MVN JETTY:RUN 报错 The plugin 'org.mortbay.jetty:maven-jetty-plugin' does not exis

pom.xml配置:

<plugin>
	<groupId>org.mortbay.jetty</groupId>
	<artifactId>maven-jetty-plugin</artifactId>
	<version>6.0.0</version>
	<configuration>
		<webDefaultXml>src/test/resources/webdefault.xml</webDefaultXml>
		<systemProperties>
			<systemProperty>
				<name>jetty.port</name>
				<value>8081</value>
			</systemProperty>
		</systemProperties>
		<contextPath>/</contextPath>
		<scanIntervalSeconds>3000</scanIntervalSeconds>
		<scanTargets>
			<scanTarget>src/main/webapp/WEB-INF</scanTarget>
			<scanTarget>../target/classes</scanTarget>
		</scanTargets>
	</configuration>
</plugin>

后来在settings.xml 加了这么一句后, 解决.
<pluginGroups>
    <pluginGroup>org.mortbay.jetty</pluginGroup>
  </pluginGroups>