解决Tomcat“Exception looking up UserDatabase under key UserDatabase”错误

最近Tomcat出现了如下错误，在网上找了好多资料也没有解决。最后把同学正常启动的Tomcat的配置文件拷了过来，Tomcat能够正常运行。经过比较然来是我的server.xml文件中少了默认的：

                     type="org.apache.catalina.UserDatabase"
       description="User database that can be updated and saved"
           factory="org.apache.catalina.users.MemoryUserDatabaseFactory"
          pathname="conf/tomcat-users.xml" />

从而导致了“Exception looking up UserDatabase under key UserDatabase”的错误：

严重: Exception looking up UserDatabase under key UserDatabase
javax.naming.NameNotFoundException: Name UserDatabase is not bound in this Cont
xt
        at org.apache.naming.NamingContext.lookup(NamingContext.java:770)
        at org.apache.naming.NamingContext.lookup(NamingContext.java:153)
        at org.apache.catalina.realm.UserDatabaseRealm.start(UserDatabaseRealm.
ava:253)
        at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1006

        at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:44
)
        at org.apache.catalina.core.StandardService.start(StandardService.java:
48)
        at org.apache.catalina.core.StandardServer.start(StandardServer.java:70
)
        at org.apache.catalina.startup.Catalina.start(Catalina.java:552)
        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:295)
        at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:433)
2009-8-25 9:38:36 org.apache.catalina.startup.Catalina start
严重: Catalina.start:
LifecycleException:  No UserDatabase component found under key UserDatabase
        at org.apache.catalina.realm.UserDatabaseRealm.start(UserDatabaseRealm.
ava:261)
        at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1006

        at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:44
)
        at org.apache.catalina.core.StandardService.start(StandardService.java:
48)
        at org.apache.catalina.core.StandardServer.start(StandardServer.java:70
)
        at org.apache.catalina.startup.Catalina.start(Catalina.java:552)
        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:295)
        at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:433)
2009-8-25 9:38:36 org.apache.catalina.startup.Catalina start
信息: Server startup in 121 ms