刚接触servlet类，按照课本的方法使用eclipse新建了一个servlet类。
新建完成后，在web.xml里面进行注册
这时候就会报错了。
五月 07, 2016 11:23:28 上午 org.apache.coyote.AbstractProtocol pause
信息: Pausing ProtocolHandler ["http-nio-8080"]
五月 07, 2016 11:23:28 上午 org.apache.coyote.AbstractProtocol pause
信息: Pausing ProtocolHandler ["ajp-nio-8009"]
五月 07, 2016 11:23:28 上午 org.apache.catalina.core.StandardService stopInternal
信息: Stopping service Catalina
五月 07, 2016 11:23:28 上午 org.apache.coyote.AbstractProtocol destroy
信息: Destroying ProtocolHandler ["http-nio-8080"]
五月 07, 2016 11:23:28 上午 org.apache.coyote.AbstractProtocol destroy
严重: Failed to destroy end point associated with ProtocolHandler ["http-nio-8080"]
java.lang.NullPointerException
at org.apache.tomcat.util.net.NioEndpoint.releaseCaches(NioEndpoint.java:305)
at org.apache.tomcat.util.net.NioEndpoint.unbind(NioEndpoint.java:481)
at org.apache.tomcat.util.net.AbstractEndpoint.destroy(AbstractEndpoint.java:831)
at org.apache.coyote.AbstractProtocol.destroy(AbstractProtocol.java:533)
at org.apache.catalina.connector.Connector.destroyInternal(Connector.java:1023)
at org.apache.catalina.util.LifecycleBase.destroy(LifecycleBase.java:297)
at org.apache.catalina.core.StandardService.destroyInternal(StandardService.java:589)
at org.apache.catalina.util.LifecycleBase.destroy(LifecycleBase.java:297)
at org.apache.catalina.core.StandardServer.destroyInternal(StandardServer.java:877)
at org.apache.catalina.util.LifecycleBase.destroy(LifecycleBase.java:297)
at org.apache.catalina.startup.Catalina.start(Catalina.java:633)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:351)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:485)
五月 07, 2016 11:23:28 上午 org.apache.coyote.AbstractProtocol destroy
信息: Destroying ProtocolHandler ["ajp-nio-8009"]
五月 07, 2016 11:23:28 上午 org.apache.coyote.AbstractProtocol destroy
严重: Failed to destroy end point associated with ProtocolHandler ["ajp-nio-8009"]
java.lang.NullPointerException
at org.apache.tomcat.util.net.NioEndpoint.releaseCaches(NioEndpoint.java:305)
at org.apache.tomcat.util.net.NioEndpoint.unbind(NioEndpoint.java:481)
at org.apache.tomcat.util.net.AbstractEndpoint.destroy(AbstractEndpoint.java:831)
at org.apache.coyote.AbstractProtocol.destroy(AbstractProtocol.java:533)
at org.apache.catalina.connector.Connector.destroyInternal(Connector.java:1023)
at org.apache.catalina.util.LifecycleBase.destroy(LifecycleBase.java:297)
at org.apache.catalina.core.StandardService.destroyInternal(StandardService.java:589)
at org.apache.catalina.util.LifecycleBase.destroy(LifecycleBase.java:297)
at org.apache.catalina.core.StandardServer.destroyInternal(StandardServer.java:877)
at org.apache.catalina.util.LifecycleBase.destroy(LifecycleBase.java:297)
at org.apache.catalina.startup.Catalina.start(Catalina.java:633)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:351)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:485)
后来我找到了原因。原来是我用了Tomcat8.0
服务器使用注解的方式帮我注册了servlet了，这时候已经不需要我再web.xml手动注册了。
我把web.xml配置的内容删了就可以了。