需求：用Session模拟登录，退出登录场景
项目结构：

Paste_Image.png

代码：
User

public class User {
    private String id;
    private String name;
    private String password;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    
}

UserDao

public class UserDao {
    public static User findByName(String name){
        Document doc = XMLUtil.getDocument();
        Element em = (Element)doc.selectSingleNode("//name[text()='"+name+"']");
        User user = null;
        if (em!=null){
            user = new User();
            user.setName(name);
            Element el = em.getParent();
            user.setId(el.attributeValue("id"));
            user.setPassword(el.elementText("password"));
        }
        return user;
    }
}

XMLUtil

public class XMLUtil {

    /**
     * 读取xml文件，返回document对象
     */
    public static Document getDocument(){
        try {
            InputStream in = Thread.currentThread().getContextClassLoader().getResourceAsStream("user.xml");
            
            Document doc = new SAXReader().read(in);
            return doc;
        } catch (DocumentException e) {
            e.printStackTrace();
            //把转换为运行时异常抛出即可！
            throw new RuntimeException(e);
        }
    }
    
    /**
     * 传如docuemnt对象，写出到xml文件中
     */
    public static void write2xml(Document doc){
        try {
            OutputStream out = new FileOutputStream("e:/user.xml");
            OutputFormat format = OutputFormat.createPrettyPrint();
            XMLWriter writer = new XMLWriter(out,format);
            writer.write(doc);
            writer.close();
        } catch (Exception e) {
            e.printStackTrace();
            //把转换为运行时异常抛出即可！
            throw new RuntimeException(e);
        }
    }
}

UserIndexServlet

public class UserIndexServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
 
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession(false);
        response.setContentType("text/html;charset=utf-8");
        if (session!=null){ //如果session为空，说明从来没有登录过
            User user = (User) session.getAttribute("name");
            if (user!=null)
                response.getWriter().write("欢迎您回来"+user.getName()+"<a href='/session_login/LogoutServlet'>[退出登录]</a>");
            else
                response.sendRedirect(request.getContextPath()+"/login.html");
                
        }else{
            response.sendRedirect(request.getContextPath()+"/login.html");
        }
    }

    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

LoginServlet

public class LoginServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");
        String name = request.getParameter("name");
        String password = request.getParameter("password");
        User user = UserDao.findByName(name);
        if (user!=null){
            if (user.getPassword().equals(password)){
                HttpSession settion = request.getSession();
                settion.setAttribute("name", user);
                response.sendRedirect(request.getContextPath()+"/UserIndexServlet");
            }else
                response.getWriter().write("用户名或密码不正确");
        }else
            response.getWriter().write("用户名或密码不正确");
        
    }

    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

LogoutServlet

public class LogoutServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public LogoutServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        if (session !=null){
            session.removeAttribute("name");
            response.sendRedirect(request.getContextPath()+"/UserIndexServlet");
        }
    }
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

效果展示：

session_login.gif