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 devel
oped 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。