Ruby程式中建立和解析XML檔案的方法_ruby專題

使用builder建立XML

builder安裝方法：

gem install builder

require 'builder'   x = Builder::XmlMarkup.new(:target => $stdout, :indent => 1) #":target =＞$stdout"參數：指示輸出內容將被寫向標準輸出控制台 #":indent =＞1"參數：XML輸出形式將被縮排一個空白字元x.instruct! :xml,:version =>'1.1',:encoding => 'gb2312' x.comment! "書本資訊"  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 #列印XML

Ruby建立XML輸出結果：

< ?xml version="1.1" encoding="gb2312"?> < !-- 書本資訊 --> < 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 是一個完全用ruby寫的processor ,他有多種api，其中兩個經典的api是通過DOM-like 和SAX-like 來進行區分的。第一種是將整個檔案讀進記憶體，然後儲存為一個分層的形式(也就是一棵樹了).而第二種是"parse as you go",當你的檔案很大，並且記憶體受到限制的時候，比較適合用這種。

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)

我們需要require rexml/document 庫，並且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 

這裡要注意的是xml中的屬性和值被表示為一個hash，因此我們能夠通過attributes[]來提取我們需要的值，元素的值還能通過類似於path的字串或者整數來取得.其中用整數取的話，是1-based而不是0-based.

2  Stream Parsing（也就是SAX-like Parsing)

這邊使用了一個小技巧，那就是定義了一個listener 類，它將會在parse的時候被回調:

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) 

這裡介紹一下StreamListener 模組，這個模組它提供了幾個空的回調方法，因此你可以為了實現你自己的功能而覆蓋它.當parser 進入一個tag時，就會調用tag_start方法.而text方法也是類似的，他只不過是當讀取到資料時會被回調,它的輸出是這樣的：

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通過XPath 類來提供Xpath的支援. 它也同時支援DOM-like和SAX-like .還是前面的那個xml檔案，我們使用Xpath可以這樣做：

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 

輸出是類似於下面的：

<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"]