gnodet opened a new issue, #13562:
URL: https://github.com/apache/maven/issues/13562
## Context
The logging PR stack (#12571) introduces several new event/callback systems
that bypass
the existing v4 `Event → Listener` hierarchy. After the stack lands, Maven
will have
**seven** distinct event/listener/callback patterns:
| System | Pattern | v3/v4 |
|--------|---------|-------|
| `ExecutionEvent` / `ExecutionListener` | Typed events on bus | v4 |
| `RepositoryEvent` / `RepositoryListener` | Typed events on bus | v4 |
| `EventSpy.onEvent(Object)` | Untyped bag | v3 internal |
| `BuildEventListener` | 11 string methods | v3 internal |
| `MojoExecutionListener` / `ProjectExecutionListener` | Direct callbacks |
v3 internal |
| `DiagnosticReporter` | Service (not on bus) | v4 (new) |
| `LogSink` | Direct callback | internal (new) |
The v4 `Event`/`Listener` model is already designed as a unified bus, but
the new
logging additions bypass it.
## Proposal
Extend the v4 `Event` hierarchy so that **all** observable build activity
flows through
a single bus. `DiagnosticReporter` remains a service (producer API for
plugins), but its
implementation emits events onto the bus so consumers subscribe uniformly.
```
Event (base — existing)
├── ExecutionEvent (17 types — existing)
├── RepositoryEvent (19 types — existing, see #13520)
├── DiagnosticEvent (new — wraps BuilderProblem)
├── TransferEvent (new — download progress, missing from v4 entirely)
├── LogEvent (new — significant log messages)
├── CacheEvent (new — for IDE embedders, see #13519)
└── BuildPhaseEvent (new — entering validate/compile/test phase)
```
### Benefits
- **Single `Listener`** can observe everything (execution + diagnostics +
downloads + logs)
- `BuildReportCollector` migrates from v3 `EventSpy` to v4 `Listener`
- Extensions (OTel, build scanners) get a unified stream
- #13520 (repository event listener SPI) fits naturally
- #13519 (cache invalidation for IDE embedders) gets an event type
### Design constraints
- `DiagnosticReporter` stays a service — it's the input API for plugins
- `DiagnosticReporter` implementation emits `DiagnosticEvent` so listeners
can react
- Backward compatibility: `EventSpy` continues to work, `BuildEventListener`
is
deprecated but not removed in 4.1.0
## Sub-issues
- [ ] #13520 — Repository event listener SPI
- [ ] `DiagnosticEvent` — emit from `DiagnosticReporter` onto the v4 event
bus
- [ ] `TransferEvent` — download progress as v4 event (currently no v4
equivalent)
- [ ] `LogEvent` as v4 `Event` subtype — unify with `build.report.LogEvent`
- [ ] `CacheEvent` — for IDE embedder cache invalidation (#13519)
- [ ] `BuildPhaseEvent` — lifecycle phase transitions as events
- [ ] Migrate `BuildReportCollector` from `EventSpy` to v4 `Listener`
## Related
- #12571 — Build output overhaul (parent tracking issue)
- #13520 — Repository event listener SPI
- #13519 — Cache invalidation/eviction SPI for IDE embedders
- #13499 — v4 API/SPI gaps for build extensions
<!-- AI-Agent-Disclosure -->
> 🤖 Drafted with Claude Code (Claude Opus 4.6). Reviewed and confirmed by
@gnodet before submission.
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