public class DirList extends HttpServlet{

    /**
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        String filePath="c:\\";
        if(request.getParameter("filePath")!=null)filePath=request.getParameter("filePath");
        
        this.showDir(filePath,response.getWriter());
    }
    
    private void showDir(String filePath,PrintWriter write) throws IOException{

        File dirFile=new File(filePath);        
        File[] fileList=dirFile.listFiles();
                
        for(int i=0;i<fileList.length;i++){
            File fileTmp=fileList[i];
            if(fileTmp.isFile()){
                write.write(fileTmp.getName()+"<br>");
            }else{
                
                write.write("<span>+</span>"+fileTmp.getName()+"<br>");
                write.write("<div style='margin-left:10;border:1px red solid;'>");
                
                
                //就是下面这句,注释掉就不报错了

                //this.showDir(fileTmp.getAbsolutePath(), write); 
            
            }

        }
        write.write("</div>");
    }

    /**
     * The doPost method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to post.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        this.doGet(request, response);
    }

}