Error assembling WAR: webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in

执行Maven Install打包的时候，出现以下错误信息：

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-war-plugin:2.2:war (default-war) on project pro-test: Error assembling WAR: webxml attribute is required (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 switch.
[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 read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

产生该错误的原因是“webapp”目录下缺少“WEB-INF”目录，缺少“WEB-INF/web.xml”文件。