ServletContext全局对象：

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

public class HelloServlet extends HttpServlet {


    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        ServletConfig config = this.getServletConfig();//创建ServletConfig对象
        System.out.println(config.getInitParameter("className"));
        System.out.println(config.getInitParameter("url"));

        ServletContext servletContext = this.getServletContext();
//        servletContext.getAttribute()//得到对象中的值 通过name的
//        servletContext.setAttribute();//加入参数  键值对
//        servletContext.removeAttribute(); 删除参数
//        servletContext.getAttributeNames(); 得到对象中的参数名称
        System.out.println("servletContext.getRealPath(\"/WEB-INF\") = " + servletContext.getRealPath("/WEB-INF"));
          //获得文件的绝对路径
        
        System.out.println("servletContext.getContextPath() = "
                + servletContext.getContextPath());
          //得到工程路径如果工程在root路径下那就是空的
//        this.getServletConfig().getServletContext() 获得SerivetContext对象方式
//        this.getServletContext() 获得SerivetContext对象方式
//        req.getServletContext() 获得SerivetContext对象方式
//        req.getSession().getServletContext() 获得SerivetContext对象方式

    }
}

过滤器：
所有资源访问之前被调用对象
1.数据库加载
2.权限验证
3.数据检查
4.数据加工
5.数据缓存

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

//@WebFilter("*")
public class AFilter implements Filter {
    //过滤器实现的是一个接口
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }

    @Override
    public void doFilter(ServletRequest servletRequest,
                         ServletResponse servletResponse,
                         FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        if (request.getRequestURL().toString().endsWith(".jpg")) {//判断条件
            //通行
            //FilterChain 中的方法
            filterChain.doFilter(servletRequest, servletResponse);
        }
    }

    @Override
    public void destroy() {

    }
}

监听器里面有监听Session创建啊销毁啊添加啊等方法

@WebListener
public class SessionListener implements HttpSessionIdListener,
        HttpSessionListener, HttpSessionAttributeListener{
    public void sessionIdChanged(HttpSessionEvent var1, String var2){
        System.out.println("sessionIdChanged..");
    }

    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        System.out.println("sessionCreated..");
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        System.out.println("sessionDestroyed..");
    }

    @Override
    public void attributeAdded(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("attributeAdded..");
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("attributeRemoved..");
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("attributeReplaced..");
    }
}