在 Maven 插件中，如何从依赖项访问文件？

Question

Background: I am trying to hook the compiler for a domain-specific language into Maven. The DSL is compiled to Java source code, so my idea was to write a Mojo that gets executed in the generate-sources phase. The compiler is currently implemented as a Gradle plugin and cannot handle multiple projects with dependencies, and while trying to support projects, I hit problems in Gradle. I'm now trying to understand if Maven handles that situation in a better way. The problems with Gradle are irrelevant to this question.

My DSL has packages like Java that get mapped to identical Java packages. The same should be true for projects (called "modules", AFAIK, if they are part of a reactor build). In that case, for each project, the DSL sources get compiled to Java source code, as well as meta-data (a JSON file per compiled class). When project A depends on B, the DSL compilation process for A needs the meta-data files from B. That meta-data should be packaged as resources into the JAR file together with the generated and compiled Java code, as well as possibly hand-written and compiled Java code.

In Maven, I can get the current MavenProject injected into the Mojo. I can ask the MavenProject for its dependencies, but I only seem to get meta-data like the Maven coordinates for the dependencies. I do not understand how to access files from the dependencies. What is necessary to do that?

I also do not understand yet if I have to make a distinction whether the dependency is a JAR from an artifact repository or another module in a reactor build. The reason for this is that, apparently, in a reactor build, it is not required for the dependency to go through the full Maven lifecycle up to "install": Suppose that project A depends on project B, and I run "mvn generate-sources" in the parent. Then the DSL compiler runs in project B, and later it runs in project A. However, the build process for B has stopped after generate-sources, so to the DSL compiler for A, B looks very different from an artifact from a repository -- starting with the fact that there is no JAR file. Is it correct that I have to make this distinction (between reactor dependencies and repository dependencies)? If so, how do I get the necessary information? Are there any best practices on how to do that?

Answer 1

I managed to access dependencies by having a "dependency resolver" injected into my Mojo (which is a thin wrapper around RepositorySystem), as well as a session for the latter:

@Component
private ProjectDependenciesResolver projectDependenciesResolver;

@Parameter(defaultValue = "${repositorySystemSession}", readonly = true)
private RepositorySystemSession repoSession;

I can use those to obtain dependencies:

DefaultDependencyResolutionRequest request = new DefaultDependencyResolutionRequest(project, repoSession);
DependencyResolutionResult result = projectDependenciesResolver.resolve(request);

The result contains the dependencies as File objects which can point to a JAR in the.m2 folder (for repository dependencies), a JAR file that was just built (for reactor dependencies when building towards a sufficiently "high" phase), a folder full of class files (for reactor dependencies when building towards a "lower" phase where JAR packaging did not yet happen), or not exist at all (for reactor dependencies when building towards a "really low" phase).

This is sufficient for me. I'll have to make a distinction based on whether I get a JAR file, folder, or nothing at all (should be pretty simple), and I have to deal with the fact that when building towards phase X, reactor dependencies will also build only up to X, so files that I need might be missing.