使用jspsmartuploader下载出现问题

package myjob.web;import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import com.jspsmart.upload.*;public class doDownload extends HttpServlet {
    private static final String CONTENT_TYPE = "text/html; charset=GBK";
    private ServletConfig config;
    //Initialize global variables
    public void init() throws ServletException {
        this.config = config;
    }
    final public ServletConfig getServletConfig(){
        return config;
    }
    //Process the HTTP Get request
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws
            ServletException, IOException {        response.setContentType(CONTENT_TYPE);
        String downfile = null;        try{
            downfile = "d:\\tmp\\a.txt";
            if (downfile!=null)
              downloadfile(request, response, downfile);
        }catch(Exception e){
          PrintWriter out = response.getWriter();
          out.print("downfile error.");
          String err=e.getMessage();
          out.print(err);
        }
    }
    //Process the HTTP Post request
     public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       doGet(request, response);
  }
    //Clean up resources
    public void destroy() {
    }
    private void downloadfile(HttpServletRequest request, HttpServletResponse response, String filename) throws Exception{
  // 新建一个SmartUpload对象
  SmartUpload mySmartUpload = new SmartUpload();
  mySmartUpload.initialize(config, request, response);   mySmartUpload.setContentDisposition("inline;"); 
    mySmartUpload.downloadFile(filename,null,"a.txt"); 
}
}源代码如上，使用时是在jsp文件中 <a href="doDownload" title="点击这里下载文件">下载</a>，点击下载。
运行后显示null，经检查，发现config为null，导致调用downloadfile时出现异常，显示错误为null。还应该做其他的设置么？