监听器

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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>

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