SpringBoot监听器的使用方法

监听器（Listener）就是监听对象的创建、销毁等状态的变化以及定义一些事件发生后接下来要进行的动作。主要监听的三个域对象为：ServletRequest域、HttpSession域和ServletContext域。本文通过几个简单的例子介绍一下监听器的用法。

监听 Servlet 上下文对象可以在系统启动的时候初始化一些数据，方便在使用的时候直接调用。使用ServletContextListener监听 Servlet上下文，监听器实现代码如下：

java">@Component
public class MyServletContextListener implements ServletContextListener {@Override    //在 ServletContext 对象创建之后马上调用，做初始化public void contextInitialized(ServletContextEvent event) {event.getServletContext().setAttribute("name","小马");System.out.println("ServletContext 对象被创建了");}@Override  // 在 ServletContext 对象销毁之后调用public void contextDestroyed(ServletContextEvent event) {System.out.println("ServletContext 对象被销毁了");}
}@GetMapping("/getServletContext")public String getServletContext(HttpServletRequest request) {ServletContext servletContext = request.getServletContext();Object name = servletContext.getAttribute("name");return String.valueOf(name);}

获取在线用户数量

使用HttpSessionListener获取在线用户数量，代码如下：

java">@Component
public class MyHttpSessionListener implements HttpSessionListener {public static Integer count = 0;@Overridepublic void sessionCreated(HttpSessionEvent event) {count++;ServletContext application = event.getSession().getServletContext();application.setAttribute("UserCount", count);System.out.println("有人上线了，现在在线人数为:" + count + "人");}@Overridepublic void sessionDestroyed(HttpSessionEvent event) {count--;ServletContext application = event.getSession().getServletContext();application.setAttribute("UserCount", count);System.out.println("有人下线了，现在在线人数为:" + count + "人");}
}
@GetMapping("/online")
public String getOnlinePersoncount(HttpServletRequest request) {Integer userCount = (Integer) request.getServletContext().getAttribute("UserCount");return (userCount == null ? "0" : userCount + "");
}
@GetMapping("/login")
public String logined(HttpSession session) {//相同的session，如果sessionid一致，只会被监听一次。session.setAttribute("username", "小马");return "success";
}
@GetMapping("/logout")
public String logout(HttpSession session) {session.invalidate();//将session设置为失效return "success";
}

统计网站访问次数

使用ServletRequestListener统计网站访问次数，代码如下：

java">@Component
public class MyServletRequestListener implements ServletRequestListener {@Overridepublic void requestInitialized(ServletRequestEvent servletRequestEvent) {Object countObject = servletRequestEvent.getServletContext().getAttribute("count");System.out.println("历史访问次数：" + countObject);Integer count = 0;if (countObject != null)count = Integer.valueOf(countObject.toString());count++;servletRequestEvent.getServletContext().setAttribute("count", count);}@Overridepublic void requestDestroyed(ServletRequestEvent servletRequestEvent) {System.out.println("当前访问次数：" + servletRequestEvent.getServletContext().getAttribute("count"));}
}

自定义监听方式

定义监听事件

java">public class ListenerEvent extends ApplicationEvent {String name = null;public String getName() {return name;}public void setName(String name) {this.name = name;}public ListenerEvent(Object source, String value) {super(source);name = value;}
}

定义监听器

java">@Component
public class MySpringBootListener implements ApplicationListener<ListenerEvent> {@Overridepublic void onApplicationEvent(ListenerEvent listenerEvent) {String eventInfo = listenerEvent.getName();System.out.println(eventInfo);}
}

发布事件

java">@RestController
public class ListenerController {@Resourceprivate ApplicationContext applicationContext;@GetMapping("/listener")public String listener() {ListenerEvent event =new ListenerEvent(this,"小马");applicationContext.publishEvent(event);return "";}
}