vary mutate-parity defends performance-oriented bytecode backends (redefine, hot-swap) against the reference fresh-loader backend. Any time the two backends classify the same mutant differently (one killed, the other survived), the parity run fails loudly so the discrepancy is caught before the performance backend can become a default.
For the backends themselves, see Strict mode → Bytecode backends.
The fresh-loader backend is the correctness source of truth. It constructs a brand-new classloader for every mutant, which is slow but gives each mutant a clean execution environment. Other backends are strictly performance optimisations: every kill-vs-survive classification must match what the reference backend would have produced. The parity gate is the continuous defence of that invariant.
vary mutate-parity <source> [-t <tests>] [--candidate redefine|hot-swap]
vary mutate-parity <directory> --corpus [--candidate redefine]
Single-file mode:
vary mutate-parity src/utils.vary -t tests/test_utils.vary
Pinned-corpus mode (CI):
vary mutate-parity programs/frugal --corpus
| Flag | Default | Purpose |
|---|---|---|
-t, --tests | auto | Test file/directory (single-file mode) |
--candidate | redefine | Candidate backend: redefine or hot-swap |
--corpus | off | Walk a project directory as a pinned corpus |
--json | off | Echo the parity report to stdout |
Human-readable text (default) and mutation-reports/parity-report.json are emitted in both modes. With --json, the JSON is also printed to stdout.
Each mismatch carries:
| Field | Meaning |
|---|---|
mutantId | Stable mutation ID (same across backends) |
location | <class>#<method><descriptor>@<instructionIndex> |
referenceOutcome / candidateOutcome | killed or survived under each backend |
candidateSwapOutcome | redefine, hotswap, fallback:*, or fresh-loader |
referenceKilledBy / candidateKilledBy | Tests that failed under each backend |
A non-empty mismatches array exits the CLI non-zero.
fresh-loader remains the default --backend on vary mutate. The redefine backend will not become the default until the parity gate is stable on the pinned corpus. Until that happens, the parity gate is the only CI signal that the redefine backend is trustworthy.
The nightly nightly-reference-parity job runs parity on a pinned corpus:
vary mutate-parity programs/frugal --corpus --json
A non-zero exit causes the nightly run to fail. The mutation-reports/parity-report.json artifact is uploaded for triage.
| Case | Why | Treatment |
|---|---|---|
| Mutant errors in either backend | Engine failure, not a classification claim | Excluded from the comparison |
| Mutant was generated by only one backend | Rare, since both use the same generator | Skipped |
If the candidate backend has no Instrumentation handle available (no -javaagent: and self-attach failed), every mutant routes through fresh-loader fallback. The parity report still runs to completion; classifications then agree by construction.