Servlet不可用是什么情况？

这是XML配置文件<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
<servlet-name>DDServlet</servlet-name>
<servlet-class>com.dd.action.DDServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>DDServlet</servlet-name>
<url-pattern>*.do</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>这是servlet里面的代码package com.dd.action;import java.io.IOException;
import java.util.List;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import tools.ObjectFactory;import com.dd.bean.Book;
import com.dd.service.BookService;
@SuppressWarnings("serial")
public class ServletForDD extends HttpServlet {
private BookService bookService ;
public void init() throws ServletException {
System.out.println("初始化");
this.bookService = (BookService) ObjectFactory.getBean("BookService");
}
public ServletForDD() {
super();
System.out.println("构造");
}
public void destroy() {
super.destroy(); 
System.out.println("销毁");
}
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doPost(request, response);

}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
String path = request.getContextPath();
String uri = request.getRequestURI();
String reName = uri.substring(path.length()+1,uri.lastIndexOf("."));
System.out.println("reName的值为："+reName);
if(reName.equals("toList")){
List<Book> list = this.bookService.findAll();
request.setAttribute("allBooks", list);
request.getRequestDispatcher("main.jsp").forward(request, response);
}
}}
求高手解答！！！！！！！！