在 Java 中使用 SAX 解析大型 XML

Question

我正在尝试解析堆栈溢出数据转储，其中一张表名为 posts.xml，其中包含大约 1000 万个条目。示例 xml：

<?xml version="1.0" encoding="utf-8"?>
<posts>
  <row Id="1" PostTypeId="1" AcceptedAnswerId="26" CreationDate="2010-07-07T19:06:25.043" Score="10" ViewCount="1192" Body="&lt;p&gt;Now that the Engineer update has come, there will be lots of Engineers building up everywhere.  How should this best be handled?&lt;/p&gt;&#xA;" OwnerUserId="11" LastEditorUserId="56" LastEditorDisplayName="" LastEditDate="2010-08-27T22:38:43.840" LastActivityDate="2010-08-27T22:38:43.840" Title="In Team Fortress 2, what is a good strategy to deal with lots of engineers turtling on the other team?" Tags="&lt;strategy&gt;&lt;team-fortress-2&gt;&lt;tactics&gt;" AnswerCount="5" CommentCount="7" />
  <row Id="2" PostTypeId="1" AcceptedAnswerId="184" CreationDate="2010-07-07T19:07:58.427" Score="5" ViewCount="469" Body="&lt;p&gt;I know I can create a Warp Gate and teleport to Pylons, but I have no idea how to make Warp Prisms or know if there's any other unit capable of transporting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would in particular like this to built remote bases in 1v1&lt;/p&gt;&#xA;" OwnerUserId="10" LastEditorUserId="68" LastEditorDisplayName="" LastEditDate="2010-07-08T00:16:46.013" LastActivityDate="2010-07-08T00:21:13.163" Title="What protoss unit can transport others?" Tags="&lt;starcraft-2&gt;&lt;how-to&gt;&lt;protoss&gt;" AnswerCount="3" CommentCount="2" />
  <row Id="3" PostTypeId="1" AcceptedAnswerId="56" CreationDate="2010-07-07T19:09:46.317" Score="7" ViewCount="356" Body="&lt;p&gt;Steam won't let me have two instances running with the same user logged in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does that mean I cannot run a dedicated server on a PC (for example, for Left 4 Dead 2) &lt;em&gt;and&lt;/em&gt; play from another machine?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way to run the dedicated server without running steam? Is there a configuration option I'm missing?&lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2010-07-07T19:27:04.777" Title="How can I run a dedicated server from steam?" Tags="&lt;steam&gt;&lt;left-4-dead-2&gt;&lt;dedicated-server&gt;&lt;account&gt;" AnswerCount="1" />
  <row Id="4" PostTypeId="1" AcceptedAnswerId="14" CreationDate="2010-07-07T19:11:05.640" Score="10" ViewCount="201" Body="&lt;p&gt;When I get to the insult sword-fighting stage of The Secret of Monkey Island, do I have to learn every single insult and comeback in order to beat the Sword Master?&lt;/p&gt;&#xA;" OwnerUserId="17" LastEditorUserId="17" LastEditorDisplayName="" LastEditDate="2010-07-08T21:25:04.787" LastActivityDate="2010-07-08T21:25:04.787" Title="Do I have to learn all of the insults and comebacks to be able to advance in The Secret of Monkey Island?" Tags="&lt;monkey-island&gt;&lt;adventure&gt;" AnswerCount="3" CommentCount="2" />

我想解析这个xml，但只加载xml的某些属性，即Id、PostTypeId、AcceptedAnswerId和其他2个属性。 SAX 中有没有办法让它只加载这些属性？如果有那怎么办？我对 SAX 很陌生，所以一些指导会有所帮助。

否则加载整个东西会很慢，而且某些属性无论如何都不会被使用，所以没用。

另一个问题是，是否可以跳转到具有行 ID X 的特定行？如果可能的话，我该怎么做？

Answer 1

“StartElement”Sax 事件允许处理单个 XML 元素。

在java代码中你必须实现这个方法

public void startElement(String uri, String localName,
    String qName, Attributes attributes)
    throws SAXException {

    if("row".equals(localName)) {
        //this code is executed for every xml element "row"
        String id = attributes.getValue("id");
        String PostTypeId = attributes.getValue("PostTypeId");
        String AcceptedAnswerId = attributes.getValue("AcceptedAnswerId");
        //others two
        // you have your att values for an "row" element
    }

 }

对于每个元素，您可以访问：

1. 命名空间 URI
2. XML QName
3. XML 本地名称
4. 属性映射，在这里你可以提取你的两个属性...

具体细节见 ContentHandler 实现。

再见

更新：改进了以前的 sn-p。

Answer 2

这与我已经回答 here 的方法几乎相同。

向下滚动到org.xml.sax Implementation 部分。您只需要一个自定义处理程序。

Answer 3

是的，你可以重写只处理你想要的元素的方法：

• http://www.javacommerce.com/displaypage.jsp?name=saxparser1.sql&id=18232
• http://www.java2s.com/Code/Java/XML/SAXDemo.htm

Answer 4

SAX 不“加载”元素。它会通知您的应用程序每个元素的开始和结束，并且完全由您的应用程序决定它需要注意哪些元素。