作用：用于将一个类声明为servlet
该注解在部署时会被容器处理，容器将根据具体的属性配置将相应的类部署为servlet
属性：下面是一些常用属性，value和url一般是必须的，但是二者不能共存，若同时指定，一般自动忽略value。

package com.item.filter;
import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;
 
//注解配置
@WebServlet(
displayName = "UserServlet" , //描述
name = "UserServlet", //servlet名称
urlPatterns = { "/user" }, //url
loadOnStartup = 1, //启动项
initParams = { @WebInitParam(name = "username", value = "张三") }
)//初始化参数
 
public class UserServlet extends HttpServlet {
 
	private String username;
 
	@Override 
	public void init(ServletConfig config) throws ServletException {  
	  //获取初始化信息：张三  
	  username = config.getInitParameter("username"); 
	} 
	  
	@Override protected 
	void doPost(HttpServletRequest req, HttpServletResponse resp)   throws ServletException, IOException {
		//主要内容操作 
	}
}