How does Maven decide what version of a plugin to use, when you don't specify any?

Question

I recognized that Maven not always uses the latest version of a plugin.

For example org.codehaus.mojo:sonar-maven-plugin version 2.7 has beed released on 19th of October but on 23th of October, 2.6 was still used by Maven (mvn sonar:sonar).

I even remember some plugins, where the latest version was several minor releases above the version that Maven decided to use.

Is there any (central) index/list/database where Maven looks up what version to use? If yes, where can it be accessed manually?

Answer 1

I know this is an ancient thread but in the interest of posterity and accuracy: all pom.xmls logically inherit from the super POM. You can always see what your "real" pom.xml looks like by typing:

mvn help:effective-pom

The resulting pom.xml that is printed is a combination of the super POM, your pom.xml, and of course any parent POMs in the mix as well.

The super POM is provided by the org.apache.maven.model.superpom.DefaultSuperPomProvider class (https://github.com/apache/maven/blob/bce33aa2662a51d18cb00347cf2fb174dc195fb1/maven-model-builder/src/main/java/org/apache/maven/model/superpom/DefaultSuperPomProvider.java#L56-L85). The resource it loads is org/apache/maven/model/pom-4.0.0.xml (https://github.com/apache/maven/blob/bce33aa2662a51d18cb00347cf2fb174dc195fb1/maven-model-builder/src/main/resources/org/apache/maven/model/pom-4.0.0.xml#L23-L149).

Answer 2

For command line execution if a version is not specified Maven looks up the latest version from the GA (group ID/artifact ID) maven-metadata.xml file.

Example: http://repo1.maven.org/maven2/org/apache/maven/plugins/maven-clean-plugin/maven-metadata.xml

If you're encoding command line execution of Maven goals into your builds you should specify a version for these, otherwise your builds may change as new versions of plugins are released.

Answer 3

As far as i know, this link will answer your question.

Automatic Plugin Version Resolution

When a plugin was invoked without an explicit version given in the POM or on the command line, Maven 2.x used to pick the latest version available where the latest version could either be a release or a snapshot. For the sake of stability, Maven 3.x prefers the latest release version over the latest snapshot version.

Given the threat of non-reproducible builds imposed by automatic plugin version resolution, this feature is scheduled for removal as far as plugin declarations in the POM are concerned. Users of Maven 3.x will find it output a warning when missing plugin versions are detected to encourage the addition of plugin versions to the POM or one of its parent POMs. The Enforcer rule requirePluginVersions can be used additionally check for missing plugin versions in the POM