Head first servlet and jsp第五章例子监听器未被调用？

Question

代码：
MyServletContextListener.java

package com.example;

import javax.servlet.*;

public class MyServletContextListener implements ServletContextListener {
    public void contexInitialized(ServletContextEvent event)
    {
        ServletContext sc = event.getServletContext();
        String dogBreed = sc.getInitParameter("breed");
        Dog d= new Dog(dogBreed);
        sc.setAttribute("dog", d);
    }
    public void contextDestroyed(ServletContextEvent event)
    {

    }
}

Dog.java

package com.example;

public class Dog {
    private String breed;
    public Dog(String breed)
    {
        this.breed=breed;
    }
    public String getBreed()
    {
        return breed;
    }
}

ListenerTester.java

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        response.setContentType("text/html");
        PrintWriter out=response.getWriter();
        out.println("test context arrtibutes set by listener<br>");
        out.println("<br>");
        Dog dog=(Dog)getServletContext().getAttribute("dog");
        out.println("Dog's breed is"+ dog.getBreed());

    }

web.xml

<servlet>
    <servlet-name>ListenerTester</servlet-name>
    <servlet-class>com.example.ListenerTester</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ListenerTester</servlet-name>
    <url-pattern>/ListenTest.do</url-pattern>
  </servlet-mapping>
  <context-param>
    <param-name>breed</param-name>
    <param-value>Great Dane</param-value>
  </context-param>
  <listener>
    <listener-class>
        com.example.MyServletContextListener
    </listener-class>
  </listener>

报错：

一月 31, 2017 9:30:24 下午 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet [ListenerTester] in context with path [/ContextListener] threw exception
java.lang.NullPointerException
    at com.example.ListenerTester.doGet(ListenerTester.java:37)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:474)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:783)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:745)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1437)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

代码上实在找不出错误 都是照着书一模一样敲的，显示500错误，在浏览器打开只显示了
test context arrtibutes set by listener
这一行
感觉监听器并没有被调用