Ruby程序中创建和解析XML文件的方法

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：

< ?xml version="1.1" encoding="gb2312"?>
< !-- 书本信息 -->
< 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"]