用maven执行mvn clean compile的时候遇到这个问题，哪位大神帮忙解决一下

[INFO]
[INFO] Parent ............................................. FAILURE [ 0.528 s]
[INFO] Hello .............................................. SKIPPED
[INFO] HelloOne ........................................... SKIPPED
[INFO] HelloTwo ........................................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 0.655 s
[INFO] Finished at: 2017-02-07T10:50:47+08:00
[INFO] Final Memory: 9M/110M
[INFO] ------------------------------------------------------------------------
[ERROR] Plugin org.apache.maven.plugins:maven-clean-plugin:2.5 or one of its dependencies could not be resolved: Failed to read artifact descriptor for org.apache.maven.plugins:maven-clean-plugin:jar:2.5: Could not transfer artifact org.apache.maven.plugins:maven-clean-plugin:pom:2.5 from/to nexus (http://localhost:8080/nexus-2.1.2/content/groups/public): Failed to transfer file: http://localhost:8080/nexus-2.1.2/content/groups/public/org/apache/maven/plugins/maven-clean-plugin/2.5/maven-clean-plugin-2.5.pom. Return code is: 503, ReasonPhrase: Service Unavailable. -> [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/PluginResolutionException