妙用Commons良药<三>

这一篇文章主要是介绍怎样用Commons处理XMl文件
1、将XML文档转化为你想要的对象
   可以使用org.apache.commons.digester.Digester来实现你想要的功能，其步骤如下：定义待匹配的模式，以及遇到之些模式时将执行的动作，通过Digester可以很容易地实现你自己的SAX解析器，并让你无须处理复杂的SAX API，即可完成你想要的功能。Digester只是SAX外面的一个轻量级外壳，其解析的速度差不多和SAX一样快的。
   看一个XML文件:

<?xml version="1.0"?>
<plays>
  <play genre="tragedy" year="1603" language="english">
    <name>Hamlet</name>
    <author>William Shakespeare</author>
    <summary>
      Prince of Denmark freaks out, talks to ghost, gets into a
      crazy nihilistic funk, and dies in a duel.
    </summary>
    <characters>
      <character protagonist="false">
        <name>Claudius</name>
        <description>King of Denmark</description>
      </character>
      <character protagonist="true">
        <name>Hamlet</name>
        <descr> Son to the late, and nephew of the present king </descr>
      </character>
      <character protagonist="false">
        <name>Horatio</name>
        <descr>friend to Hamlet </descr>
      </character>
    </characters>
  </play>
</plays>

为了解析这一个配置文件的内容，相对应地我们写出下面的类：

public class Play {
    private String genre;
    private String year;
    private String language;
    private String name;
    private String author;
    private String summary;
    private List characters = new ArrayList( );
    // accessors omitted for brevity
    // Add method to support adding elements to characters.
    public void addCharacter(Character character) {
        characters.add( character );
    }
}
public class Character {
    private String name;
    private String description;
    private boolean protagonist;
    // accessors omitted for brevity
}

下一步，写出Digester的配置文件:

<?xml version="1.0"?>
<digester-rules>
  <pattern value="plays/play">
    <object-create-rule classname="xml.digester.Play"/>
    <set-next-rule methodname="add" paramtype="java.lang.Object"/>
    <set-properties-rule/>
    <bean-property-setter-rule pattern="name"/>
    <bean-property-setter-rule pattern="summary"/>
    <bean-property-setter-rule pattern="author"/>
    <pattern value="characters/character">
      <object-create-rule classname="xml.digester.Character"/>
      <set-next-rule methodname="addCharacter" 
                     paramtype="xml.digester.Character"/>
      <set-properties-rule/>
      <bean-property-setter-rule pattern="name"/>
      <bean-property-setter-rule pattern="descr" 
                                 propertyname="description"/>
    </pattern>
  </pattern>
</digester-rules>

下面是其解析的过程,是不是相对于SAX来说简单多了啊,呵呵:

import org.apache.commons.digester.Digester;
import org.apache.commons.digester.xmlrules.DigesterLoader;

List plays = new ArrayList( );
// Create an instance of the Digester from the XML rule set
URL rules = getClass( ).getResource("./play-rules.xml");
Digester digester = DigesterLoader.createDigester(rules);
// Push a reference to the plays List on to the Stack
digester.push(plays);

// Parse the XML document
InputStream input = getClass( ).getResourceAsStream("./plays.xml");
Object root = digester.parse(input);

// The XML document contained one play "Hamlet"
Play hamlet = (Play) plays.get(0);
List characters = (List) hamlet.getCharacters( );

2、将Beans转化为XML文档
使用Commons Betwixt里面的org.apache.commons.betwixt.io.BeanWriter类可以把某个bean转换为一个xml文档,看下面的例子：

Play play = new Play( );
// populatePlay populates all properties and nested Character objects
populatePlay( play );
// Write XML document
BeanWriter beanWriter = new BeanWriter( );
beanWriter.enablePrettyPrint( );
beanWriter.write( play );
System.out.println( beanWriter.toString( ) );

短短的几行代码就可以实现这一个功能了，简单吧;注意一下，beanWriter.enablePrettyPrint( )是启用缩进格式.

BeanWriter 类还用于向某个OutputStream或Writer写入XML文档，只要向其构造函数传一个Writer或OutputStream实例即可.

import org.apache.commons.betwixt.io.BeanWriter

Play play = new Play( );
populatePlay( play );

Writer outputWriter = new FileWriter("test.dat");
BeanWriter beanWriter = new BeanWriter( outputWriter );
beanWriter.setEndOfLine( "\r\n" );
beanWriter.setIndent( "\t" );
beanWriter.enablePrettyPrint( );
beanWriter.write( play );
outputWriter.close( );

另外提示一下，beanWriter.setEndOfLine()方法可接受一个String作为行结束序列。beanWriter.setIndent()方法接受一个String参数，该String将在BeanWriter中enablePrettyPrint()开启缩进时用作缩进字符串。

3、转换ＸＭＬ文档为Beans
其实这一部分的内容和第一部分的内容差不多有点类似的，有兴趣的朋友可以对比一下.
Commons Betwixt使用Commons Digester解析XML，而BeanReader即为Digester的一个子类。BeanReader使用自省机制以及类路径中的Betwixt映射文件来创建了一组Digester规则.
举例，有一个XMl文件就为前面第一个ＸＭＬ文件

import org.apache.commons.betwixt.io.BeanReader;

InputStream customPlay = 
    getClass( ).getResourceAsStream("./customized-play.xml");
BeanReader beanReader = new BeanReader( );
beanReader.getXMLIntrospector( ).setWrapCollectionsInElement(false);
beanReader.registerBeanClass(Play.class);
Play play = (Play) beanReader.parse( customPlay );

注：些文章中的代码来自<<Jakarta Commons Cookbook>>第六章内容