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/groupIdmetadata/artifactIdmetadata/versioning/latestmetadata/versioning/releasemetadata/versioning/versions/*metadata/versioning/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/groupIdmetadata/artifactIdmetadata/versioning/snapshot/*metadata/versioning/snapshotVersions/*