In Java Web development, Listener (listener) and interface callback are very important concepts. They can be used to listen to various events in order to take appropriate measures when the event occurs.
In Java Web applications, Listener is a component that can be used to listen to events that occur in the Web application. Listener can listen to the following events:
Creation and destruction of ServletContext
Creation and destruction of HttpSession
Creation and destruction of ServletRequest
Listener can listen to the corresponding events by implementing the following interface:
ServletContextListener
HttpSessionListener
ServletRequestListener
For example, the following code demonstrates how to monitor the creation and creation of ServletContext by implementing the ServletContextListener interface Destruction event:
public class MyServletContextListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent event) {
// ServletContext创建时执行的代码
}
public void contextDestroyed(ServletContextEvent event) {
// ServletContext销毁时执行的代码
}
}The callback function is a programming pattern that allows a function to be passed as a parameter to another function and then called inside that function to execute the callback . In Java, callbacks can be implemented using interfaces.
For example, the following code demonstrates how to use interface callbacks to implement event listening:
public interface MyEventListener {
void onEvent(Event event);
}
public class EventSource {
private MyEventListener listener;
public void setEventListener(MyEventListener listener) {
this.listener = listener;
}
public void fireEvent(Event event) {
if (listener != null) {
listener.onEvent(event);
}
}
}
public class MyEventListenerImpl implements MyEventListener {
public void onEvent(Event event) {
// 处理事件的代码
}
}In the above code, the EventSource class represents the event source, which can set the event through the setEventListener() method listener. The MyEventListener interface represents an event listener, which has an onEvent() method, which is called when an event occurs. The MyEventListenerImpl class implements the MyEventListener interface, which contains the implementation of the onEvent() method for handling events.
A special type of listener used to listen to events in Web applications is called a Web listener. Web listeners can listen to the following events:
Creation and destruction of ServletContext
Creation and destruction of HttpSession
Creation and destruction of ServletRequest
Attribute state changes in HttpSession
For example, the following code demonstrates how to implement HttpSessionAttributeListener Interface to monitor changes in attribute status in HttpSession:
public class MyHttpSessionAttributeListener implements HttpSessionAttributeListener {
public void attributeAdded(HttpSessionBindingEvent event) {
// HttpSession中添加属性时执行的代码
}
public void attributeRemoved(HttpSessionBindingEvent event) {
// HttpSession中删除属性时执行的代码
}
public void attributeReplaced(HttpSessionBindingEvent event) {
// HttpSession中替换属性时执行的代码
}
}In Java Web applications, there are three scopes: ServletContext, HttpSession and ServletRequest. You can listen to these scope creation and destruction events by writing appropriate listeners.
For example, the following code demonstrates how to monitor the creation and destruction events of these three scopes by implementing the ServletContextListener, HttpSessionListener and ServletRequestListener interfaces:
public class MyServletContextListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent event) {
// ServletContext创建时执行的代码
}
public void contextDestroyed(ServletContextEvent event) {
// ServletContext销毁时执行的代码
}
}
public class MyHttpSessionListener implements HttpSessionListener {
public void sessionCreated(HttpSessionEvent event) {
// HttpSession创建时执行的代码
}
public void sessionDestroyed(HttpSessionEvent event) {
// HttpSession销毁时执行的代码
}
}
public class MyServletRequestListener implements ServletRequestListener {
public void requestInitialized(ServletRequestEvent event) {
// ServletRequest创建时执行的代码
}
public void requestDestroyed(ServletRequestEvent event) {
// ServletRequest销毁时执行的代码
}
}In addition to being able to monitor the creation and destruction events of the three scopes, you can also monitor changes in their attribute states. You can listen to these events by implementing the corresponding Listener.
For example, the following code demonstrates how to monitor changes in the status of attributes in these three scopes by implementing the ServletContextAttributeListener, HttpSessionAttributeListener and ServletRequestAttributeListener interfaces:
public class MyServletContextAttributeListener implements ServletContextAttributeListener {
public void attributeAdded(ServletContextAttributeEvent event) {
// ServletContext中添加属性时执行的代码
}
public void attributeRemoved(ServletContextAttributeEvent event) {
// ServletContext中删除属性时执行的代码
}
public void attributeReplaced(ServletContextAttributeEvent event) {
// ServletContext中替换属性时执行的代码
}
}
public class MyHttpSessionAttributeListener implements HttpSessionAttributeListener {
public void attributeAdded(HttpSessionBindingEvent event) {
// HttpSession中添加属性时执行的代码
}
public void attributeRemoved(HttpSessionBindingEvent event) {
// HttpSession中删除属性时执行的代码
}
public void attributeReplaced(HttpSessionBindingEvent event) {
// HttpSession中替换属性时执行的代码
}
}
public class MyServletRequestAttributeListener implements ServletRequestAttributeListener {
public void attributeAdded(ServletRequestAttributeEvent event) {
// ServletRequest中添加属性时执行的代码
}
public void attributeRemoved(ServletRequestAttributeEvent event) {
// ServletRequest中删除属性时执行的代码
}
public void attributeReplaced(ServletRequestAttributeEvent event) {
// ServletRequest中替换属性时执行的代码
}
}In addition to monitoring the change events of the attribute status in the HttpSession, you can also monitor the status change events of the values stored in the HttpSession. To listen to these events, you can implement the HttpSessionBindingListener interface.
For example, the following code demonstrates how to monitor the state change events of the value stored in HttpSession by implementing the HttpSessionBindingListener interface:
public class MySessionBindingListener implements HttpSessionBindingListener {
public void valueBound(HttpSessionBindingEvent event) {
// HttpSession中存值时执行的代码
}
public void valueUnbound(HttpSessionBindingEvent event) {
// HttpSession中删除值时执行的代码
}
}Listener application scenarios Very extensive. For example, Listener can be used to implement the following functions:
Load the configuration file when the ServletContext is initialized
Record the information of the logged in user when the HttpSession is created
Record the requested IP address when ServletRequest is created
Perform corresponding cleanup work when creating and destroying scopes
When using Listener, you need to pay attention to the following matters:
The implementation class of Listener must implement the corresponding interface.
Listener implementation class must be configured in the web.xml file.
The implementation class of Listener must be in the same ClassLoader as the application.
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