没有servlet能单独存在。在当前的现代Web应用中，许多组件都是在一起协作共同完成一个目标。怎么让这些组件共享信息？如何隐藏信息？怎样让信息做到线程安全？

1 属性和监听者

1.1 初始化

容器初始化一个servlet时，会为这个servlet建一个唯一的ServletConfig。容器从web.xml 配置文件中“读出”servlet初始化参数，并把这些参数交给ServletConfig。然后把ServletConfig传递给servlet的init()方法。

一旦参数置于ServletConfig中，就不会再读了，除非重写部署了servlet。

1.1.1 上下文初始化参数

上下文初始化参数与servlet初始化参数很类似，只不过上下文参数对整个Web应用可用，而不只针对一个servlet。

public class InitServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletConfig servletConfig = getServletConfig();String servletName = servletConfig.getInitParameter("servletName");ServletContext servletContext = getServletContext();String contextName = servletContext.getInitParameter("contextName");resp.setCharacterEncoding("gbk");PrintWriter writer = resp.getWriter();writer.println("servlet初始化参数:" + servletName);writer.println("上下文初始化参数:" + contextName);writer.close();}
}

<web-app xmlns="http://java.sun.com/xml/ns/j2ee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"version="2.4"><servlet><servlet-name>initServlet</servlet-name><servlet-class>servlet.InitServlet</servlet-class><init-param><param-name>servletName</param-name><param-value>initServlet</param-value></init-param></servlet><context-param><param-name>contextName</param-name><param-value>day01</param-value></context-param><servlet-mapping><servlet-name>initServlet</servlet-name><url-pattern>/init</url-pattern></servlet-mapping></web-app>

web.xml

1.2 监听者

接口	使用场景
ServletContextAttributeListener	监听Web应用上下文中是否增加、删除或替换了一个属性。
HttpSessionListener	监听会话的创建。
ServletRequestListener	用户请求监听。
ServletRequestAttributeListener	监听请求增加、删除或替换属性。
HttpSessionBindingListener	属性类对象在绑定到一个会话或从会话删除时触发给属性类对象。
HttpSessionAttributeListener	监听会话增加、删除或替换属性。
ServletContextListener	监听Web应用上下文创建或销毁。
HttpSessionActivationListener	监听会话迁移到另一个JVM。

表 servlet常见的8种监听者

public class MyAtt implements HttpSessionBindingListener {@Overridepublic void valueBound(HttpSessionBindingEvent event) {System.out.println("myAtt 添加到:" + event.getSession());}@Overridepublic void valueUnbound(HttpSessionBindingEvent event) {System.out.println("myAtt 移除");}
}public class MyHttpSessionAttributeListener implements HttpSessionAttributeListener {@Overridepublic void attributeAdded(HttpSessionBindingEvent event) {System.out.println("添加session属性：" + event.getName() + ",其值为:" + event.getValue());}@Overridepublic void attributeRemoved(HttpSessionBindingEvent event) {System.out.println("删除session属性：" + event.getName() + ",其值为:" + event.getValue());}@Overridepublic void attributeReplaced(HttpSessionBindingEvent event) {System.out.println("替换属性：" + event.getName() + ",其值为:" + event.getValue());}}public class MyHttpSessionListener implements HttpSessionListener {@Overridepublic void sessionCreated(HttpSessionEvent event) {System.out.println("有个会话被创建了:" + event.getSession());}@Overridepublic void sessionDestroyed(HttpSessionEvent event) {System.out.println("有个会话被销毁了:" + event.getSession());}
}public class MyServletContextAttributeListener implements ServletContextAttributeListener {@Overridepublic void attributeAdded(ServletContextAttributeEvent event) {System.out.println("添加上下文属性：" + event.getName() + ",其值为:" + event.getValue());}@Overridepublic void attributeRemoved(ServletContextAttributeEvent event) {System.out.println("删除上下文属性：" + event.getName() + ",其值为:" + event.getValue());}@Overridepublic void attributeReplaced(ServletContextAttributeEvent event) {System.out.println("替换上下文属性：" + event.getName() + ",其值为:" + event.getValue());}
}public class MyServletContextListener implements ServletContextListener {@Overridepublic void contextInitialized(ServletContextEvent event) {System.out.println("上下文创建：" + event.getServletContext());}@Overridepublic void contextDestroyed(ServletContextEvent event) {System.out.println("上下文销毁:" + event.getServletContext());}}public class MyServletRequestAttributeListener implements ServletRequestAttributeListener {@Overridepublic void attributeAdded(ServletRequestAttributeEvent event) {System.out.println("添加请求属性：" + event.getName() + ",其值为:" + event.getValue());}@Overridepublic void attributeRemoved(ServletRequestAttributeEvent event) {System.out.println("删除请求属性：" + event.getName() + ",其值为:" + event.getValue());}@Overridepublic void attributeReplaced(ServletRequestAttributeEvent event) {System.out.println("替换请求属性：" + event.getName() + ",其值为:" + event.getValue());}
}public class MyServletRequestListener implements ServletRequestListener {@Overridepublic void requestDestroyed(ServletRequestEvent event) {System.out.println("请求被销毁:" + event.getServletRequest());}@Overridepublic void requestInitialized(ServletRequestEvent event) {System.out.println("请求初始化:" + event.getServletRequest());}
}

监听者

public class ListenerServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {req.setAttribute("reqAtt","hello req");req.getSession().setAttribute("sessionAtt", "hello session");req.getSession().setAttribute("binding",new MyAtt());getServletContext().setAttribute("contextAtt", "hello context");}
}

<web-app xmlns="http://java.sun.com/xml/ns/j2ee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"version="2.4"><servlet><servlet-name>listenerServlet</servlet-name><servlet-class>servlet.ListenerServlet</servlet-class></servlet><servlet-mapping><servlet-name>listenerServlet</servlet-name><url-pattern>/listener</url-pattern></servlet-mapping><listener><listener-class>listener.MyHttpSessionAttributeListener</listener-class></listener><listener><listener-class>listener.MyHttpSessionListener</listener-class></listener><listener><listener-class>listener.MyServletContextAttributeListener</listener-class></listener><listener><listener-class>listener.MyServletContextListener</listener-class></listener><listener><listener-class>listener.MyServletRequestAttributeListener</listener-class></listener><listener><listener-class>listener.MyServletRequestListener</listener-class></listener></web-app>

web.xml