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如何在Ruby程序中创建和解析XML文件?

2016-03-04 14:05:25 来源:www.45fan.com 【

如何在Ruby程序中创建和解析XML文件?

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"] 

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