Listener总结(2)---域对象属性改变监听器

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之前总结的第一类 Listener,监听的是 ServletContextHttpSessionServletRequest 这三个对象的创建和销毁,接下来要介绍的第二类 Listener分别是ServletRequestAttributeListenerServletContextAttributeListenerHttpSessionAttributeListener

​从名字不难看出,他们是监听各自对象中属性的Listener,每当对象的属性发生变化(增加、修改或者删除),相应的监听器将会收到通知。XXXXXXAttributeListener 中方法的名称相同,以HttpSessionAttributeListener接口为例:

package javax.servlet.http;

import java.util.EventListener;
import javax.servlet.http.HttpSessionBindingEvent;

public interface HttpSessionAttributeListener extends EventListener {
    default void attributeAdded(HttpSessionBindingEvent se) {
    }

    default void attributeRemoved(HttpSessionBindingEvent se) {
    }

    default void attributeReplaced(HttpSessionBindingEvent se) {
    }
}

attributeAdded()attributeRemoved()attributeReplaced分别在 Session 中的属性被添加、移除和改变时被调用,从参数HttpSessionBindingEvent中可以获得 Session 的详细信息:

public String login(){
    HttpSession session = ServletActionContext.getRequest().getSession();
    session.setAttribute("UserName", username);
    session.setAttribute("UserName", "defaultUserName");
    session.removeAttribute("UserName");
    return SUCCESS;
}

//自定义MyHttpSessionAttributeListener
public class MyHttpSessionAttributeListener implements HttpSessionAttributeListener {

    private Logger logger = Logger.getLogger(MyHttpSessionAttributeListener.class);

    public void attributeAdded(HttpSessionBindingEvent se) {
        logger.debug("attributeAdded: Name: "+ se.getName()+"; Value: "+ se.getValue());
    }

    public void attributeRemoved(HttpSessionBindingEvent se) {
        logger.debug("attributeRemoved: Name: "+ se.getName()+"; Value: "+ se.getValue());
    }

    public void attributeReplaced(HttpSessionBindingEvent se) {
        logger.debug("attributeReplaced: Name: "+ se.getName()+"; Value: "+ se.getValue());
    }
}

​在模拟登陆的函数中对 Session 进行一系列操作,可以看到控制台输出信息:

HttpSessionAttributeListener.png

​与 HttpSessionAttributeListener 的作用类似,ServletContextAttributeListener和ServletContextAttributeListener接口都提供了以上三个方法,用法也大致相同,下面直接给出实现了ServletContextAttributeListener和ServletContextAttributeListener接口的两个自定义 Listener 类。

MyServletContextAttributeListener.java:

//...
public class MyServletContextAttributeListener implements ServletContextAttributeListener {

    private Logger logger = Logger.getLogger(MyServletContextAttributeListener.class);

    public void attributeReplaced(ServletContextAttributeEvent scae) {
        logger.debug("attributeReplaced: Name: "+ scae.getName()+"; Value: "+ scae.getValue());
    }

    public void attributeRemoved(ServletContextAttributeEvent scae) {
        logger.debug("attributeRemoved: Name: "+ scae.getName()+"; Value: "+ scae.getValue());
    }

    public void attributeAdded(ServletContextAttributeEvent scae) {
        logger.debug("attributeAdded: Name: "+ scae.getName()+"; Value: "+ scae.getValue());
    }
}

MyServletRequestAttributeListener.java:

public class MyServletRequestAttributeListener implements ServletContextAttributeListener {

    private Logger logger = Logger.getLogger(MyServletRequestAttributeListener.class);

    public void attributeReplaced(ServletContextAttributeEvent scae) {
        logger.debug("attributeReplaced: Name: "+ scae.getName()+"; Value: "+ scae.getValue());
    }

    public void attributeRemoved(ServletContextAttributeEvent scae) {
        logger.debug("attributeRemoved: Name: "+ scae.getName()+"; Value: "+ scae.getValue());
    }

    public void attributeAdded(ServletContextAttributeEvent scae) {
        logger.debug("attributeAdded: Name: "+ scae.getName()+"; Value: "+ scae.getValue());
    }
}

将之前login函数中的代码进行修改,使其操作 ServletContext 、Request 和 Session 中的 Attribute,再看控制台的输出:

函数 login():

public String login(){
    ServletContext servletContext = ServletActionContext.getServletContext();
    HttpServletRequest request = ServletActionContext.getRequest();
    HttpSession session = request.getSession();
  
    session.setAttribute("UserName", username);
    session.setAttribute("UserName", "defaultUserName");
    session.removeAttribute("UserName");
  
    servletContext.setAttribute("UserName", username);
    servletContext.setAttribute("UserName", "defaultUserName");
    servletContext.removeAttribute("UserName");
  
    request.setAttribute("UserName", username);
    request.setAttribute("UserName", "defaultUserName");
    request.removeAttribute("UserName");
    return SUCCESS;
}

​进行一次 Request 请求时控制台的输出:

一次完整的 Request 请求.png

​由于在调用 login()方法之前页面已经进行过一次访问,第二次访问时并没有创建或销毁 Session,所以这里只有 RequestListener 进行了响应。
​基于上一篇文章的监听对象创建和销毁的 Listener,再加上监听对象属性的 Listener,实际使用中可以将自定义 Listener 同时继承两种接口,将对象的监听事件放于一处,提高代码的可维护性:

​以 HttpSession 对象的监听为例:

//...
public class MyHttpSessionListener implements HttpSessionListener, HttpSessionAttributeListener{

    private static Logger logger = Logger.getLogger(MyHttpSessionListener.class);

    public void sessionCreated(HttpSessionEvent se) {
        logger.debug("sessionCreated");
        logger.debug(se.getSession().getId());
    }

    public void sessionDestroyed(HttpSessionEvent se) {
        logger.debug("sessionDestroyed");
    }

    public void attributeReplaced(HttpSessionBindingEvent se) {
        
    }

    public void attributeRemoved(HttpSessionBindingEvent se) {
        
    }

    public void attributeAdded(HttpSessionBindingEvent se) {
        
    }
}

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