Let the DefaultBuildContext also delegate getValue and setValue to the legacy build-api

[Volodjam]

Since #48 (PR #50), DefaultBuildContext stores objects in an internal contextMap.

Starting with PR #71, DefaultBuildContext delegates all methods to the legacy build API except getValue() and setValue(). As a result, objects are stored in the singleton DefaultBuildContext instead of the thread-local build context (ThreadBuildContext).

This causes problems in Eclipse when updating multiple projects that use plugins based on the new build API (for example, bnd-maven-plugin) to store and retrieve objects from the build context. In this situation, the same contextMap is shared across projects, leading to unintended cross-project state sharing.

It would be better to also delegate getValue() and setValue() to the legacy build API so that objects are stored in the thread-local ThreadBuildContext rather than the singleton DefaultBuildContext.

[laeubi]

It would be better to also delegate getValue() and setValue() to the legacy build API so that objects are stored in the thread-local ThreadBuildContext rather than the singleton DefaultBuildContext.

Should we not maybe better than make the DefaultBuildContext thread aware? I'm not sure this is documented but it seems a reasonable behavior.

In any case I doubt a ThreadLocal would help here, it should be per execution isn't it?

[Volodjam]

You're right - DefaultBuildContext should ideally define a proper context state scope on its own as well (for command-line builds).

According to the documentation of setValue(), its primary (and only) purpose is to allow preservation of state needed for proper incremental behaviour between consecutive executions of the same mojo needed to.

In m2e, MavenBuilder creates a BuildContext per project build (iteration) and shares it across all build participants (executors) involved in that Maven build. See MavenBuilderImpl.java#L131. However, the context state is initialized during a full build of the project and then reused across subsequent incremental builds of the same project. See PlexusBuildAPI.java#L42.

For backwards compatibility, DefaultBuildContext should therefore delegate setValue() and getValue() to the legacy build API.

It seems to me that if the intended behavior for command-line builds should match Eclipse full builds, then the context state would likely need to be stored per project.
If instead the behavior should follow the second part of the setValue() documentation, then it should be scoped per execution.

That said, I'm not sure whether DefaultBuildContext can currently support either of these scopes, or what the best way would be to implement them.