Compilation software: IntelliJ IDEA 2019.2.4 x64
Operating system: win10 x64-bit Home Edition
Server software: apache-tomcat-8.5.27
Table of contents
1. What is a Servlet listener?
In official terms, "It is an object based on the Java Servlet specification, and is mainly used to monitor the creation and destruction events of domain objects such as ServletContext, HttpSession and HttpServletRequest in web applications , and to monitor the occurrence of properties in these domain objects. Modified event", a very incisive expression.
But these words may not be very understandable to novice Xiaobai. Don't worry, I personally understand that it is like a bug in our real life . It eavesdrops on certain groups and responds accordingly based on specific behavioral events of the target , such as the notorious US prism door hacking incident.
2. What are the functions of the Servlet listener?
2.1 Creation and destruction of listening domain objects
2.1.1 ServletContextListener interface
effect:
Monitor the creation and destruction of ServletContext objects (global context objects)
| method name | effect |
|---|---|
| contextInitialized(ServletContextEvent sce) | Called when ServletContext is created |
| contextDestroyed(ServletContextEvent sce) | Called when the ServletContext is destroyed |
Note:
ServletContext对象It is created when the web project is loaded (the Tomcat server is turned on) , but it is destroyed when the web project is unloaded (the Tomcat server is turned off)
2.1.2 HttpSessionListener interface
effect:
Monitor the creation and destruction of HttpSession objects (session domain objects)
| method name | effect |
|---|---|
| sessionCreated(HttpSessionEvent hse) | Called when the HttpSession object is created |
| sessionDestroyed(HttpSessionEvent hse) | Called when the HttpSession object is destroyed |
Note:
HttpSession对象It is created when the getSession method is called for the first time , but the object is destroyed when the client is closed, forcibly invalidated, and automatically invalidated when the maximum idle time is reached.
2.1.3 ServletRequestListener interface
effect:
Monitor the creation and destruction of ServletRequest objects (request domain objects)
| method name | effect |
|---|---|
| requestInitialized(ServletRequestEvent sre) | Called when the ServletRequest object is created |
| requestDestroyed(ServletRequestEvent sre) | Called when the ServletRequest object is destroyed |
Note:
ServletRequest对象It is created when there is a request , but destroyed when the response ends.
2.2 Adding, modifying, and deleting shared data in monitoring domain objects
2.2.1 ServletContextAttributeListener interface
effect:
Monitor the addition, removal and modification of attributes in the ServletContext object
| method name | effect |
|---|---|
| attributeAdded(ServletContextAttributeEvent scab) | Called when an attribute is added to the ServletContext |
| attributeRemoved(ServletContextAttributeEvent scab) | Called when an attribute is removed from the ServletContext |
| attributeReplaced(ServletContextAttributeEvent scab) | Called when an attribute in the ServletContext is modified |
The ServletContextAttributeEvent object represents an attribute change event, and it contains the following methods:
| method name | effect |
|---|---|
| getName() | Get the modified or added attribute name |
| getValue() | Get the attribute value that was modified or added |
| getServletContext() | Get the ServletContext object |
2.2.2 HttpSessionAttributeListener interface
effect:
Monitor the addition, removal and modification of attributes in the HttpSession object
| method name | effect |
|---|---|
| attributeAdded(HttpSessionBindingEvent se) | Called when adding attributes to HttpSession |
| attributeRemoved(HttpSessionBindingEvent se) | Called when an attribute is removed from the HttpSession |
| attributeReplaced(HttpSessionBindingEvent se) | Called when an attribute in the HttpSession is modified |
The HttpSessionBindingEvent object represents an attribute change event, and it contains the following methods:
| method name | effect |
|---|---|
| getName() | Get the modified or added attribute name |
| getValue() | Get the attribute value that was modified or added |
| getSession() | Get the HttpSession object that triggered the event |
2.2.3 ServletRequestAttributeListener interface
effect:
Monitor the addition, removal and modification of attributes in the ServletRequest object
| method name | effect |
|---|---|
| attributeAdded(ServletRequestAttributeEvent srae) | Called when an attribute is added to the ServletRequest |
| attributeRemoved(ServletRequestAttributeEvent srae) | Called when an attribute is removed from a ServletRequest |
| attributeReplaced(ServletRequestAttributeEvent srae) | Called when an attribute in the ServletRequest is modified |
The ServletRequestAttributeEvent object represents an attribute change event, and it contains the following methods:
| method name | effect |
|---|---|
| getName() | Get the modified or added attribute name |
| getValue() | Get the attribute value that was modified or added |
| getServletRequest () | Get the ServletRequest object that triggered the event |
3. How to create a Servlet listener?
step:
①Create a class
② Implement an interface
According to the needs of business scenarios, you can choose to implement one of the three (ServletContextListener, HttpSessionListener, or ServletRequestListener) interfaces to use
③ Implement the abstract method in the interface
④ Register the listener (the most primitive approach)
Location:
Register in web.xml
The code example is as follows:
//注册刚创建的监听器MyListener
<listener>
<listener-class>Listenner.MyListener</listener-class>
</listener>
4. Hands-on case
Case: Create a listener MyListener class, inherit the ServletContextListener interface, to monitor the creation and destruction of the global context object, and register the listener in web.xml. The presentation listener observes the creation and destruction of the global context object.
①Create the MyListener class and inherit the ServletContextListener interface
The code example is as follows:
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
//监听ServletContext应用域对象
public class MyListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent servletContextEvent) {
System.out.println("ServletContext对象创建后执行");
}
@Override
public void contextDestroyed(ServletContextEvent servletContextEvent) {
System.out.println("ServletContext对象销毁后执行");
}
}
② Register the listener in web.xml
The code example is as follows:
<listener>
<listener-class>Listener.MyListener</listener-class>
</listener>
