No plugin found for prefix 'jetty' in the current project and in the plugin groups (local, central)

In order to easily run my webapp, I decided to add Jetty to my single POM file.

Following the official documentation, I added this to my <plugins>:

  <plugin>
    <groupId>org.eclipse.jetty</groupId>
    <artifactId>jetty-maven-plugin</artifactId>
    <version>9.4.0-SNAPSHOT</version>
  </plugin>

PROBLEM: mvn jetty:run fails:

$ mvn jetty:start
[INFO] Scanning for projects...
[WARNING] The POM for org.eclipse.jetty:jetty-maven-plugin:jar:9.4.0-SNAPSHOT is missing, no dependency information available
[WARNING] Failed to retrieve plugin descriptor for org.eclipse.jetty:jetty-maven-plugin:9.4.0-SNAPSHOT: Plugin org.eclipse.jetty:jetty-maven-plugin:9.4.0-SNAPSHOT or one of its dependencies could not be resolved: Could not find artifact org.eclipse.jetty:jetty-maven-plugin:jar:9.4.0-SNAPSHOT
Downloading: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-metadata.xml
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/mojo/maven-metadata.xml
Downloaded: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-metadata.xml (13 KB at 2.1 KB/sec)
Downloaded: https://repo.maven.apache.org/maven2/org/codehaus/mojo/maven-metadata.xml (20 KB at 3.2 KB/sec)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 7.013 s
[INFO] Finished at: 2016-08-17T16:49:28+09:00
[INFO] Final Memory: 14M/307M
[INFO] ------------------------------------------------------------------------
[ERROR] No plugin found for prefix 'jetty' in the current project and in the plugin groups [org.apache.maven.plugins, org.codehaus.mojo] available from the repositories [local (/home/nico/.m2/repository), central (https://repo.maven.apache.org/maven2)] -> [Help 1]

Another approach at https://mvnrepository.com/artifact/org.eclipse.jetty/jetty-maven-plugin/9.4.0.M0 suggests to add this in <dependencies>:

            <dependency>
                <groupId>org.eclipse.jetty</groupId>
                <artifactId>jetty-maven-plugin</artifactId>
                <version>9.4.0.M0</version>
            </dependency>

It fails too, mvn jetty:start says:

[ERROR] No plugin found for prefix 'jetty' in the current project and in the plugin groups [org.apache.maven.plugins, org.codehaus.mojo] available from the repositories [local (/home/nico/.m2/repository), central (https://repo.maven.apache.org/maven2)] -> [Help 1]

I removed my .m2 folder and let Maven recreate it, no better. I have no settings.xml in ~/.m2 and here is the content of ~/.m2/repository/org/eclipse/jetty/jetty-maven-plugin/9.4.0.M0/:

-rw-rw-r-- 1 nico nico 101524  8月 24 17:29 jetty-maven-plugin-9.4.0.M0.jar
-rw-rw-r-- 1 nico nico     40  8月 24 17:29 jetty-maven-plugin-9.4.0.M0.jar.sha1
-rw-rw-r-- 1 nico nico   5526  8月 24 17:28 jetty-maven-plugin-9.4.0.M0.pom
-rw-rw-r-- 1 nico nico     40  8月 24 17:28 jetty-maven-plugin-9.4.0.M0.pom.sha1
-rw-rw-r-- 1 nico nico    215  8月 24 17:29 _remote.repositories

Note: There are several questions on the same topic, all have outdated answers containing Maven ids (mortbay, codehaus) dating from before Jetty moved to Eclipse, or suggest adding the <plugin> block seen at the top of my question.

Solution

You are using the Plugin version 9.4.0-SNAPSHOT. This version is not available in the central repo (available versions).

To add a dependency doesn't solve the problem because a dependency is not the same as a plugin. A dependency is used or required by the code you want to compile and a plugin is something that would compile, build or analyze your code.

In short:

you don't need a dependency on jetty-maven-plugin

you have to change the version of the plugin to a version which is available in the central or your local repo.
For example:

<plugin>
    <groupId>org.eclipse.jetty</groupId>
    <artifactId>jetty-maven-plugin</artifactId>
    <version>9.4.0.M0</version>
</plugin>