XStream详解

XStream的作用

XStream可以把JavaBean对象转换成XML！

通常服务器向客户端响应的数据都是来自数据库的一组对象，而我们不能直接把对象响应给客户端，所以我们需要把对象转换成XML再响应给客户端，这时就需要使用XStream组合了

XStream相关JAR包

我们可以到http://xstream.codehaus.org/地址去下载XStream安装包！

XStream的必导JAR包：

核心JAR包：xstream-1.4.7.jar；

必须依赖包：xpp3_min-1.1.4c（XML Pull Parser，一款速度很快的XML解析器）

XStream举例

首先创建两个JavaBean对象

City类----------------------------
public class City {

    private String name;
    private String description;
    
    public City(String name,String description) {
        this.name=name;
        this.description=description;
    }
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    @Override
    public String toString() {
        return "City [name=" + name + ", description=" + description + "]";
    }


}

Province类--------------------------
public class Province {

    private String name;
    private List<City> cities = new ArrayList<City>();
    
    public Province(String name) {
        this.name=name;
    }
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<City> getCities() {
        return cities;
    }
    
    public void addCity(City name) {
        cities.add(name);
    }
    @Override
    public String toString() {
        return "Province [name=" + name + ", cities=" + cities + "]";
    }


}

接下来，我们需要写一个main()，创建一个List，List中存放两个Province对象！最终我们把List转换成XML

下面是使用XStream转换list为XML的代码

        Province p1 = new Province("辽宁省");
        p1.addCity(new City("沈阳", "shenyang"));
        p1.addCity(new City("大连", "dalian"));
        
        Province p2 = new Province("吉林省");
        p2.addCity(new City("长春", "changchen"));
        p2.addCity(new City("白城", "baicheng"));
        
        List<Province> list = new ArrayList<Province>();

        list.add(p1);
        list.add(p2);
//--------------------------------------------------------

　　　　   XStream xstream = new XStream();

        String s = xstream.toXML(list);

        System.out.println(s);

下面是运行结果

<list>
  <cn.itcast.xstream.demo1.Province>
    <name>辽宁省</name>
    <cities>
      <cn.itcast.xstream.demo1.City>
        <name>沈阳</name>
        <description>shenyang</description>
      </cn.itcast.xstream.demo1.City>
      <cn.itcast.xstream.demo1.City>
        <name>大连</name>
        <description>dalian</description>
      </cn.itcast.xstream.demo1.City>
    </cities>
  </cn.itcast.xstream.demo1.Province>
  <cn.itcast.xstream.demo1.Province>
    <name>吉林省</name>
    <cities>
      <cn.itcast.xstream.demo1.City>
        <name>长春</name>
        <description>changchen</description>
      </cn.itcast.xstream.demo1.City>
      <cn.itcast.xstream.demo1.City>
        <name>白城</name>
        <description>baicheng</description>
      </cn.itcast.xstream.demo1.City>
    </cities>
  </cn.itcast.xstream.demo1.Province>
</list>

使用alias，在生成的XML中，与类名对应的元素名称包含了包名部分，这很不好看！若想自定义生成的元素名称，需要使用XStream为类名提供别名：

xstream.alias("province", Province.class);
        xstream.alias("china", List.class); 
        xstream.alias("city", City.class);
---------------------------------------------------------------------------------------
结果为：
<china>
  <province>
    <name>辽宁省</name>
    <cities>
      <city>
        <name>沈阳</name>
        <description>shenyang</description>
      </city>
      <city>
        <name>大连</name>
        <description>dalian</description>
      </city>
    </cities>
  </province>
  <province>
    <name>吉林省</name>
    <cities>
      <city>
        <name>长春</name>
        <description>changchen</description>
      </city>
      <city>
        <name>白城</name>
        <description>baicheng</description>
      </city>
    </cities>
  </province>
</china>

把子元素变为元素属性，例如我们需要把<province>的子元素<name>变成：<province name=””>样式，那么需要调用如下方法

xstream.useAttributeFor(Province.class, "name");
--------------------------------------------------------------
结果为
<china>
  <province name="辽宁省">
    <cities>
      <city>
        <name>沈阳</name>
        <description>shenyang</description>
      </city>
      <city>
        <name>大连</name>
        <description>dalian</description>
      </city>
    </cities>
  </province>
  <province name="吉林省">
    <cities>
      <city>
        <name>长春</name>
        <description>changchen</description>
      </city>
      <city>
        <name>白城</name>
        <description>baicheng</description>
      </city>
    </cities>
  </province>
</china>

去除集合属性对应元素

因为Pronvice类有一个cities成员，所以生成了<cities>元素，但这个元素对XML文档而言没有什么意义，所以我们希望把它去除！

xstream.addImplicitCollection(Province.class, "cities");
--------------------------------------------------------------------------
结果为
<china>
  <province name="辽宁省">
    <city>
      <name>沈阳</name>
      <description>shenyang</description>
    </city>
    <city>
      <name>大连</name>
      <description>dalian</description>
    </city>
  </province>
  <province name="吉林省">
    <city>
      <name>长春</name>
      <description>changchen</description>
    </city>
    <city>
      <name>白城</name>
      <description>baicheng</description>
    </city>
  </province>
</china>

让类的成员不生成对应XML元素

每个类，每个成员都有对应的元素（或属性）存在，但有时我们并不希望某些类的成员在对应的XML文档中出现，例如我们不希望City类的description成员出现在XML文档中，可以使用下面方法

xstream.omitField(City.class, "description"); //不希望description属性出现
----------------------------------------------------------
结果为
<china>
  <province name="辽宁省">
    <city>
      <name>沈阳</name>
    </city>
    <city>
      <name>大连</name>
    </city>
  </province>
  <province name="吉林省">
    <city>
      <name>长春</name>
    </city>
    <city>
      <name>白城</name>
    </city>
  </province>
</china>