Maven JAR Plugin 3.0.2 Error: You have to use a classifier to attach supplemental artifacts to the project instead of replacing them

Question

Maven JAR plugin (version 3.0.2) keeps throwing the following error, even for a single invocation of the jar goal:

[ERR

Answer 1

If your logs shows something like:

[INFO] --- maven-jar-plugin:3.0.2:jar (default-jar) @ test --
[WARNING] JAR will be empty - no content was marked for inclusion!

Adding a single useless class in src/main/java seems to solve the issue see:

http://maven.40175.n5.nabble.com/quot-mvn-clean-verify-deploy-quot-causes-jar-plugin-to-execute-twice-td5877166.html

Answer 2

In my case , I have setup the execution's ID as default-jar, then the error gone. e.g.

<execution>
    <id>default-jar</id>
    <phase>package</phase>
    <goals>
        <goal>jar</goal>
    </goals>
</execution>

Answer 3

The Jar Plugin is actually getting executed twice with the configuration:

<plugin>
  <artifactId>maven-jar-plugin</artifactId>
  <version>3.0.2</version>
  <executions>
    <execution>
      <goals>
        <goal>jar</goal>
      </goals>
    </execution>
  </executions>
</plugin>

If you check the logs with such a configuration, you will have something like:

[INFO] --- maven-jar-plugin:3.0.2:jar (default-jar) @ test ---
[INFO] Building jar: ...\test\target\test-0.0.1-SNAPSHOT.jar
[INFO] 
[INFO] --- maven-jar-plugin:3.0.2:jar (default) @ test ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------

meaning that the plugin was in fact executed twice. What happens, is that the Jar Plugin, in a project that has a packaging of jar has a default execution bound to the package phase. This default execution is the one mentioned in the logs with the ID of default-jar.

When you configured an <execution> in the plugin, you actually configured a new execution, where the jar goal of the plugin is to be invoked. Since the jar goal binds by default to the package phase, that execution is getting executed at that phase, after the default binding inherent to the jar packaging. And since the plugin ran already, it is failing because running it again would actually replace the main artifact already produced during the first run. This error was added in version 3.0.0 of the plugin in MJAR-198, because such a thing happening is very likely a mis-configuration which should be detected early.

As such, the fix is simple: don't have an execution that specifies the goal jar, and let the default one (coming from the jar packaging) do the work. The JAR will still be created, even without the explicit configuration of the jar goal, thanks to the default execution. If you want a test JAR as well, you will still need to configure the plugin to do that with:

<plugin>
  <artifactId>maven-jar-plugin</artifactId>
  <version>3.0.2</version>
  <executions>
    <execution>
      <goals>
        <goal>test-jar</goal>
      </goals>
    </execution>
  </executions>
</plugin>

But note that the goal jar is not specified.