java-web11-过滤器、监听器思想简单了解

11.Filter过滤器

11.1 Filter过滤器介绍

• filter：过滤器，用来过滤网站的数据。
• 应用场景：处理中文乱码，登录验证。
  
• 过滤器和servlet一样都需要request和response对象。

11.2 编写过滤器步骤

• 导包
  
• 编写过滤器，实现过滤器接口，重写方法

public class CharacterEncoding  implements Filter {
    /*初始化：web服务器启动时初始化*/
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("过滤器初始化");
    }
    /*过滤所有的代码，在过滤特定的请求时会执行
    * 必须要让过滤器继续执行*/
    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        /*多个过滤器链交接，让请求继续向下传送，否则被拦截在此*/
        chain.doFilter(req,resp);
    }
    /*销毁：web服务器关闭时销毁*/
    @Override
    public void destroy() {
        System.out.println("过滤器销毁");
    }
}

• 在web.xml中配置filter过滤器

<filter>
        <filter-name>CharacterEncoding</filter-name>
        <filter-class>com.zk.filter.CharacterEncoding</filter-class>
    </filter>
    <!--尽量不用/*对所有请求过滤，可以写多个过滤路径-->
    <filter-mapping>
        <filter-name>CharacterEncoding</filter-name>
        <url-pattern>/ds01</url-pattern>
<!--        <url-pattern>/*</url-pattern>-->
    </filter-mapping>

11.3 常用场景

• 登录成功后进入主页，否则回退到登录页

• • 用户登录之后，向session中存入用户数据
  • 进入主页时判断用户是否已经登录，要求在过滤器中实现

  @Override
  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
      HttpServletRequest req = (HttpServletRequest)request;
      HttpServletResponse resp = (HttpServletResponse)response;
      String session_id = (String) req.getSession().getAttribute(Constant.Session_id);
      if (session_id == null) {
          System.out.println("过滤时返回login.jsp");
          resp.sendRedirect("/error.jsp");
      }
      chain.doFilter(req,resp);
  }
  

• 优化将常量转为实体类静态常量进行维护

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        if (username.equals("admin")) {
            req.getSession().setAttribute(Constant.Session_id,req.getSession().getId());
            resp.sendRedirect("/sys/success.jsp");
        }else {
            resp.sendRedirect("/error.jsp");
        }
    }

public class Constant {
    public static final String Session_id = "Session_id";
}

• 早期权限分离的思想
  

12.监听器

12.1 监听器编写

• 实现监听器接口

• 重写里面的方法

public class ListenterDemo0 implements HttpSessionListener {
    /*创建session监听
    * 观察者模式：一旦创建session就会触发这个事件*/
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext servletContext = se.getSession().getServletContext();
        System.out.println(se.getSession().getId());
        Integer usercount = (Integer) servletContext.getAttribute("usercount");
        if (usercount==null) {
            usercount = new Integer(1);
        }else {
            usercount +=1;
        }
        servletContext.setAttribute("usercount",usercount);
    }
    /*一旦销毁session就会触发这个事件*/
    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext servletContext = se.getSession().getServletContext();
        Integer usercount = (Integer) servletContext.getAttribute("usercount");
        if (usercount==null) {
            usercount = new Integer(0);
        }else {
            usercount -=1;
        }
        servletContext.setAttribute("usercount",usercount);
    }
}

• web.xml中配置监听器

<listener>
    <listener-class>com.zk.listenter.ListenterDemo0</listener-class>
</listener>

• 多人在线是因为关闭浏览器session不会销毁，只是浏览器cookie存的sessionid没了，所以再访问会重新创建一个session，原来的session要等超时或者重启服务器才会消失

12.2 常见场景

• GUI编程中使用到

public class FrameDemo02 {
    public static void main(String[] args) {
        Frame frame = new Frame("xx游戏");
        Panel panel = new Panel(null);
        panel.setLayout(null);
        frame.setBounds(20,20,300,300);
        frame.setBackground(Color.LIGHT_GRAY);
        panel.setBounds(50,50,200,200);
        panel.setBackground(new Color(0,30,0));
        frame.add(panel);
        frame.setVisible(true);
        /*frame.addWindowListener(new WindowListener() {
            @Override
            public void windowOpened(WindowEvent e) {
                System.out.println("打开");
            }

            @Override
            public void windowClosing(WindowEvent e) {
                System.out.println("关闭中");
                System.exit(0);  //0~1正常、非正常关闭
            }

            @Override
            public void windowClosed(WindowEvent e) {
                System.out.println("关闭后");
            }

            @Override
            public void windowIconified(WindowEvent e) {
                System.out.println("Iconified");
            }

            @Override
            public void windowDeiconified(WindowEvent e) {
                System.out.println("Deiconified");
            }

            @Override
            public void windowActivated(WindowEvent e) {
                System.out.println("激活");
            }

            @Override
            public void windowDeactivated(WindowEvent e) {
                System.out.println("未激活");
            }
        });*/
        /*适配器模式：在接口上加一层来继承其几个方法即可*/
        frame.addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                System.exit(0);  //0~1正常、非正常关闭
            }
        });
    }
}

13.junit单元测试

• 导入依赖

<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.11</version>
</dependency>

• 在要测试的方法上加注解@Test即可直接运行方法（只能在方法上使用，看此注解源码上的源注解）