13.Listener

监听器，监听对象的变化情况

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class ContextLoaderListenerr implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        System.out.println("App has been loaded");
    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        System.out.println("App has been destroyed");
    }
}