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"]