XStream别名指南

1，存在的问题
设想我们的客户端定义了一个用于XStream读写的XML文件：
我们将设计一些模型类并配置XStream按照这个XML文件格式执行读写操作。

<blog author="Guilherme Silveira"> 
  <entry> 
    <title>first</title> 
    <description>My first blog entry.</description> 
  </entry> 
  <entry> 
    <title>tutorial</title> 
    <description> 
        Today we have developed a nice alias tutorial. Tell your friends! NOW! 
    </description> 
  </entry> 
</blog>

2，模型：
首先，建立一个简单的Blog对象：

package com.thoughtworks.xstream;  
public class Blog { 
        private Author author; 
        private List entries = new ArrayList();  
        public Blog(Author author) { 
                this.author = author; 
        }  
        public void add(Entry entry) { 
                entries.add(entry); 
        }  
        public List getContent() { 
                return entries; 
        } 
}

然后是一个带有名字的作者对象：

package com.thoughtworks.xstream;  
public class Author { 
        private String name; 
        public Author(String name) { 
                this.name = name; 
        } 
        public String getName() { 
                return name; 
        } 
}

具体的blog内容对象：

package com.thoughtworks.xstream;  
public class Entry { 
        private String title, description; 
        public Entry(String title, String description) { 
                this.title = title; 
                this.description = description; 
        } 
}

虽然我们没有创建getters/setters方法，但这并不影响XStream对XML->Object文件的解析。
3，简单的测试
首先初始化一个blog实例，然后使用XStream来序列化

public static void main(String[] args) {  
        Blog teamBlog = new Blog(new Author("Guilherme Silveira")); 
        teamBlog.add(new Entry("first","My first blog entry.")); 
        teamBlog.add(new Entry("tutorial", 
                "Today we have developed a nice alias tutorial. Tell your friends! NOW!")); 
        XStream xstream = new XStream(); 
        System.out.println(xstream.toXML(teamBlog)); 
}

由该Blog实例解析出的XML文件为： 

<com.thoughtworks.xstream.Blog> 
  <author> 
    <name>Guilherme Silveira</name> 
  </author> 
  <entries> 
    <com.thoughtworks.xstream.Entry> 
      <title>first</title> 
      <description>My first blog entry.</description> 
    </com.thoughtworks.xstream.Entry> 
    <com.thoughtworks.xstream.Entry> 
      <title>tutorial</title> 
      <description> 
        Today we have developed a nice alias tutorial. Tell your friends! NOW! 
      </description> 
    </com.thoughtworks.xstream.Entry> 
  </entries> 
</com.thoughtworks.xstream.Blog>

4，为类取别名
首先我们来改变XStream对com.thoughtworks.xstream.Blog的输出名称。我们只想使用一个简单的blog来取代。下面为Blog类创建一个别名：
Xstream.alias("blog",Blog.class);
同样的，为Entry类创建一个别名：
Xstream.alias("entry",Entry.class);
好，到此输出的XML变为：
<blog> 
  <author> 
    <name>Guilherme Silveira</name> 
  </author> 
  <entries> 
    <entry> 
      <title>first</title> 
      <description>My first blog entry.</description> 
    </entry> 
    <entry> 
      <title>tutorial</title> 
      <description> 
        Today we have developed a nice alias tutorial. Tell your friends! NOW! 
      </description> 
    </entry> 
  </entries> 
</blog>


  

5，去掉entries标记

下面，我们将实施叫做"implicit collection"的过程（即取消标记）：所有的集合类型，都不需要显示他的根标签(root tag)，你可以直接使用一个implicit collection去映射。

在我们的例子里面，我们不希望出现entries标签，只需要一个接一个的列出所有的entry标签即可。

要做到这点，只需要简单的调用XStream对象上的addImplicitCollection方法，就可以配置XStream取消对entries的输出：
package com.thoughtworks.xstream; 
import java.util.ArrayList; 
import java.util.List; 
public class Test { 
        public static void main(String[] args) { 
                Blog teamBlog = new Blog(new Author("Guilherme Silveira")); 
                teamBlog.add(new Entry("first","My first blog entry.")); 
                teamBlog.add(new Entry("tutorial", 
                        "Today we have developed a nice alias tutorial. Tell your friends! NOW!")); 
                XStream xstream = new XStream(); 
                xstream.alias("blog", Blog.class); 
                xstream.alias("entry", Entry.class); 
                xstream.addImplicitCollection(Blog.class, "entries"); 
                System.out.println(xstream.toXML(teamBlog)); 
        } 
}


注意addImplicitCollection方法的调用，需要描述在某个类上的某个成员变量不需要被显示。

得到的结果基本上达到了要求：

<blog> 
  <author> 
    <name>Guilherme Silveira</name> 
  </author> 
  <entry> 
    <title>first</title> 
    <description>My first blog entry.</description> 
  </entry> 
  <entry> 
    <title>tutorial</title> 
    <description> 
        Today we have developed a nice alias tutorial. Tell your friends! NOW! 
    </description> 
  </entry> 
</blog>

7，为属性添加别名
下一步是要把author成员变量设置为XML的属性。要做到这点，我们需要告诉XStream将author属性作为Blog类的"author"属性。
xstream.useAttributeFor(Blog.class,"author")；
现在留给我们一个问题，XStream怎么讲一个Author转换成一个String对象让他在Blog节点中以author属性显示？
只需要使用SimpleValeConverter并且实现我们自己的Author转换器：
class AuthorConverter implements SingleValueConverter {
}
第一个需要实现的方法是告诉XStream该转化器是用来转换什么类型的对象：
        public boolean canConvert(Class type) {
                return type.equals(Author.class);
        }
接下来是将一个Author实例转化成字符串：
public String toString(Object obj) {
                return ((Author) obj).getName();
        }
最后是相反的工作：怎么从一个字符串中得到Author实例
        public Object fromString(String name) {
                return new Author(name);
        }
最后，该转化器看起来是这样：
class AuthorConverter implements SingleValueConverter {
        public String toString(Object obj) {
                return ((Author) obj).getName();
        }
        public Object fromString(String name) {
                return new Author(name);
        }
        public boolean canConvert(Class type) {
                return type.equals(Author.class);
        }
 }
然后将这个转化器注册到XStream：

public class Test { 
        public static void main(String[] args) { 
                Blog teamBlog = new Blog(new Author("Guilherme Silveira")); 
                teamBlog.add(new Entry("first","My first blog entry.")); 
                teamBlog.add(new Entry("tutorial", 
                        "Today we have developed a nice alias tutorial. Tell your friends! NOW!")); 
                XStream xstream = new XStream(); 
                xstream.alias("blog", Blog.class); 
                xstream.alias("entry", Entry.class); 
                xstream.addImplicitCollection(Blog.class, "entries"); 
                xstream.useAttributeFor(Blog.class, "author"); 
                xstream.registerConverter(new AuthorConverter()); 
                System.out.println(xstream.toXML(teamBlog)); 
        } 
}

最后的输出：

<blog author="Guilherme Silveira"> 
  <entry> 
    <title>first</title> 
    <description>My first blog entry.</description> 
  </entry> 
  <entry> 
    <title>tutorial</title> 
    <description> 
        Today we have developed a nice alias tutorial. Tell your friends! NOW! 
    </description> 
  </entry> 
</blog>

在这里，useAttributeFor方法被其他几个相似功能的方法重载，包括一个接受一个额外的字符串(Class , String, String)的版本，该版本告诉XStream将该属性创建为另一个别名，比如在这里，如果使用useAttributeFor(Blog.class , "author", "auth")会将"author"属性在XML中映射成"auth"属性。