使用builder创建XML

builder安装方法：

gem install builder

require 'builder' x = Builder::XmlMarkup.new(:target => $stdout, :indent => 1) #":target =＞$stdout"参数：指示输出内 容将被写向标准输出控制台 #":indent =＞1"参数：XML输出形式将被缩 进一个空格字符x.instruct! :xml, :version =>'1.1',:encoding => 'gb2312' x.comment! "书本信息" x.library("shelf" => "Recent Acquisitions") { x.section("name" => "ruby"){ x.book("isbn" => "0672310001"){ x.title "Programming Ruby" x.author "Yukihiro " x.description "Programming Ruby - The Pragmatic Programmer's Guide" } } }

p x #打印XML

Ruby创建XML输出结果：

< "1.1" encoding="gb2312"?> < > < library shelf="Recent Acquisitions"> < section name="ruby"> < book isbn="0672310001"> < title>Programming Ruby< /title> < author>Yukihiro < /author> < description>Programming Ruby - The Pragmatic Programmer's Guide < /description> < /book> < /section> < /library> < inspect/> #< IO:0x2a06ae8>

使用ReXML解析XML

REXML 是一个完全用ruby写的processor ,他有多种api，其中两个经典的api是通过DOM-like 和SAX-like 来进行区分的。第一种是将整个文件读进内存，然后存储为一个分层的形式(也就是一棵树了).而第二种是"parse as you go",当你的文件很大，并且内存受到限制的时候，比较适合用这种。

看下面的book.xml:

引用

<library shelf="Recent Acquisitions"> <section name="Ruby"> <book isbn="0672328844"> <title>The Ruby Way</title> <author>Hal Fulton</author> <description> Second edition. The book you are now reading. Ain't recursion grand? </description> </book> </section> <section name="Space"> <book isbn="0684835509"> <title>The Case for Mars</title> <author>Robert Zubrin</author> <description>Pushing toward a second home for the human race. </description> </book> <book isbn="074325631X"> <title>First Man: The Life of Neil A. Armstrong</title> <author>James R. Hansen</author> <description>Definitive biography of the first man on the moon. </description> </book> </section> </library>

1 Tree Parsing(也就是DOM-like)

我们需要require rexml/document 库，并且include REXML :

require 'rexml/document' include REXML input = File.new("books.xml") doc = Document.new(input) root = doc.root puts root.attributes["shelf"] # Recent Acquisitions doc.elements.each("library/section") { |e| puts e.attributes["name"] } # Output: # Ruby # Space doc.elements.each("*/section/book") { |e| puts e.attributes["isbn"] } # Output: # 0672328844 # 0321445619 # 0684835509 # 074325631X sec2 = root.elements[2] author = sec2.elements[1].elements["author"].text # Robert Zubrin

这里要注意的是xml中的属性和值被表示为一个hash，因此我们能够通过attributes[]来提取我们需要的值，元素的值还能通过类似于path的字符串或者整数来取得.其中用整数取的话，是1-based而不是0-based.

2 Stream Parsing（也就是SAX-like Parsing)

这边使用了一个小技巧，那就是定义了一个listener 类，它将会在parse的时候被回调:

require 'rexml/document' require 'rexml/streamlistener' include REXML class MyListener include REXML::StreamListener def tag_start(*args) puts "tag_start: #{args.map {|x| x.inspect}.join(', ')}" end def text(data) return if data =~ /^w*$/ # whitespace only abbrev = data[0..40] + (data.length > 40 ") puts " text : #{abbrev.inspect}" end end list = MyListener.new source = File.new "books.xml" Document.parse_stream(source, list)

这里介绍一下StreamListener 模块，这个模块它提供了几个空的回调方法，因此你可以为了实现你自己的功能而覆盖它.当parser 进入一个tag时，就会调用tag_start方法.而text方法也是类似的，他只不过是当读取到数据时会被回调,它的输出是这样的：

tag_start: "library", {"shelf"=>"Recent Acquisitions"} tag_start: "section", {"name"=>"Ruby"} tag_start: "book", {"isbn"=>"0672328844"} tag_start: "title", {} text : "The Ruby Way" .........................................

3 XPath

REXML通过XPath 类来提供Xpath的支持. 它也同时支持DOM-like和SAX-like .还是前面的那个xml文件，我们使用Xpath可以这样做：

book1 = XPath.first(doc, "//book") # Info for first book found p book1 # Print out all titles XPath.each(doc, "//title") { |e| puts e.text } # Get an array of all of the "author" elements in the document. names = XPath.match(doc, "//author").map {|x| x.text } p names

输出是类似于下面的：

<book isbn='0672328844'> ... </> The Ruby Way The Case for Mars First Man: The Life of Neil A. Armstrong ["Hal Fulton", "Robert Zubrin", "James R. Hansen"]