Maven Metadata

Repositories contain metadata (aka repository metadata) files as well, that enables several discovery and resolution-like operations for Maven. These metadata files are not Artifacts, and hence, are not addressable by Maven users. They are instead transparently operated and handled by Maven itself in automatic manner. These files are XML files named as maven-metadata.xml (are deployed with checksums just like artifacts are).

Links:

We distinguish 3 different kinds of metadata files (using GAV coordinates):

  • G level metadata
  • A level metadata
  • V level metadata

These metadata XML files share same XML schema (model), and the reason of that is simple: one XML file may carry multiple kinds of metadata (for different Artifacts)! Consider these two artifacts: org.foo:bar:1.0 and org.foo.bar:baz:1.0. With default layout, the repository path org/foo/bar is once A level for first, and G level for second artifact. Note: this is extreme example, and such artifact naming should be avoided!

The G Level Metadata

What How
Location Path corresponding to groupId
Repositories containing it Release and Snapshots
Artifacts related to them Only maven-plugin packaged

The group level metadata (found in directory of groupId) serves purpose only in case of Maven Plugins (POM packaging: maven-plugin). This metadata contains mapping of plugin prefix to artifactId, and the XML bit considered for this kind of metadata is metadata/plugins/plugin[] path only.

Example: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-metadata.xml

When user uses some plugin prefix on CLI, Maven will go through registered pluginGroups groupIds, download their G level metadata, and look for prefix. If found, Maven has obtained G (from pluginGroups) and A (from metadata) coordinates of plugin, and will go for given version, or attempt to discover latest version.

The A Level Metadata

What How
Location Path corresponding to artifactId
Repositories containing it Release and Snapshots
Artifacts related to them All

The artifactId level metadata (found in directory of artifactId) serves purpose of version discovery. This metadata contains list of versions of given GA coordinates. The XML bit considered for this kind of metadata are only:

  • metadata/groupId
  • metadata/artifactId
  • metadata/versionining/latest
  • metadata/versionining/release
  • metadata/versionining/versions/*
  • metadata/versionining/lastUpdated

Example: https://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-jar-plugin/maven-metadata.xml

The V Level Metadata

What How
Location Path corresponding to baseVersion
Repositories containing it Snapshots only
Artifacts related to them Only Snapshots

Exists only in Snapshot Remote repositories. The version level metadata (found in directory of baseVersion) serves the purpose of snapshot timestamped version resolution. This metadata contains mapping of all artifacts in this baseVersion directory with corresponding timestamps. The XML bit considered for this kind of metadata are only:

  • metadata/groupId
  • metadata/artifactId
  • metadata/versionining/snapshot/*
  • metadata/versionining/snapshotVersions/*

Example: https://repository.apache.org/content/repositories/snapshots/org/apache/maven/plugins/maven-jar-plugin/3.3.0-SNAPSHOT/maven-metadata.xml