Struts2 ParamsPrepareParamsStack & Partial Update

在struts-default.xml裡面有一組名為paramsPrepareParamsStack的interceptor-stack，如下：

struts-default.xml

<interceptor-stack name="paramsPrepareParamsStack">
    <interceptor-ref name="exception"/>
    <interceptor-ref name="alias"/>
    <interceptor-ref name="i18n"/>
    <interceptor-ref name="checkbox"/>
    <interceptor-ref name="multiselect"/>
    <interceptor-ref name="params">
        <param name="excludeParams">dojo\..*,^struts\..*,^session\..*,^request\..*,^application\..*,^servlet(Request|Response)\..*,parameters\...*</param>
    </interceptor-ref>
    <interceptor-ref name="servletConfig"/>
    <interceptor-ref name="prepare"/>
    <interceptor-ref name="chain"/>
    <interceptor-ref name="modelDriven"/>
    <interceptor-ref name="fileUpload"/>
    <interceptor-ref name="staticParams"/>
    <interceptor-ref name="actionMappingParams"/>
    <interceptor-ref name="params">
        <param name="excludeParams">dojo\..*,^struts\..*,^session\..*,^request\..*,^application\..*,^servlet(Request|Response)\..*,parameters\...*</param>
    </interceptor-ref>
    <interceptor-ref name="conversionError"/>
    <interceptor-ref name="validation">
        <param name="excludeMethods">input,back,cancel,browse</param>
    </interceptor-ref>
    <interceptor-ref name="workflow">
        <param name="excludeMethods">input,back,cancel,browse</param>
    </interceptor-ref>
</interceptor-stack>

官方說明

An example of the paramsPrepareParams trick. This stack is exactly the same as the defaultStack, except that it includes one extra interceptor before the prepare interceptor: the params interceptor.

This is useful for when you wish to apply parameters directly to an object that you wish to load externally (such as a DAO or database or service layer), but can't load that object until at least the ID parameter has been loaded. By loading the parameters twice, you can retrieve the object in the prepare() method, allowing the second params interceptor to apply the values on the object.

意思就是如果想要利用Struts2幫忙將partial parameters直接塞到model裡面的話，可以先利用第一次的ParametersInterceptor拿到id值，然後用PrepareInterceptor取得完整的model，再利用第二次的ParametersInterceptor讓Struts2將user要變更的資料塞到完整的model之中，這樣就可以處理頁面上user只做partial update的操作行為，好處就是在Action裡面可以直接將model往後端logic / data tier傳就可以了。