报错如下，很简单的学习maven的小例子。

[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.913 s
[INFO] Finished at: 2015-10-31T23:58:15+08:00
[INFO] Final Memory: 16M/179M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-war-plugin:2.2:war
(default-war) on project maven01: Error assembling WAR: webxml attribute is req
uired (or pre-existing WEB-INF/web.xml if executing in update mode) -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e swit
ch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please rea
d the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionE
xception