gnodet opened a new issue, #13460:
URL: https://github.com/apache/maven/issues/13460
## Goal
Progressively migrate Maven's internal implementation from v3 API types
(`MavenProject`, `MavenSession`, v3 `Artifact`, etc.) to v4 API types
(`Project`, `Session`, v4 `Artifact`, etc.), while **preserving full backward
compatibility** with existing Maven 3 plugins.
This is a long-term effort toward Maven 5, where the `compat/` modules can
be dropped entirely.
## Current State
The **model builder** has already been successfully migrated: the real logic
lives in `impl/maven-impl` using v4 types natively, and the v3
`DefaultModelBuilder` in `compat/` delegates to it. This is the pattern to
follow for all other subsystems.
However, the **entire build orchestration pipeline** still runs on v3 types
internally:
```
DefaultMaven.doExecute(MavenExecutionRequest) ← v3
→ DefaultGraphBuilder.build(MavenSession) ← v3
→ Collection strategies → v3 ProjectBuilder ← v3
→ builds List<MavenProject> ← v3
→ DefaultProjectDependencyGraph ← v3
→ LifecycleStarter.execute(MavenSession) ← v3
→ BuildPlanExecutor / MultiThreadedBuilder ← v3
→ LifecycleModuleBuilder → MojoExecutor ← v3
→ BuildPluginManager / MavenPluginManager ← v3
```
In `impl/maven-core`, **~209 of 384 Java files** still import v3 types:
- `MavenProject`: ~95 files
- `MavenSession`: ~59 files
- v3 `Artifact`: ~44 files
### Already v4-native (in `impl/maven-impl`)
- Model builder + all sub-services (interpolator, normalizer, inheritance
assembler, profile selector, etc.)
- Artifact services (factory, resolver, installer, deployer)
- Dependency resolver
- Settings/toolchains builders
- Repository services
- Version resolver/range resolver
## Strategy: Inversion, Not Removal
The migration follows the **inversion pattern** proven by the model builder:
1. **Create a v4-native implementation** in `impl/maven-impl`
2. **Invert the delegation**: make the v3 compat class delegate to the v4
implementation (not the other way around)
3. **Keep v3 types as wrappers** for plugin compatibility — plugins still
inject `MavenProject`, `MavenSession`, etc.
**Today:**
```
v4 API wrapper → delegates to → v3 implementation (real logic)
```
**After migration:**
```
v3 compat wrapper → delegates to → v4 implementation (real logic)
```
Plugins don't notice the change — they keep using v3 types that are now thin
wrappers around v4 objects.
## Migration Phases
Each phase is tracked as a separate sub-issue that can be picked up
independently (though some have ordering dependencies noted below).
### Phase 1 — Project Builder inversion
Move project-building logic from
`maven-core/.../project/DefaultProjectBuilder` (v3, ~1420 lines) into a
v4-native implementation in `maven-impl`. The v3 project builder already
delegates to the v4 model builder internally, so the hardest sub-problem is
solved.
### Phase 2 — Session inversion
Invert so v3 `MavenSession` wraps a v4 `Session` (currently it's the
opposite — v4 `DefaultSession` wraps v3 `MavenSession`).
### Phase 3 — Graph builder / reactor (v4 API design + implementation)
Define a v4 API for the project dependency graph (no equivalent exists
today), then implement it in `maven-impl`.
### Phase 4 — Lifecycle engine
Migrate the lifecycle execution pipeline (~44 files) to v4 types. This is
the largest phase and likely needs to be broken into sub-phases. Depends on
phases 1-3.
### Phase 5 — Plugin system
Migrate internal plugin resolution, caching, and management (~28 files) to
v4 types. Plugin-facing API (`AbstractMojo`, `PluginDescriptor`, etc.) stays in
`compat/`.
### Phase 6 — Orchestrator and CLI
Once everything below is v4-native, `DefaultMaven` and the CLI can work with
v4 types directly.
### Quick wins (can start now, no ordering dependency)
- Event loggers (~6 files) — migrate to v4 `ExecutionEvent`
- `DefaultProject` inversion — hold v4 types natively instead of wrapping
`MavenProject`
- Collection strategies — small classes, easy to migrate once project
builder is done
## Public API Boundary (must stay for compat until Maven 5)
These types are injected into plugins and **cannot be removed** until Maven
5:
- `org.apache.maven.project.MavenProject`
- `org.apache.maven.execution.MavenSession`
- `org.apache.maven.plugin.AbstractMojo` / `Mojo`
- `org.apache.maven.plugin.descriptor.PluginDescriptor` / `MojoDescriptor`
- `org.apache.maven.model.*` (v3 model types)
- `org.apache.maven.artifact.Artifact`
- `org.apache.maven.settings.Settings`
They become thin wrappers over v4 types, not removed.
_Claude Code on behalf of Guillaume Nodet_
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