Blastradius

Most "test impact analysis" tools guess from a static, per-module dependency graph, or train something probabilistic on historical flakiness. Blastradius does neither: a -javaagent observes every class actually loaded while each test runs, records it, and uses that real, per-test dependency map to decide what to run next time. No training data, no heuristics, no opaque score.

Proven on real projects, not just fixtures

Before any of this was trusted to actually skip tests in CI, it ran in shadow mode against real open-source history and had every decision checked against ground truth:

Project	Real history	Commit pairs analyzed	Would-miss cases	Savings
commons-io	6,230 commits	5	0	41.2% of test executions correctly skipped
jsoup	2,455 commits	100	0	2.0% skipped (83% of this window was non-source maintenance commits — correctly triggering the safe fallback, not a mechanism weakness)

105 real commit pairs across two independent, unmodified production codebases. Zero missed test failures. Full analysis and how three real mechanism bugs were found and fixed along the way (a hardcoded argLine, a JaCoCo collision, a parameterized-test name mismatch) is in SESSION.md.

How it works

1. Track. On a build of your base branch, a java.lang.instrument agent watches every class actually loaded while each test runs and records which production classes it really touched — ground truth, not a guess.
2. Diff. On every other build, the current commit is diffed against your base reference: Java source changes vs. everything else (config, resources, pom.xml, migrations).
3. Select. A test runs if one of its tracked dependencies changed, it's new or was itself modified, or a non-source change triggered the conservative "just run everything" fallback.
4. Gate. The selection narrows Surefire/Failsafe via the standard -Dtest= filter — nothing exotic, nothing that fights JaCoCo or a custom argLine.

Modules

Module	What it is	Status
blastradius-core	The shared engine — the dependency-tracking agent and the selection rules (dependency match, conservative fallback, always-select-new/modified). Built and proven first; reused unmodified by both modules below.	Complete, 41 tests
blastradius-maven-plugin	The product. A real, installable blastradius:select Maven goal that gates CI by actually skipping tests during a live build. See its own README for adoption, configuration, and console output reference.	Complete, 46 tests
blastradius-validator	The shadow-mode harness that produced the real-project numbers above — replays a project's own commit history, compares what would have been skipped against ground truth, and reports would-miss cases. Still here if you want to validate the mechanism against a project of your own before adopting the plugin.	Complete, 60 tests

Quick start

<plugin>
  <groupId>io.github.baokhang83.blastradius</groupId>
  <artifactId>blastradius-maven-plugin</artifactId>
  <version>0.1.0-SNAPSHOT</version>
  <executions>
    <execution>
      <phase>process-test-classes</phase>
      <goals><goal>select</goal></goals>
    </execution>
  </executions>
  <configuration>
    <baseRef>main</baseRef>
  </configuration>
</plugin>

No other change required — Surefire/Failsafe stay configured exactly as they already are. See blastradius-maven-plugin/README.md for the full configuration reference, what each build mode (TRACK/SELECT/FALLBACK) prints, and how to set it up in CI.

git clone https://github.com/baokhang83/blastradius.git
cd blastradius
mvn clean install   # builds and tests all three modules

Multi-module reactors

Fully supported, without extra bookkeeping. Because tracking is based on actual class loads rather than a static per-module dependency graph, a change in one module correctly selects a dependent test living in another module — attribution falls out of the mechanism itself.

Why this is safe to use

The selection mechanism is sound by default, not by absolute guarantee — see the real-project numbers above for what "sound by default" has actually measured out to. We recommend every adopting team also run their full test suite portfolio on a regular cadence (recommended: daily) as a complementary safety net, so even an occasional gap is caught within a day rather than never. That combination — fast, sound-by-default selection on every build, backstopped by a full run — is the intended trust model, not either one alone.

Design principles (project constitution, v2.0.0)

• Test-Driven Development is non-negotiable. Every piece of engine code was built red → green → refactor; a tool that decides which tests to skip cannot itself be undertested.
• Clean code & simplicity. No speculative abstraction — blastradius-core was extracted only once a second real consumer (the plugin) needed it.
• Safety over speed. Sound, conservative selection is the strong default, complemented by the recommended daily full-suite run above, not a substitute for one.
• Deterministic core before ML. Selection is pure, explainable dependency tracking, requiring zero historical/training data and correct from a project's very first run — no machine learning, no probabilistic shortcuts.
• Explainability. Every decision carries a concrete reason — which changed class a test's tracked dependencies intersect with, or which fallback rule fired — never an opaque score.
• Maintainable, modern foundations. JUnit 5 Platform, current JDK, no deprecated APIs or abandoned tooling.

Full text and rationale: .specify/memory/constitution.md.

Known limitations

• A class loaded only inside a JUnit 5 @BeforeAll is never attributed to any specific test — tracking only attributes loads to tests that are actually executing. If such a class changes and breaks a test that depended on it only via @BeforeAll setup, that dependency is invisible to selection. Narrow, deterministic, and documented — not a bug being hidden. Lean on the recommended daily full-suite run if this matters for your project.
• Refreshing the dependency index (a "track" build) runs the full suite once; correct, but not optimized for very slow suites. It only happens on base-reference builds, never on every PR build.

Project layout

blastradius-core/           the engine: tracking agent + selection rules
blastradius-maven-plugin/   the product: the blastradius:select goal
blastradius-validator/      shadow-mode validation harness (real-project evidence above)
specs/                      spec, plan, research (ADR-style), contracts, tasks — per feature
.specify/memory/            project constitution
SESSION.md                  narrative log of how T061's real-project validation went

License

Apache License 2.0.