过滤器Filter

实现javax.servlet.Filter;

//只要是/servlet下的任何请求，都会经过此过滤器，
@WebFilter(filterName = "encodingFilter", value = {"/servlet/*"})
public class EncodingFilter implements Filter {
    @Override
    //初始化,服务器启动时初始化
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("过滤前");
    }

    @Override
    //chain:链
    /*
     * 1.过滤器中的所有代码，在过滤特定请求时都会执行
     * 2.必须要让过滤器继续通行，chain.doFilter(req, resp);
     * */
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        chain.doFilter(req, resp);
    }

    @Override
    //销毁，服务器关闭时销毁
    public void destroy() {
        System.out.println("销毁");
    }
}

监听器Listener

需要实现什么功能的监听器，就要实现相应的类

//统计在线人数，通过session,所以implements HttpSessionListener
@WebListener(value = "onlineCountListener")
public class OnlineCountListener implements HttpSessionListener {
    @Override
    //创建session监听，创建session时触发
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext context = se.getSession().getServletContext();
        Object c = context.getAttribute("count");
        System.out.println("666==" + c);
        int count;
        if (c == null) {
            count = 1;
        } else {
            count = (int) c + 1;
        }
        context.setAttribute("count", count);
    }

    @Override
    //销毁session时触发
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext context = se.getSession().getServletContext();
        int count = (int) context.getAttribute("count");
        count -= 1;
        context.setAttribute("count", count);

    }
}

    <%--三个等价--%>
    <%= getServletConfig().getServletContext().getAttribute("count")%>online
    ${applicationScope.count}online
    ${applicationScope.get("count")}online