spring boot 监听器

1.监听全局唯一对象 ServletContextListener,可以在项目启动时将数据写入缓存，结束时将数据保存到数据库

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class ServletContextListenerimplements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent){
        System.err.println("ServletContext 上下文初始化");
    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent){
        System.err.println("ServletContext 上下文销毁");
    }

}

2.监听session，和request请求

(1)session,可以用来统计在线人数

import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.*;
import java.util.HashSet;

@WebListener()
public class SessionListener implements HttpSessionListener {
    
    @Override
    public void sessionCreated(HttpSessionEvent event) {
        System.err.println("---sessionCreated----");
        HttpSession session = event.getSession();
        ServletContext application = session.getServletContext();
        // 在application范围由一个HashSet集保存所有的session
        HashSet sessions = (HashSet) application.getAttribute("sessions");
        if (sessions == null) {
            sessions = new HashSet();
            application.setAttribute("sessions", sessions);
        }
        // 新创建的session均添加到HashSet集中
        sessions.add(session);
        // 可以在别处从application范围中取出sessions集合
        // 然后使用sessions.size()获取当前活动的session数，即为“在线人数”
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent event) throws ClassCastException {
        System.err.println("---sessionDestroyed----");
        HttpSession session = event.getSession();
        System.err.println("deletedSessionId: "+session.getId());
        System.out.println(session.getCreationTime());
        System.out.println(session.getLastAccessedTime());
        ServletContext application = session.getServletContext();
        HashSet sessions = (HashSet) application.getAttribute("sessions");
        // 销毁的session均从HashSet集中移除
        sessions.remove(session);
    }
}

(2)request

@WebListener()
public class RequestListener implements ServletRequestListener {
    @Override
    public void requestInitialized(ServletRequestEvent sre){
        HttpServletRequest request=(HttpServletRequest) sre.getServletRequest();
        String s=request.getParameter("s");
        System.err.println("request initialized");
    }
    @Override
    public void requestDestroyed(ServletRequestEvent sre){
        System.err.println("request destroyed");
    }
}

3.监听全局上下文属性(ServletContextAttributeListener),session属性(HttpSessionAttributeListener)，request属性(ServletRequestAttributeListener)

import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.*;
import java.util.HashSet;

@WebListener()
public class SessionListener implements HttpSessionAttributeListener{
    @Override
    public void  attributeAdded(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.err.println("--attributeAdd--");
        HttpSession session=httpSessionBindingEvent.getSession();
        System.err.println(httpSessionBindingEvent.getName());
        System.err.println(httpSessionBindingEvent.getValue());

    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.err.println("--attributeRemoved--");
        System.err.println(httpSessionBindingEvent.getName());
        System.err.println(httpSessionBindingEvent.getValue());
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.err.println("--attributeReplaced--");
        System.err.println(httpSessionBindingEvent.getName());
        System.err.println(httpSessionBindingEvent.getValue());
    }
}