【Web入门系列】初探监听器Listener

定义

监听器主要用来监一些特定对象的创建或销毁以及属性变化的，是一个实现特定接口的普通java类。

按照监听的对象划分，可以分为ServletContext对象监听器，HttpSession对象监听器，ServletRequest对象监听器；按照监听的事件划分，可以分为对象自身的创建和销毁的监听器，对象中属性的创建和消除的监听器，session中某个对象的状态变化的监听器。

ServletRequestListener、ServletContextListener、HttpSessionListener

<listener>
    <listener-class>ysdrzp.listener.MyServletRequestListener</listener-class>
</listener>
<listener>
    <listener-class>ysdrzp.listener.MyServletContextListener</listener-class>
</listener>
<listener>
    <listener-class>ysdrzp.listener.MyHttpSessionListener</listener-class>
</listener>

/**
 * 监听ServletRequest对象的创建和销毁
 */
public class MyServletRequestListener implements ServletRequestListener {

    @Override
    public void requestInitialized(ServletRequestEvent servletRequestEvent) {
        System.out.println("MyServletRequestListener.requestInitialized()");
    }

    @Override
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
        System.out.println("MyServletRequestListener.requestDestroyed()");
    }

}

/**
 * 监听ServletContext对象的创建和销毁
 */
public class MyServletContextListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        System.out.println("MyServletContextListener.contextInitialized()");
    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        System.out.println("MyServletContextListener.contextDestroyed()");
    }
    
}

/**
 * 监听HttpSession对象的创建和销毁
 */
public class MyHttpSessionListener implements HttpSessionListener {

    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        System.out.println("MyHttpSessionListener.sessionCreated()");
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        System.out.println("MyHttpSessionListener.sessionDestroyed()");
    }
    
}

 ServletRequestAttributeListener、ServletContextAttributeListener、HttpSessionAttributeListener

/**
 * ServletRequest域对象属性监听器
 */
public class MyServletRequestAttributeListener implements ServletRequestAttributeListener {

    @Override
    public void attributeAdded(ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println(servletRequestAttributeEvent.getName()+"属性增加");
        System.out.println(servletRequestAttributeEvent.getValue());
    }

    @Override
    public void attributeRemoved(ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println(servletRequestAttributeEvent.getName()+"属性移除");
        System.out.println(servletRequestAttributeEvent.getValue());
    }

    @Override
    public void attributeReplaced(ServletRequestAttributeEvent servletRequestAttributeEvent) {
        System.out.println(servletRequestAttributeEvent.getName()+"属性改变");
        System.out.println(servletRequestAttributeEvent.getValue());
    }
}

/**
 * ServletContext域对象属性改变监听器
 */
public class MyServletContextAttributeLinstener implements ServletContextAttributeListener {

    @Override
    public void attributeAdded(ServletContextAttributeEvent servletContextAttributeEvent) {

    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent servletContextAttributeEvent) {

    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent servletContextAttributeEvent) {

    }
}

/**
 * HttpSession域对象属性监听器
 */
public class MyHttpSessionAttributeListener implements HttpSessionAttributeListener {

    @Override
    public void attributeAdded(HttpSessionBindingEvent httpSessionBindingEvent) {

    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent httpSessionBindingEvent) {

    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent httpSessionBindingEvent) {

    }
}

HttpSessionBindingListener

public class User implements HttpSessionBindingListener {

    private String name;

    private String sex;

    public User(String name,String sex){
        this.name=name;
        this.sex=sex;
    }

    @Override
    public void valueBound(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("User对象已经放入session中");
    }

    @Override
    public void valueUnbound(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("User对象从session中移除");
    }
}