTOTHELANGUAGEITSELF,ANDYOUSHOULDBEFAMILIARWITHANXMLDOCUMENTTHISCHAPTERTAKESTHENEXTSTEP,ANDTHEFIRSTONOURPATHOFJAVAPROGRAMMING,BYDEMONSTRATINGHOWANXMLDOCUMENTISPARSEDANDHOWWECANACCESSTHEPARSEDDATAFROMWITHINJAVACODEONEOFTHEFIRSTTHINGSYOUWILLHAVETODOWHENDEALINGWITHXMLPROGRAMMATICALLYISTAKEANXMLDOCUMENTANDPARSEITASTHEDOCUMENTISPARSED,THEDATAINTHEDOCUMENTBECOMESAVAILABLETOTHEAPPLICATIONUSINGTHEPARSER,ANDSUDDENLYWEAREWITHINANXMLAWAREAPPLICATIONIFTHISALLSOUNDSALITTLETOOSIMPLETOBETRUE,ITALMOSTISINTHISCHAPTER,WEWILLLOOKCLOSELYATHOWANXMLDOCUMENTISPARSEDUSINGAPARSERWITHINANAPPLICATIONANDHOWTOFEEDTHATPARSERYOURDOCUMENTSDATAWILLBECOVEREDTHENWEWILLLOOKATTHEVARIOUSCALLBACKSTHATAREAVAILABLEWITHINTHEPARSINGLIFECYCLETHESEEVENTSARETHEPOINTSWHEREAPPLICATIONSPECIFICCODECANBEINSERTEDANDDATAMANIPULATIONCANOCCURINADDITIONTOLOOKINGATHOWPARSERSWORK,WEWILLALSOBEGINOUREXPLORATIONOFTHESIMPLEAPIFORXMLSAXINTHISCHAPTERSAXISWHATMAKESTHESEPARSINGCALLBACKSAVAILABLETHEINTERFACESPROVIDEDINTHESAXPACKAGEWILLBECOMEANIMPORTANTPARTOFOURTOOLKITFORHANDLINGXMLEVENTHOUGHTHESAXCLASSESARESMALLANDFEWINNUMBER,EVERYTHINGELSEINOURDISCUSSIONSOFXMLISBASEDONTHESECLASSESASOLIDUNDERSTANDINGOFHOWTHEYHELPUSACCESSXMLDATAISCRITICALTOEFFECTIVELYLEVERAGINGXMLINYOURJAVAPROGRAMS31GETTINGPREPAREDTHEREARESEVERALITEMSTHATWESHOULDTAKECAREOFBEFOREBEGINNINGTOCODEFIRST,YOUMUSTOBTAINANXMLPARSERWRITINGAPARSERFORXMLISASERIOUSTASK,ANDTHEREARESEVERALEFFORTSGOINGONTOPROVIDEEXCELLENTXMLPARSERSWEARENOTGOINGTODETAILTHEPROCESSOFACTUALLYWRITINGANXMLPARSERHERERATHER,WEWILLDISCUSSTHEAPPLICATIONSTHATWRAPTHISPARSINGBEHAVIOR,FOCUSINGONUSINGEXISTINGTOOLSTOMANIPULATEXMLDATATHISRESULTSINBETTERANDFASTERPROGRAMS,ASWEDONOTSEEKTOREINVENTWHATISALREADYAVAILABLEAFTERSELECTINGAPARSER,WEMUSTENSURETHATACOPYOFTHESAXCLASSESISONHANDTHESEAREEASYTOLOCATE,ANDAREKEYTOOURJAVACODEBEINGABLETOPROCESSXMLFINALLY,WEWILLNEEDANXMLDOCUMENTTOPARSETHEN,ONTOTHECODEJAVAANDXML311OBTAININGAPARSERTHEFIRSTSTEPINGETTINGREADYTOCODEJAVATHATUSESXMLISLOCATINGANDOBTAININGTHEPARSERYOUWANTTOUSEWEBRIEFLYTALKEDABOUTTHISPROCESSINCHAPTER1,ANDLISTEDVARIOUSXMLPARSERSTHATCOULDBEUSEDTOENSURETHATYOURPARSERWORKSWITHALLOFTHEEXAMPLESINTHEBOOK,YOUSHOULDVERIFYYOURPARSERSCOMPLIANCEWITHTHEXMLSPECIFICATIONBECAUSEOFTHEVARIETYOFPARSERSAVAILABLEANDTHERAPIDPACEOFCHANGEWITHINTHEXMLCOMMUNITY,ALLOFTHEDETAILSABOUTWHICHPARSERSHAVEWHATCOMPLIANCELEVELSAREBEYONDTHESCOPEOFTHISBOOKYOUSHOULDCONSULTTHEPARSERSVENDORANDVISITTHEWEBSITESPREVIOUSLYGIVENFORTHISINFORMATIONINTHESPIRITOFTHEOPENSOURCECOMMUNITY,ALLOFTHEEXAMPLESINTHISBOOKWILLUSETHEAPACHE,THISCANDJAVAXERCESPARSERFREELYAVAILABLEINBINARYANDSOURCEFORMATHTTP/XMLAPACHEORGBASEDPARSERISALREADYONEOFTHEMOSTWIDELYCONTRIBUTEDTOPARSERSAVAILABLEINADDITION,USINGANOPENSOURCEPARSERSUCHASXERCESALLOWSYOUTOSENDQUESTIONSORBUGREPORTSTOTHEPARSERSAUTHORS,RESULTINGINABETTERPRODUCT,ASWELLASHELPINGYOUUSETHESOFTWAREQUICKLYANDCORRECTLYTOSUBSCRIBETOTHEGENERALLISTANDREQUESTHELPONTHEXERCESPARSER,SENDABLANKEMAILTOXERCESDEVSUBSCRIBEXMLAPACHEORGTHEMEMBERSOFTHISLISTCANHELPIFYOUHAVEQUESTIONSORPROBLEMSWITHAPARSERNOTSPECIFICALLYCOVEREDINTHISBOOKOFCOURSE,THEEXAMPLESINTHISBOOKALLRUNNORMALLYONANYPARSERTHATUSESTHESAXIMPLEMENTATIONCOVEREDHEREONCEYOUHAVESELECTEDANDDOWNLOADEDANXMLPARSER,MAKESURETHATYOURJAVAENVIRONMENT,WHETHERITBEANIDEINTEGRATEDDEVELOPMENTENVIRONMENTORACOMMANDLINE,HASTHEXMLPARSERCLASSESINITSCLASSPATHTHISWILLBEABASICREQUIREMENTFORALLFURTHEREXAMPLES312GETTINGTHESAXCLASSESANDINTERFACESONCEYOUHAVEYOURPARSER,YOUNEEDTOLOCATETHESAXCLASSESTHESECLASSESAREALMOSTALWAYSINCLUDEDWITHAPARSERWHENDOWNLOADED,ANDXERCESISNOEXCEPTIONIFTHISISTHECASEWITHYOURPARSER,YOUSHOULDBESURENOTTODOWNLOADTHESAXCLASSESEXPLICITLY,ASYOURPARSERISPROBABLYPACKAGEDWITHTHELATESTVERSIONOFSAXTHATISSUPPORTEDBYTHEPARSERATTHETIMEOFTHISWRITING,SAX20HADJUSTGONEFINALTHESAX20CLASSESAREUSEDTHROUGHOUTTHISBOOK,ANDSHOULDCOMEBUNDLEDWITHTHELATESTVERSIONOFTHEAPACHEXERCESPARSERIFYOUARENOTSUREWHETHERYOUHAVETHESAXCLASSES,LOOKATTHEJARFILEORCLASSSTRUCTUREUSEDBYYOURPARSERTHESAXCLASSESAREPACKAGEDINTHEORGXMLSAXSTRUCTURETHELATESTVERSIONOFTHESEINCLUDES17CLASSESINTHISROOTDIRECTORY,ASWELLAS9CLASSESINORGXMLSAXHELPERSAND2ORGXMLSAXEXTIFYOUAREMISSINGANYOFTHESECLASSES,YOUSHOULDTRYTOCONTACTYOURINPARSERSVENDORTOSEEWHYTHECLASSESWERENOTINCLUDEDWITHYOURDISTRIBUTIONITISPOSSIBLETHATSOMECLASSESMAYHAVEBEENLEFTOUTIFTHEYARENOTSUPPORTEDINWHOLETHESECLASSCOUNTSAREFORSAX20ASWELLFEWERCLASSESMAYAPPEARIFONLYSAX10ISSUPPORTED1SUPPORTINGSAXINWHOLEISAVERYIMPORTANTITEMFORAPARSERALTHOUGHYOUARECERTAINLYWELCOMETOUSEANYPARSERYOULIKE,IFYOURPARSERDOESNOTHAVECOMPLETESAX20SUPPORT,MANYOFTHEEXAMPLESINTHISBOOKWILLNOTWORKINADDITION,YOURPARSERISNOTKEEPINGUPWITHTHELATESTXMLDEVELOPMENTSFOREITHERORBOTHREASONS,YOUMAYWANTTOCONSIDERATLEASTTRYINGTHEXERCESPARSERFORTHEDURATIONOFTHISBOOKFINALLY,YOUMAYWANTTOEITHERDOWNLOADORBOOKMARKTHESAXAPIJAVADOCSONTHEWEBTHISDOCUMENTATIONISEXTREMELYHELPFULINUSINGTHESAXCLASSES,ANDTH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用XML编写JAVA代码的第一步，是设置并得到想使用的解析器。在第一章中我们已经讨论了这个过程，并且列举了可供使用的各种XML解析器。为了确保你的解析器能够应用本书中的所有例子，就应该用XML规范来检验解析器是否合适。由于可用的解析器的多样性和XML社区快速的变化，本书将不讨论哪个解析器具有什么级别的适应性这样的细节，你应该咨询解析器厂商，访问前面提供的网站，以得到这些信息。在原代码公开的宗旨下，本书中的所有规范将使用APACHEXERCES解析器。由于HTTP/XMLAPACHEORG可免费地得到其二进制文件。这个以C和JAVA语言为基础的解析器已经成为贡献最大的解析器之一，而且使用XERCERS这样的开源解析器，你可以把问题或出错报告直接发给解析器的作者，这样，既可以帮助作者开发出更好的产品，又可以帮助你快速正确地使用此软件。若要订阅一个普通的标单并且请求帮助，请发送一个空的电子邮件到XERCESDEVSUBSCRIBEXMLAPACHEORG如果你对解析器有疑问或难题而本书又未涉及，名单上的成员会对你有所帮助。当然，所有使用了这里讨论的SAX工具的解析器上，都能正常运行本书上的例子。一旦选择并下载了一个XML解析器，不管JAVA的环境是IDE（集成开发环境），还是命令行，都要确保此XML解析器与其在同一路径上。这也对后面所有例子的基本要求。获得SAX类和接口当有了接续其后，就需要设置SAX类，这些类通常在下载时都包括解析器，XERCES也不例外。如果解析器是这种情况，就不必明确地下载SAX类，因为该解析器很可能已经有解析器支持的最新版本的SAX包了，在写到这得时候，SAX20最终开始流行起来。本书全部使用SAX20，而且已经与最新版本的APACHEXERCES解析器捆绑其来。如果你不能确定是否有SAX类，请查看解析器使用的JAR文件或类结构，SAX类被封装在ORGXMLSAX结构中，最新版本有17个类在其目录，同时又9个在ORGXMLSAXHELPER,两个在ORGXMLSAXEXT中，如果其中有缺少的类，你应该与解析器厂商联系，看看为什么该发布版本中没有这些类，如果有些类不能被全部支持，则有可能是被厂商保留了。这些类也适用于SAX20；如果只支持SAX10，类会更少。最后，你可能想下载SAXAPIJAVADOCS或将其做成书签。在使用SAX类时，这份资料极有帮助，而且JAVADOCS结构提供了一个标准，简单的方法来查询关于这些类的额外信息和他们的行为。这些资料在HTTP/WWWMEGGINSONCOM/SAX/SAX2/JAVADOC/INDEXHTOIL中得到。你也可以使用包含解析器的源代码，或从HTTP/WWWMEGGINSONCOM/SAX/SAX2下载完整的源代码，再从SAX源代码中生成JAVADOC获得XML文档要保证手中的有一份需进行解析的XML文档，本例中的解析输出基于在第二章“创建XML”中曾讨论过的XML文档，将此文件命名为CONTENT,XML，并存于硬盘上。我们强烈推荐你遵循示例进行，你可以从书中录入，也可以从网上下载，本书所在的网址为HTTP/WWWOREILLYCOM/CATALOG/JAVAXML然而，我们希望你能花点时间从书中录入，因为，这样做比快速下载会让你更加熟悉XML的语法。除在下载或创建文档之外，我们还必须进行一些小的改动，因为还未涉及到文档约束和文档转换，本章中的程序仅涉及解析XML。为避免错误，我们去掉了XML文档中指相外部DTD的引用，这个外部DTD约束了XML和XSL样式表转化。你应该把XML文档中的这两行，以及COCOON请求XSL转换的处理指令暂时变为注释。注意这些变为注释的行为中有XML文档的完整路径。在本章和以后各章节的程序中也必须提供完整的路径。最后，我们标出了OREILLYCOPYRIGHT外部实体的引用，这其中包含了从文件系统中装载一个文件所需的版权信息。相反，如果没有DTD来定义如何解决这种实体引用问题，将会得到意想不到的错误，在下一章节将会看到在XML文档中是如何解决这类问题的。SAX阅读器为节省时间，让我们马上就开始编程。第一个程序将以一个XML文件为命令行参数来解析这个文件，将会建立反映解析过程情况的文档，来显示解析过程的种种事件，这将有助于更好地了解底层正在发生的事。首先必须得到一个符合SAXORGXMLSAXXMLREADER接口规范的例子。这个接口定义了解析行为并允许设置某些特征和属性，我们将在第五章“验证XML”中看到这一点。对那些熟知SAX10的人而言，该接口替换了ORGXMLSAXPARSER阅读器SAX提供了一个所有支持SAX的XML解析器都必须支持的接口，这样使得SAX明白在一个应用中什么是可以回调的，什么是可以使用的。例如，XERCES的主SAX解析器类，ORGAPACHEXERCESPARSERSSXPARSER支持ORGXMLSAXREADER接口。如果分析过解析器的源码，就会发现在他的主SAX解析器类中有相同的接口支持，每一解析器必须有一个支持此接口的类（有时会更多）。正因为如此可以用举例子来解析XML文档；XMLREADERPARSERNEWSAXPARSER/DOSOMETHINGWITHTHEPARSERPARSERPARSEURI那些对SAX完全陌生的人，会不明白为什么有些例子叫READER，而有些却叫XMLREADER，SAX10类将主解析接口定义为解析器。由这种命名方式，许多继承代码被冠以不同的解析器名。这是普通的约定，这种接口有时遭非议，因为他的许多名子空间的特征和属性都还需改进。然而他应然是一种好的接口，它确实能指出各种例子的真正目的。因此，我们开始用一个小程序来对SAX解析器做出说明，这并不能真正解析一个文档，实际上它只是给出一个框架对本章后续内容服务，实际解析过程将在下一章里具体描述。例31SAX解析器范例IMPORTORGXMLSAXXMLREADER/IMPORTYOURVENDORSXMLREADERIMPLEMENTATIONHEREIMPORTORGAPACHEXERCESPARSERSSAXPARSER/SAXPARSERDEMOWILLTAKEANXMLFILEANDPARSEITUSINGSAX,DISPLAYINGTHECALLBACKSINTHEPARSINGLIFECYCLEAUTHORBRETTMCLAUGHLINVERSION10/PUBLICCLASSSAXPARSERDEMO/THISPARSESTHEFILE,USINGREGISTEREDSAXHANDLERS,ANDOUTPUTSTHEEVENTSINTHEPARSINGPROCESSCYCLEPARAMURISTRINGURIOFFILETOPARSE/PUBLICVOIDPERFORMDEMOSTRINGURISYSTEMOUTPRINTLN“PARSINGXMLFILE“URI“NN“/INSTAN