Listener & Filter

Listener

// Three steps to use a listener

// Step 1.  Implement a listener interface

// Step 2. Write its callable methods

// Step 3. Register in the web.xml

ServletContextListener (Right now the only one we‘ll use)

when the ServletContext object is created, it has public void contextInitialized(ServletContextEvent sce)
when the ServletContext object is destroyed, is has public void contextDestroyed(ServletContextEvent sce)

// here is MyServletContextlistenerImpl.java
package com.truman.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class MyServletContextListenerImpl implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        System.out.println("The ServletContext object has been created.");
    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        System.out.println("The ServletContext object has been destroyed.");
    }
}

here is registration in web.xml

<listener>
        <listener-class>com.truman.listener.MyServletContextListenerImpl</listener-class>
</listener>

here is the output:

deployed, please wait...
21-Jun-2020 09:39:48.426 WARNING [RMI TCP Connection(2)-127.0.0.1] org.apache.tomcat.util.descriptor.web.WebXml.setVersion Unknown version string [4.0]. Default version will be used.
The ServletContext object has been created.   // this line
[2020-06-21 09:39:48,598] Artifact thirdTry:war exploded: Artifact is deployed successfully

//*************************************************************************************************
21-Jun-2020 09:39:57.924 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/Users/zhuya/apache-tomcat-8.5.38/webapps/manager] has finished in [42] ms
The ServletContext object has been destroyed.  // and this line
21-Jun-2020 09:39:57.938 INFO [main] org.apache.coyote.AbstractProtocol.stop Stopping ProtocolHandler ["http-nio-8080"]

Listener & Filter

原文：https://www.cnblogs.com/nedrain/p/13171464.html