过滤器Filter

过滤器基础

学习：https://www.jianshu.com/p/33827fba6429

编码过滤器

过滤器java

public class CharacterEncodingFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("初始化");
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
            throws IOException, ServletException {
        //过滤操作
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        servletResponse.setContentType("text/html;charset=UTF-8");

        System.out.println("过滤前");
        filterChain.doFilter(servletRequest,servletResponse);   //可能会有多个过滤器，让程序继续过滤。必须加上这个代码
        System.out.println("过滤后");
    }
    @Override
    public void destroy() {
        System.out.println("销毁");
    }
}

xml配置

    <filter>
        <filter-name>filter</filter-name>
        <filter-class>com.yuan.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>filter</filter-name>
        <url-pattern>/servlet/*</url-pattern>
    </filter-mapping>

模拟权限拦截

简单工具类

public class Name {
    public static String USERNAME = "username";
}

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
  <form action="/servlet/login" method="get">
      <input type="text" name="username">
      <input type="submit">
  </form>
</body>
</html>

loginServlet

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

UserFilter

public class UserFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest)servletRequest;
        HttpServletResponse resp = (HttpServletResponse) servletResponse;

        Object attribute = req.getSession().getAttribute(Name.USERNAME);
        if(attribute==null){
           resp.sendRedirect("/error.jsp");
        }
        filterChain.doFilter(req,resp);
    }
    @Override
    public void destroy() {
    }
}

sys.successful

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <h1>登陆成功</h1>
    <P><a href="/servlet/logout">注销</a> </P>
</body>
</html>

logoutServlet

public class logoutServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.getSession().removeAttribute(Name.USERNAME);
        resp.sendRedirect("/login.jsp");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

error.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <h1>权限不够</h1>
    <p>
        <a href="login.jsp">注销</a>
    </p>
</body>
</html>

xml配置

    <filter>
        <filter-name>UserFilter</filter-name>
        <filter-class>com.yuan.filter.UserFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>UserFilter</filter-name>
        <url-pattern>/sys/*</url-pattern>
    </filter-mapping>

监听器

监听器基础

学习：https://www.cnblogs.com/xing901022/p/4378727.html

监听在线人数

监听器java
监听在线人数，创建session时人数+1，销毁session时人数-1

public class OnlineCounterListener implements HttpSessionListener {
    @Override
    //session创建监听
    //有session创建时就会触发
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        //用ServletContext存放数据
        ServletContext context = httpSessionEvent.getSession().getServletContext();
        Integer count = (Integer) context.getAttribute("count");
        if(count==null){
            count = new Integer(1);
        }else{
            int i = count.intValue();
            count = new Integer(i+10);
        }
        context.setAttribute("count",count);
    }
    @Override
    //session销毁监听
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        //用ServletContext存放数据
        ServletContext context = httpSessionEvent.getSession().getServletContext();
        Integer count = (Integer) context.getAttribute("count");
        if(count==null){
            count = new Integer(0);
        }else{
            int i = count.intValue();
            count = new Integer(i-1);
        }
        context.setAttribute("count",count);
    }
}

xml配置

    <listener>
        <listener-class>com.yuan.listener.OnlineCounterListener</listener-class>
    </listener>

jsp
显示数据

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
    <h1>在线观看人数为：<span style="color: brown"><%=this.getServletConfig().getServletContext().getAttribute("count")%></span></h1>
  </body>
</html>