XStream別名指南

原文地址：http://ahang8415.javaeye.com/blog/319902

1，存在的問題
設想我們的用戶端定義了一個用於XStream讀寫的XML檔案：
我們將設計一些模型類並配置XStream按照這個XML檔案格式執行讀寫操作。

<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，建立一個簡單的Blog對象：

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;         } }

然後是一個帶有名字的作者對象：

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

具體的blog內容對象：

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

雖然我們沒有建立getters/setters方法，但這並不影響XStream對XML->Object檔案的解析。
3，簡單的測試
首先初始化一個blog執行個體，然後使用XStream來序列化

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)); }

由該Blog執行個體解析出的XML檔案為：

<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，為類取別名首先我們來改變XStream對com.thoughtworks.xstream.Blog的輸出名稱。我們只想使用一個簡單的blog來取代。下面為Blog類建立一個別名：Xstream.alias("blog",Blog.class);同樣的，為Entry類建立一個別名：Xstream.alias("entry",Entry.class);好，到此輸出的XML變為：
<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標記
下面，我們將實施叫做"implicit collection"的過程（即取消標幟）：所有的集合類型，都不需要顯示他的根標籤(root tag)，你可以直接使用一個implicit collection去映射。
在我們的例子裡面，我們不希望出現entries標籤，只需要一個接一個的列出所有的entry標籤即可。
要做到這點，只需要簡單的調用XStream對象上的addImplicitCollection方法，就可以配置XStream取消對entries的輸出：
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方法的調用，需要描述在某個類上的某個成員變數不需要被顯示。
得到的結果基本上達到了要求：
<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，為屬性添加別名
下一步是要把author成員變數設定為XML的屬性。要做到這點，我們需要告訴XStream將author屬性作為Blog類的"author"屬性。
xstream.useAttributeFor(Blog.class,"author")；
現在留給我們一個問題，XStream怎麼講一個Author轉換成一個String對象讓他在Blog節點中以author屬性顯示？
只需要使用SimpleValeConverter並且實現我們自己的Author轉換器：
class AuthorConverter implements SingleValueConverter { 
} 
第一個需要實現的方法是告訴XStream該轉化器是用來轉換什麼類型的對象：
        public boolean canConvert(Class type) { 
                return type.equals(Author.class); 
        } 
接下來是將一個Author執行個體轉化成字串：
public String toString(Object obj) { 
                return ((Author) obj).getName(); 
        } 
最後是相反的工作：怎麼從一個字串中得到Author執行個體
        public Object fromString(String name) { 
                return new Author(name); 
        } 
最後，該轉化器看起來是這樣：
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); 
        } 
} 
然後將這個轉化器註冊到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));         } }
最後的輸出：
<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>
在這裡，useAttributeFor方法被其他幾個相似功能的方法重載，包括一個接受一個額外的字串(Class , String, String)的版本，該版本告訴XStream將該屬性建立為另一個別名，比如在這裡，如果使用useAttributeFor(Blog.class , "author", "auth")會將"author"屬性在XML中映射成"auth"屬性。