About the listener listener in Java EE

Listener is designed based on the observer pattern, listener provides a means of controlling control and data for servlet applications. Registration of Listener

Java Web Listener Registration is simpler because the objects and events that they listen to are created by the servlet container, and if their listeners are needed, they need to be configured in Web.xml:

[Java] view plain copy <!--web.xml--> <listener> <listener-class> ..... Implements custom listeners for specific interfaces 1</listener-class> <listener-class> .... Implement custom listener 2</listener-class> for specific interfaces ... </listener>

for multiple listener that implement the same interface, the execution is determined in the order in which they are registered in Web.xml .

Listener can also be added dynamically in the application.

Listener Related Interface

Several types of listener are defined in the servlet specification, and they are used to listen to the three domain objects of servletcontext,httpsession and ServletRequest, respectively .

The servlet specification divides this listener into three types for operations on these three objects:

event listeners listening for the creation and destruction of three domain objects

Listener class	The interface that contains	Interface description
Servletcontextlistener	Contextinitialized (Servletcontextevent SCE); Contextdestroyed (Servletcontextevent SCE);	triggered when the context container is initialized, the Contextinitialized interface is first invoked before all of the filter and servlet init method calls; The context container is destroyed, and the Contextdestroyed interface is invoked after all the filter and servlet destroy method calls;
Httpsessionlistener	sessioncreated (httpsessionevent se); sessiondestroyed (htppsessionevent se);	triggered when a session object is created; triggered when a session object is invalidated;
Servletrequestlistener	Requestinitialized (Servletrequestevent SRE); Requestdestroyed (Servletrequestevent SRE);	The method is triggered before the HttpServletRequest object is passed to the service method of the user's servlet; The method is triggered when the HttpServletRequest object is called after the service method of the user's servlet is invoked;

Note: Servletcontextlistener can no longer add new after the container is started, because the event it listens to will no longer appear.

Monitor the increase of attributes in three domain objects, delete, change the event listener

Listener class	The interface that contains	Interface description
Servletcontextattributelistener	attributeadded (servletcontextattributeevent scab); Attributeremoved (servletcontextattributeevent scab); attributereplaced (servletcontextattributeevent scab);	This method is triggered when the Servletcontext.setattribute method is invoked; This method is triggered when the Servletcontext.removeattribute method is invoked; If the attribute already exists before the call to Servletcontext.setattribute, this method is triggered when the attribute is replaced.
Httpsessionattributelistener	attributeadded (httpsessionbindingevent se); attributeremoved (httpsessionbindingevent se); attributereplaced (httpsessionbindingevent se);	The method is triggered when the Session.setattribute method is invoked; The method is triggered when the Session.removeattribute method is invoked; If the attribute already exists before the call to Session.setattribute, the method is triggered when the attribute is replaced;
Servletrequestattributelistener	attributeadded (Servletrequestattributeevent srae); Attributeremoved (Servletrequestattributeevent srae); attributereplaced (Servletrequestattributeevent srae);	This method is triggered when the Request.setattribute method is invoked; This method is triggered when the Request.removeattribute method is invoked; If the attribute already exists before the call to Request.setattribute, the method is triggered when the attribute is substituted;

Perceptual listeners

This listener does not need to be registered.

When a Java bean implements this interface, it can listen for when it is bound, untied, or deactivated.

Httpsessionbindinglistener: The class that implements this interface, can detect oneself when be httpsession bind, unbind;

Httpsessionactivationlistener: The class that implements this interface, can detect when oneself with httpsession activates and passivation together;

examples of listener applications

Like what:

Initialization of the

application--an application-level resource initialization in the Contextinitialized method of a custom inherited Servletcontextlistener listener for increased efficiency, Frees the application-level resources in the Contextdestroyed method ;