== Single dependency version, single classpath target platform
Target platform is expected to have single version of any GA directly or indirectly referenced by the project. The build will fail if any of the referenced GAs has more than one version configured in the target platform.
As a side note, target platform configuration allows multiple versions of that same GA. This is necessary to support multiple versions of parent pom artifacts, e.g., org.apache:apache, which can be referenced by different, otherwise unrelated artifacts.
The target platform artifact version will be forced during dependency resolution. This is similar to how works in traditional Maven 3 project, however the target platform covers all project dependencies (i.e. it is not optional) and does not require explicit pom.xml configuration.
Generally, project dependencies are expressed in terms of their GA coordinates, and elements are redundant and can be omitted from project pom.xml files. This applies both to and pom.xml sections. If s are provided, however, they are validated to match the artifact version present in the target platform, either using exact version match or version range match, if element represents a version range.
To maintain compatibility with Maven 3 and other tools, "versionless" dependencies versions are injected in pom.xml files as elements during the build. This will guarantee consistent dependencies versions when the artifacts are consumed.
| Dependency type | element | Resolved version |
|---|---|---|
| project-project | allowed, validated ** | reactor project |
| project-external | not allowed | TP version |
| external-external | ignored | TP version |
** this is not desired in the long term, but is necessary to provide partial build compatibility. will be changed to "not allowed" once ProjectDependencyGraph builder is able to cope with versionless reactor dependencies.
== Possible future extension, multiple dependency versions
Target platform allows multiple versions of artifacts directly or indirectly referenced from the project.
For artifacts present in a single version, the target platform behaves as implicit , that is, the version will be used everywhere the artifact GA is referenced, regardless of the specified by the reference.
Project pom.xml may omit elements for single-version target platform artifacts. If element is present, it is validated to match the artifact version present in the target platform, either using exact version match or version range match, if element represents version range.
For artifacts present in multiple versions, the target platform behaves as a filter that hides other artifact versions, but otherwise normal Maven 3 dependency specification and resolution rules apply. I.e., all and elements that correspond to multi-versioned artifact must have element, "nearest wins" rule is used to resolve version conflict, etc.
Going from single- to multi-versioned target platform artifact will require changes to project pom.xml.
To maintain compatibility with Maven 3 and other tools, versions of "versionless" dependencies are injected in pom.xml files as elements during the build. This will guarantee consistent dependencies versions when the artifacts are consumed.
| Dependency type | element | Resolved version |
|---|---|---|
| project-project | allowed, validated | reactor project |
| project-external | allowed, validated | TP version |
| external-external | required, ignored | TP version |
Note that this alternative behaviour is backwards compatible with 'single-version' approach and can be implemented in the future based on user feedback.