对HttpServletRequest的一点疑惑,求解惑

Question

设置了一个filter,在用户访问jsp时读取xml里面的配置,然后根据配置转发给action.
在读取xml的class: XmlReader内试图用
[code="java"]
HttpServletRequest request = ServletActionContext.getRequest();
[/code]
获取request时报空指针错误.
个人理解是Struts2中,HttpServletRequest 的传递路线(?描述似乎有问题) 是 用户请求(访问jsp)->Filter->Action.
即:

      HttpServletRequest                    HttpServletRequest 

JSP ------------------------------> Filter ------------------------------> Action
|
| Null
|-----------------------------> XmlReader --->NullPointerException

在filter中试图得到HttpServletRequest 都会报空指针错误.
不知道这种理解是否正确?

filter:
[code="java"]
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {

    HttpServletRequest re = (HttpServletRequest) request;
    HttpSession session = re.getSession();
    XmlReader xr = new XmlReader();
    String companyid = xr.getSettingValueByNode("CompanyId");
    String url = re.getServletPath().toString();

    // redirect to action accroding to the request url      
          ......
}

[/code]
XmlReader:
[code="java"]
public class XmlReader {
public XmlReader() {
};

public String getSettingValueByNode(String node) {
    SAXReader reader = new SAXReader();
    String value = "";
    try {   
             //Alert NullPointException here:
        HttpServletRequest request = ServletActionContext.getRequest;
             // Get real path           
        String realpath = request.getSession().getServletContext().getRealPath("/");
        // Get the real path of SystemConfig.xml
        Document doc = reader.read(new File(realpath + "\\SystemConfig.xml"));
        List nodes = doc.selectNodes("/SystemConfig/Setting/" + node);

        Iterator it = nodes.iterator();
        while (it.hasNext()) {
            Element elm = (Element) it.next();
            value = elm.getText();
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    System.out.println(value);
    return value;
}

}
[/code]
[b]问题补充：[/b]
谢谢holan ,
可以的,实际上也就这样做了,只是想确认一下这样的理解对不对而已.
[b]问题补充：[/b]
"因为struts2的filter在此时没有执行，所以上下文中就不可能有request对象了"

--我的filter是在struts2的FilterDispatcher之前执行的,但是在Filter中可以
[code="java"]
public void doFilter(ServletRequest request, ServletResponse response,

FilterChain chain) throws IOException, ServletException {

    HttpServletRequest re = (HttpServletRequest) request;   

......
[/code]
应该还是有request对象的吧? request不是应该在访问地址之后就产生的吗?

Answer 1

你没有明白我的意思，我不是说filter中无法访问到request对象，我的意思是说你通过ServletActionContext来获取request对象时，不一定能拿到，首先要明白ServletActionContext中的request对象是哪里来的呢，不用想也是在struts的filter中把request放进去的。如果你在struts的filter执行之前，使用ServletActionContext去获取request，当然是获取不到的。