请教拦截器问题

我要写个Interceptor，目的是让输入的值改变，比如：在jsp中，

在传到action之前，我要把“biaoti”到值“biaoti1111” 改成 “biaoti2222”
在action直接取biaoti值就得到“biaoti2222” 请问。我在拦截器中该咋写啊？？？

我写的是String ss=request.getParameter("biaoti");
但request没有setParameter()方法啊？
我还有别的办法吗，难道我只有去action中把得到的值改一般？那样工作量太大了，我写了N个action，都需要此操作！！天啊！！

[code="java"]
package interceptor;

import action.EAction;
import com.opensymphony.xwork.Action;
import com.opensymphony.xwork.ActionInvocation;
import com.opensymphony.xwork.interceptor.AroundInterceptor;

public class ConvertInterceptor extends AroundInterceptor {

public String intercept(ActionInvocation invocation) throws Exception {
    before(invocation);
    String result = invocation.invoke();
    after(invocation, result);

    return result;
}

protected void after(ActionInvocation actionInvocation, String s) throws Exception {
}


protected void before(ActionInvocation actionInvocation) throws Exception {
     Action action = actionInvocation.getAction();
     if(action instanceof EAction) {
         EAction eAction = (EAction) action;
         eAction.setParam("hello, world");  //你要修改的参数名称
     }

}

}
[/code]
假如说下面就是你要修改的Action
[code="java"]
package action;

import com.opensymphony.xwork.ActionSupport;

public class EAction extends ActionSupport {

public String execute() throws Exception {
    System.out.println("param = " + param);
    return SUCCESS;
}

public String getParam() {
    return param;
}

public void setParam(String param) {
    this.param = param;
}

private String param;

}
[/code]

另外，把这个interceptor放到你的所有的interceptor之后就可以了，例如
[code="xml"]

        <interceptor-stack name="myStack">
            <interceptor-ref name="completeStack"/>
            <interceptor-ref name="convertInterceptor"/>
         </interceptor-stack>

[/code]