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前面一篇实现了用户登录过程，这篇来学习下如何把用户注销。用户注销，实际上就是把session对象给销毁。但是我们前面一篇没有使用session对象，所以这里我们先把前面文章使用request域对象保存user对象改成存储在session中，然后来看用户注销功能的实现过程。
1.优化前一篇代码
前面一篇，我们把用户登录成功的用户对象存在request域中，这个有两点不好。第一，存session更好，多个页面能拿到这个用户对象。第二个没有判断登录失败的情况。现在把LoginServlet.java代码修改如下
package com.anthony.web.servlet;import java.io.IOException;
import java.lang.reflect.InvocationTargetException;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import org.apache.commons.beanutils.BeanUtils;import com.anthony.domain.User;
import com.anthony.service.UserService;
import com.anthony.service.impl.UserServiceImpl;public class LoginServlet extends HttpServlet {protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {req.setCharacterEncoding("UTF-8");resp.setContentType("text/html;charset=UTF-8");//获取表单数据User user = new User();try {BeanUtils.populate(user, req.getParameterMap());// 调用业务方法UserService us = new UserServiceImpl();User u = us.login(user);//判断u是否为空if(u !=null) {//登录成功//分发转向req.getSession().setAttribute("u", u);req.getRequestDispatcher("/index.jsp").forward(req, resp);}else {//登录失败就让用户跳转登录页面resp.sendRedirect(req.getContextPath() +"/login.jsp");}} catch (IllegalAccessException e) {e.printStackTrace();} catch (InvocationTargetException e) {e.printStackTrace();}}protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req,resp);}}
添加判断，登录成功就把u对象存到session对象中，如果u等于空，也就是登录失败，就跳转到登录页面。
2.注销功能实现
2.1index.jsp链接添加跳转到LogoutServlet.java
<%@ page language="java" contentType="text/html; charset=UTF-8"pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>网站首页</title>
</head>
<body><!-- 如果u(来自request域对象)为空，显示登录和注册按钮，如果不为空，显示欢迎用户消息 --><c:if test="${empty u}"><a href="login.jsp">登录</a><a href="register.jsp">注册</a></c:if><c:if test="${not empty u}">欢迎您：${u.username}<a href="${pageContext.request.contextPath}/servlet/logoutServlet">注销</a></c:if>
</body>
</html>下面就来写LogoutServlet.java代码
2.2 新建LogoutServlet.java
在web.servlet包新建一个LogoutServlet.java， servlet代码内容如下。
package com.anthony.web.servlet;import java.io.IOException;
import java.lang.reflect.InvocationTargetException;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import org.apache.commons.beanutils.BeanUtils;import com.anthony.domain.User;
import com.anthony.service.UserService;
import com.anthony.service.impl.UserServiceImpl;public class LogoutServlet extends HttpServlet {protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {req.setCharacterEncoding("UTF-8");resp.setContentType("text/html;charset=UTF-8");//点击注销，意味session对象要销毁req.getSession().invalidate();//往首页跳转resp.sendRedirect(req.getContextPath()+"/index.jsp");}protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req,resp);}}
重新部署之后，登录成功，点击注销，会跳转到主页。
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