maven-enforcer-plugin_maven不指定版本会怎么样

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.4.1:enforce (enforce-versions) on project spark-parent_2.11: Some Enforcer rules
 have failed. Look above for specific messages explaining why the rule failed. -> [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

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-enforcer-plugin</artifactId>
  <version>1.4.1</version>
  <executions>
    <execution>
      <id>enforce-versions</id>
      <goals>
        <goal>enforce</goal>
      </goals>
      <configuration>
        <rules>
          <requireMavenVersion>
            <version>${maven.version}</version>
          </requireMavenVersion>
          <requireJavaVersion>
            <version>${java.version}</version>
          </requireJavaVersion>
          <bannedDependencies>
            <excludes>
              <!--
                Akka depends on io.netty:netty, which puts classes under the org.jboss.netty
                package. This conflicts with the classes in org.jboss.netty:netty
                artifact, so we have to ban that artifact here. In Netty 4.x, the classes
                are under the io.netty package, so it's fine for us to depend on both
                io.netty:netty and io.netty:netty-all.
              -->
              <exclude>org.jboss.netty</exclude>
              <exclude>org.codehaus.groovy</exclude>
            </excludes>
            <searchTransitive>true</searchTransitive>
          </bannedDependencies>
        </rules>
      </configuration>
    </execution>
  </executions>
</plugin>