Java监听器详解-优快云博客

1.监听器作用

监听器Listener就是在application、session、request三个对象创建、销毁或者往其中添加、修改、删除属性时自动执行的功能组件。

2.监听器分类

（1）ServletContext监听

1） ServletContextListener

用于监听ServletContext的创建和销毁

public interface ServletContextListener extends EventListener 
{
	//ServletContext创建时候会调用该方法
    public void contextInitialized ( ServletContextEvent sce );
    
    //ServletContext销毁时会调用
    public void contextDestroyed ( ServletContextEvent sce );
}

其中的事件：ServletContextEvent

public class ServletContextEvent extends java.util.EventObject 
{ 
    public ServletContextEvent(ServletContext source) 
    {
    	super(source);
    }
    
    //获得ServletContext对象
    public ServletContext getServletContext () 
    { 
    	return (ServletContext) super.getSource();
    }
}

2） ServletContextAttributeListener

用于监听ServletContext中属性的增加、删除、修改

public interface ServletContextAttributeListener extends EventListener 
{
	/**当一个属性被添加的时候调用.在添加之后被调用.*/
	public void attributeAdded(ServletContextAttributeEvent scab);
	/** 当一个属性被删除的时候调用.在删除之后被调用.*/
	public void attributeRemoved(ServletContextAttributeEvent scab);
	/** 当一个属性被替换（也就是第二次替换原来的值）的时候调用.在替换之后被调用 */
	public void attributeReplaced(ServletContextAttributeEvent scab);
}

其中的事件ServletContextAttributeEvent

public class ServletContextAttributeEvent extends ServletContextEvent 
{ 
	private String name;
	private Object value;

	public ServletContextAttributeEvent(ServletContext source, String name, Object value) 
	{
	    super(source);
	    this.name = name;
	    this.value = value;
	}
	
	//获得触发事件的属性名臣
	public String getName() 
	{
		return this.name;
	}
	
	//获得触发事件的属性的值
	public Object getValue() 
	{
	    return this.value;   
	}
}

（2）Session监听

1） HttpSessionListener

对session的创建和销毁监听

public interface HttpSessionListener extends EventListener 
{
	//创建Session时调用
    public void sessionCreated ( HttpSessionEvent se );
    
    //销毁Session时调用
    public void sessionDestroyed ( HttpSessionEvent se );
}

其中的事件HttpSessionEvent

public class HttpSessionEvent extends java.util.EventObject 
{
	/** Construct a session event from the given source.*/
	 public HttpSessionEvent(HttpSession source) 
	 {
		super(source);
	}
	 
	/** 返回触发事件的Session*/
    public HttpSession getSession () 
    { 
    	return (HttpSession) super.getSource();
    }
}

2） HttpSessionAttributeListener

对session中属性的添加、删除、替换监听

public interface HttpSessionAttributeListener extends EventListener 
{
	/** Notification that an attribute has been added to a session. Called after the attribute is added.*/
    public void attributeAdded ( HttpSessionBindingEvent se );
	/** Notification that an attribute has been removed from a session. Called after the attribute is removed. */
    public void attributeRemoved ( HttpSessionBindingEvent se );
	/** Notification that an attribute has been replaced in a session. Called after the attribute is replaced. */
    public void attributeReplaced ( HttpSessionBindingEvent se );

}

其中的事件HttpSessionBingdingEvent

public class HttpSessionBindingEvent extends HttpSessionEvent 
{
    private String name;

    private Object value;
    
   	/** Return the session that changed. */
    public HttpSession getSession () 
    { 
    	return super.getSession();
    }

    public String getName() 
    {
    	return name;
    }
    
	public Object getValue() 
	{
	    return this.value;   
	}
}

注意：

Session的销毁有2种情况：

I.session超时，这个在web.xml中可以配置

<session-config>
    <session-timeout>120</session-timeout><!--session120分钟后超时销毁-->
</session-config>

II.手工使session失效

public void invalidate();//使session失效方法。session.invalidate();

（3）request监听

1）ServletRequestListener

对request的创建和销毁监听

public interface ServletRequestListener extends EventListener 
{
    /** The request is about to go out of scope of the web application. */
    public void requestDestroyed ( ServletRequestEvent sre );

    /** The request is about to come into scope of the web application. */
    public void requestInitialized ( ServletRequestEvent sre );
}

其中的事件ServletRequestEvent

public class ServletRequestEvent extends java.util.EventObject 
{ 
    private ServletRequest request;
    public ServletRequestEvent(ServletContext sc, ServletRequest request) 
    {
        super(sc);
        this.request = request;
    }
    
    //返回事件相关的Request
    public ServletRequest getServletRequest () 
    { 
        return this.request;
    }

    //返回事件相关的ServletContext
    public ServletContext getServletContext () { 
        return (ServletContext) super.getSource();
    }
}

2）ServletRequestAttributeListener

对request中属性的添加、删除、替换监听

public interface ServletRequestAttributeListener extends EventListener 
{
    /** Notification that a new attribute was added to the
     ** servlet request. Called after the attribute is added.
     */
    public void attributeAdded(ServletRequestAttributeEvent srae);

    /** Notification that an existing attribute has been removed from the
     ** servlet request. Called after the attribute is removed.
     */
    public void attributeRemoved(ServletRequestAttributeEvent srae);

    /** Notification that an attribute was replaced on the
     ** servlet request. Called after the attribute is replaced.
     */
    public void attributeReplaced(ServletRequestAttributeEvent srae);
}

其中触发的事件ServletRequestAttributeEvent

public class ServletRequestAttributeEvent extends ServletRequestEvent { 
    private String name;
    private Object value;
    public ServletRequestAttributeEvent(ServletContext sc, ServletRequest request, String name, Object value) {
        super(sc, request);
        this.name = name;
        this.value = value;
    }

    public String getName() {
        return this.name;
    }
    public Object getValue() {
        return this.value;   
    }
}

3.监听器使用
（1）创建监听器类，实现上面的一个或者多个接口
（2）在web.xml中配置

<listener>
    <listener-class>com.listener.class</listener-class>
</listener>

注意： Listener的配置信息必须写在Filter和Servlet之前，因为初始化顺序为Listener->Filter->Servlet

4.应用

（1）使用监听器记录在线人数

public class MaxUserCountListener implements HttpSessionListener {
    public MaxUserCountListener() 
    {
    }

    public void sessionCreated(HttpSessionEvent se) 
    {
    	System.out.println("enter");
    	ServletContext sc = se.getSession().getServletContext();
    	Object count =  sc.getAttribute("count");
    	if(count==null)
    	{
    		sc.setAttribute("count", 1);
    	}
    	else
    	{
    		int newCount = Integer.valueOf(count.toString());
    		newCount++;
    		sc.setAttribute("count", newCount);
    	}
    }

    //session注销、超时时候调用，停止tomcat不会调用
    public void sessionDestroyed(HttpSessionEvent se)
    {
    	ServletContext sc = se.getSession().getServletContext();
    	int count = Integer.valueOf(sc.getAttribute("count").toString());
    	count--;
    	sc.setAttribute("count", count);
    }
	
}

（2）Spring使用ContextLoaderListener加载ApplicationContext配置信息

ContextLoaderListener的作用就是在启动Web容器时，自动装配ApplicationContext的配置信息。因为它实现了ServletContextListener这个接口，在web.xml配置这个监听器，容器启动时，就会默认执行他的实现方法。
ContextLoaderListener如何查找ApplicationContext.xml的配置位置以及配置多个xml文件：如果web.xml中没有写任何参数配置信息，默认路径是“/WEB-INF/applicationContext.xml”，在WEB-INF目录下创建的xml文件名称必须是applicationContext.xml。如果需要自定义文件名，可以在web.xml中加入contextConfigLocation这个context参数

<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:spring/applicationContext-*.xml</param-value><!-- 采用的是通配符方式，查找WEB-INF/spring目录下xml文件。如有多个xml文件，以“,”分隔。 -->
</context-param>

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

（3）Spring使用Log4jConfigListener配置Log4j日志

Spring使用Log4jConfigListener的好处：

1）动态的改变记录级别和策略，不需要重启Web应用

2）把log文件定在/WEB-INF/logs/而不需要写绝对路径。因为系统把web目录的路径压入webapp.root的系统变量。

3）可以把log4j.properties和其他properties一起放在/WEB-INF

<context-param>
    <param-name>webAppRootKey</param-name>
    <param-value>project.root</param-value><!-- 用于定位log文件输出位置在web应用根目录下，log4j配置文件中写输出位置：log4j.appender.FILE.File=${project.root}/logs/project.log -->
</context-param>
<context-param>
    <param-name>log4jConfigLocation</param-name>
    <param-value>classpath:log4j.properties</param-value><!-- 载入log4j配置文件 -->
</context-param>
<context-param>
    <param-name>log4jRefreshInterval</param-name>
    <param-value>60000</param-value><!--Spring刷新Log4j配置文件的间隔60秒,单位为millisecond-->
</context-param>

<listener>
    <listener-class>org.springframework.web.util.Log4jConfigListener</listener-class>
</listener>