转换器（Converter）——Struts 2.0中的魔术师

http://www.35java.com/zhibo/forum.php?mod=viewthread&tid=376&extra=page%3D3
开发Web应用程序与开发传统桌面应用程序不同，Web应用程序实际上是分布个不同的主机（当然也可以同一个主机，不过比较少见）上的两个进程之间互交。这种互交建立在HTTP之上，它们互相传递是都是字符串。换句话说，        服务器可以的接收到的来自用户的数据只能是字符串或字符数组，而在服务器上的对象中，这些数据往往有多种不同的类型，如日期（Date），整数（int），浮点数（float）或自定义类型（UDT）等，如图1所示。因此，我们需要服务器端将字符串转换为适合的类型。
                                                                               
                                图1　UI与服务器对象关系               
                        同样的问题也发生在使用UI展示服务器数据的情况。HTML的Form控件不同于桌面应用程序可以表示对象，其值只能为字符串类型，所以我们需要通过某种方式将特定对象转换成字符串。
                        要实现上述转换，Struts 2.0中有位魔术师可以帮到你——Converter。有了它，你不用一遍又一遍的重复编写诸如此类代码：
                        Date birthday = DateFormat.getInstance(DateFormat.SHORT).parse(strDate);
        <input type="text" value="<%= DateFormat.getInstance(DateFormat.SHORT).format(birthday)        %>" />
                        好了，现在让我们来看一个例子。
                        转换器——Hello World                        在我的上一篇文章《在Struts            2.0中国际化(i18n)您的应用程序》的最后我举了一个可以让用户方便地切换语言的例子，下面例子与其相似，但实现方法不同。
                        首先，如《在Struts            2.0中国际化(i18n)您的应用程序》的第一个例子一样，创建和配置默认的资源文件；
                        接着，新建源代码文件夹下的tutorial包创建HelloWorld.java文件，代码如下：

                                                                                package                                tutorial;

                                import                                java.util.Locale;

                                import                                com.opensymphony.xwork2.ActionSupport;
                                import                                com.opensymphony.xwork2.util.LocalizedTextUtil;

                                public                               
                                class                                HelloWorld                                                                                extends                                ActionSupport                                                                                                                                                                                                                                                {
                                                                                                           private                                                                                                            String msg;
                                                                                                               private                                                                                                                Locale loc                                                 =                                                                                                                    Locale.US;
                                                                     
                                                                                                                       public                                                                                                                        String getMsg()                                                                                                                                                                                                                                                                                 {
                                                                                                                                                       return                                                                msg;       
                                                                                       }                                                                                                                                                               
                                                                                                                                                          
                                                                                                                                           public                                                                                                                                            Locale getLoc()                                                                                                                                                                                                                                                                                 {
                                                                                                                                                                           return                                                                loc;
                                                                                                           }                                                                                                                                                               
                                                                                                                                                                              
                                                                                                                                                               public                                               
                                                void                                                setLoc(Locale                                                                                                                loc)                                                                                                                                                                                                                                                                                 {
                                                                                                                                                                                               this                                                                .loc                                                                                                                                                                                        =                                                                loc;
                                                                                                                               }                                                                                                                                                               
                                                                                                                                                                                                  
                                                                                                                                   @Override
                                                                                                                                                                                   public                                                                                                                                                                                    String execute()                                                                                                                                                                                                                                                                                 {
                                                                                                                                       //                                                                                                                                    LocalizedTextUtil是Struts 2.0中国际化的工具类，<s:text>标志就是通过调用它实现国际化的                                                                                                                                               
                                                                                                                                                                                                                        msg                                                                                                                                                 =                                                                LocalizedTextUtil.findDefaultText(                                                                "                                                                HelloWorld                                                                "                                                                , loc);
                                                                                                                                                                   return                                                                SUCCESS;
                                                                                                   }                                                                                                                                                               
                                                                }                                               
                        然后，在源代码文件夹下的struts.xml加入如下代码新建Action：

                                                <                                package                                 name                                ="ConverterDemo"                                extends                                ="struts-default"                                >                               
                                <                                action                                                    name                                ="HelloWorld"                                class                                ="tutorial.HelloWorld"                                >                               
                                <                                result                                >                                /HelloWorld.jsp                                </                                result                                >                               
                                </                                action                                >                               
                                </                                package                                >               
                        再在Web文件夹下，新建 HelloWorld.jsp，代码如下：

                                                <                                %@ page                                  contentType                                ="text/html; charset=UTF-8"                                %                                >                               
                                <                                %@taglib                                                prefix                                ="s"                                uri                                ="/struts-tags"                                %                                >                               
                                <                                html                                >                               
                                <                                head                                >                               
                                <                                title                                >                                Hello World                                </                                title                                >                               
                                </                                head                                >                               
                                <                                body                                >                               
                                <                                s:form                                                                                            action                                ="HelloWorld"                                theme                                ="simple"                                >                                                                                                                  
                                                                                Locale:                                 <                                s:textfield                                 name                                ="loc"                               
                                />                                &nbsp;                                <                                s:submit                                 />                               
                                </                                s:form                                >                               
                                <                                h2                                ><                                s:property                                 value                                ="msg"                               
                                /></                                h2                                >                               
                                </                                body                                >                               
                                </                                html                                >               
                        接下来，在源代码文件夹的tutorial包中新建LocaleConverter.java文件，代码如下：

                                                                                package                                tutorial;

                                import                                java.util.Locale;
                                import                                java.util.Map;

                                public                               
                                class                                LocaleConverter                                                                        extends                                ognl.DefaultTypeConverter                                                                                                                                                                                                                                        {
                                                   @Override
                                                                                                   public                                                                                                    Object convertValue(Map context, Object value, Class toType)                                                                                                                                                                                                                                                                                 {
                                                                                                                                   if                                                                (toType                                                                                                                            ==                                                                Locale.                                                                class                                                                )                                                                                                                                                                                                                                                                                                                                                                 {
                                                                                   String locale                                                                                                                                                             =                                                                                ((String[]) value)[                                                                                0                                                                                ];
                                                                                                                                                                           return                                                                               
                                                                                new                                                                                                                                                                        Locale(locale.substring(                                                                                0                                                                                ,                                                                                                                                                                        2                                                                                ), locale.substring(                                                                                3                                                                                ));
                                                                                                       }                                                                                                                               
                                                                else                                                               
                                                                if                                                                (toType                                                                 ==                                                                String.                                                                class                                                                )                                                                                                                                                                                                                                                                                                                                                                 {
                                                                                   Locale locale                                                                                                                                                             =                                                                                (Locale) value;
                                                                                                                                                                           return                                                                                locale.toString();
                                                                                           }                                                                                                                                                                                                               
                                                                                                                                                                                                                                               return                                                               
                                                                null                                                                ;
                                                                                                   }                                                                                                                                                               
                                                                }                                               
                            再接下来，在源代码文件夹下新建xwork-conversion.properties，并在其中添加如下代码：

                                                java.util.Locale                                =                                tutorial.LocaleConverter               
                        发布运行应用程序，在浏览器中键入http://localhost:8080/Struts2_Converter/HelloWorld.action，输出页面如图2所示：

图2 HelloWorld英文输出
                        在Locale输入框中输入“zh_CN”，按“Submit”提交，出现如图3所示页面：

图3 HelloWorld中文输出
                        上述例子中，Locale文本输入框对应是Action中的类型为java.util.Locale的属性loc，所以需要创建一个自定义转变器实现两者间的转换。所有的Struts        2.0中的转换器都必须实现ognl.TypeConverter接口。        为了简单起见，OGNL包也为你提供了ognl.DefaultTypeConverter类去帮助您实现转换器。在例子中，LocaleConverter继承了ognl.DefaultTypeConverter，重载了其方法原型为“public        Object convertValue(Map context, Object value, Class toType)”的方法。下面简单地介绍一下函数的参数：