j2py-converter 0.8.0

Java-to-Python source translator with line-level structural correspondence for side-by-side review

j2py

j2py is a Java-to-Python source translator. It converts Java classes to Python that preserves line-level structural correspondence — same method order, same control flow, camelCase -> snake_case naming — so reviewers can audit output against the original Java side by side. The goal is reviewable equivalence, not a fully idiomatic rewrite. After deterministic conversion a file can be passed to an LLM for an additional conversion attempt.

How it works

Java source
  → parse (tree-sitter-java)
  → analyze (symbols, dependency graph)
  → translate (deterministic rule layer, then optional LLM completion)
  → validate (syntax, lint, types)
  → Python output

The rule layer handles common language constructs deterministically (~70% of typical code). Where rules stop, an optional configured LLM provider fills gaps using disk-cached prompts. Every file gets a confidence score based on rule-layer coverage, validation status, and semantic warnings, plus structured diagnostics for anything left unhandled. An optional LLM review pass can run after translation as a non-mutating second opinion: it records structured findings without changing generated Python, coverage, or confidence.

Status

Beta. The library is usable for experimentation, fixture-driven development, and batch translation of real Java projects. The deterministic rule layer reaches high node coverage on pinned dense corpus samples, but behavioral equivalence at library scale is still early, some cross-file/framework boundaries require project policy or manual fixups, and output on large enterprise codebases will contain review warnings and known correctness gaps.

Deterministic support today includes:

• tree-sitter Java parsing and symbol extraction
• class, nested class, local/anonymous class helpers, interface, enum, and record skeletons
• interface abstract methods, default methods, and static methods
• fields, constructors, methods, and overloads: chained constructor delegation, builder-style forwarding merged into default parameters, and type-dispatch overload groups via a vendored @overloaded runtime dispatcher (ADR 0009)
• common expressions: literals, identifiers, field access, arrays, class literals, assignments, updates, ternaries, null checks, collection calls, string concat, String.charAt, and typed get(...) lowering for lists, maps, indexed-predicate APIs, and common API receivers
• stream pipelines: map, filter, flatMap, distinct, sorted, collectors such as toList, toSet, joining, groupingBy/mapping, toMap, and block lambdas
• control flow: if/else, enhanced and classic for, while, do while, safe switch forms, try/catch/finally, throw, break, and continue
• configured import emission, naming policy, type maps, exception maps, and comment flags
• dependency-ordered directory translation
• structured diagnostics, confidence scoring, validation, post-LLM structural verification, and optional Anthropic, Gemini, or OpenAI-compatible completion
• side-by-side Java/Python review via j2py compare
• rule-only project assessment via j2py doctor with JSON/HTML reports and conservative config suggestions

Known gaps include:

• overload groups whose erased Python signatures collide (e.g. int vs long) and other ambiguous overload groups that still fall back to manual-dispatch TODOs
• complex enum static initialization beyond translated enum constant class bodies
• annotation semantics beyond syntactic metadata shells
• runtime/framework behavior (dependency injection, persistence mappings, container lifecycle) — j2py translates source structure, not application frameworks

For a concise statement of where j2py helps and where enterprise framework semantics remain manual, see docs/POSITIONING.md.

Which surface should I use?

j2py has several user-facing surfaces, but they are one pipeline rather than separate products:

doctor -> config -> translate -> sidecars -> wire -> validate/review

For simple Java, start with the core translator and review output:

j2py translate Foo.java
j2py compare Foo.java Foo.py

For enterprise or framework-heavy migrations, use the advanced path:

j2py doctor project/
# create and review config
j2py translate project/ --config j2py_config.py --output translated_py
j2py-wire list translated_py
j2py-wire generate translated_py --target fastapi
j2py-wire validate translated_py

The layers are: core translator, configuration, framework plugins, wiring, and assessment. See Positioning and enterprise scope, Getting Started, Assessment, and Wiring for the full guide.

For Spring migrators, start with the Spring conversion guide. It covers the opt-in Spring config, SpringWiringPlugin sidecars, j2py-wire generate, j2py-wire validate, the PetClinic smoke gate, and the corpus checks that show whether Spring translation improved or regressed. The Spring -> FastAPI/SQLAlchemy mapping cookbook documents detailed annotation_map recipes (controllers, DI, JPA entities, @Transactional), Spring JDBC/RowMapper SQLAlchemy scaffolding, and explicit manual-port callouts. For framework metadata extraction or source transforms beyond one-to-one mappings, see the framework plugin guide. Install j2py-converter[spring] only when you need the optional Spring/FastAPI/SQLAlchemy runtime packages; installing that extra does not enable Spring behavior without explicit configuration.

Quick start

Install the beta pre-release from PyPI:

pip install --pre j2py-converter
j2py --help

The PyPI distribution is j2py-converter; the import package and CLI command are j2py. (The bare j2py name on PyPI is owned by an unrelated project.)

For a full user walkthrough, start with docs/GETTING_STARTED.md. For install variants and troubleshooting, see docs/INSTALL.md. For command details, see docs/CLI.md. For the full docs map by audience, use docs/README.md, which separates User Docs, Java enterprise framework guides, Developer Docs, and Repo Hygiene records. For the 0.8.0 release story and current known limits, see docs/releases/0.8.0/RELEASE_NOTES.md.

First installed-package smoke:

mkdir -p /tmp/j2py-smoke/src/main/java/demo
cat > /tmp/j2py-smoke/src/main/java/demo/HelloWorld.java <<'JAVA'
package demo;

public class HelloWorld {
    private final String name;

    public HelloWorld(String name) {
        this.name = name;
    }

    public String greeting() {
        return "Hello, " + name;
    }
}
JAVA

j2py translate /tmp/j2py-smoke/src/main/java \
  --output /tmp/j2py-smoke/translated_py \
  --no-llm \
  --no-validate

python -m py_compile /tmp/j2py-smoke/translated_py/demo/HelloWorld.py

Local development:

uv sync --locked
make check

Translate a file without LLM completion:

uv run j2py translate tests/fixtures/java/HelloWorld.java --no-llm --no-validate --dry-run

Assess a Java source tree before migration:

uv run j2py doctor path/to/java/root --json j2py-assessment.json --html j2py-assessment.html
uv run j2py doctor path/to/java/root --config-suggestions j2py.suggested.yaml
uv run j2py doctor diff before.json after.json
uv run j2py sarif j2py-assessment.json --output j2py.sarif

See docs/DOCTOR.md for the assessment layer guide, report format, roadmap, and limitations, and docs/SARIF.md for code-scanning export.

Translate a directory in dependency order:

uv run j2py translate path/to/java/root --output translated_py --no-llm

Skip unchanged files on repeated directory runs:

uv run j2py translate path/to/java/root --output translated_py --incremental

Generate review reports:

uv run j2py translate path/to/java/root --output translated_py --dashboard dashboard.html
uv run j2py translate SomeClass.java --report review.html
uv run j2py translate SomeClass.java --no-llm --llm-review --review-report review.json

Watch a source tree and incrementally re-translate changed Java files:

uv run j2py watch path/to/java/root --output translated_py --no-llm

Side-by-side review in VS Code:

uv run j2py compare tests/fixtures/java/HelloWorld.java --no-llm

Print compare paths without opening an editor:

uv run j2py compare tests/fixtures/java/HelloWorld.java --no-open --no-llm

See docs/OUTPUT_REVIEW.md for how to interpret confidence, warnings, validation, TODO markers, and generated review artifacts.

VS Code support is experimental beyond j2py compare. The repository includes a small extension package under packages/j2py-vscode for editor-triggered translation, side-by-side review, TODO diagnostics, and status-bar confidence. See docs/VS_CODE.md before relying on it in a migration workflow.

LLM completion with the default Anthropic provider (requires ANTHROPIC_API_KEY):

ANTHROPIC_API_KEY=... uv run j2py translate SomeClass.java

LLM completion with Gemini Flash requires the optional Gemini extra plus GEMINI_API_KEY:

pip install --pre "j2py-converter[gemini]"
GEMINI_API_KEY=... uv run j2py translate SomeClass.java \
  --llm-provider gemini --model gemini-3.5-flash

Selecting --llm-provider gemini without the extra installed fails with an install hint instead of a raw Python import traceback. Contributor installs that use the dev extra also include the Gemini SDK so live Gemini probes and harvest commands remain available.

LLM completion with OpenAI-compatible endpoints requires the optional OpenAI extra, OPENAI_API_KEY, and an explicit endpoint model ID. Set OPENAI_BASE_URL, configure llm_base_url, or pass --llm-base-url for non-default endpoints:

pip install --pre "j2py-converter[openai]"
OPENAI_API_KEY=... uv run j2py translate SomeClass.java \
  --llm-provider openai \
  --llm-base-url https://openai-compatible.example/v1 \
  --model provider-model-id

Selecting --llm-provider openai without the extra installed fails with an install hint. openai-compatible is accepted as a config/CLI alias for openai.

Configuration can live in j2py.yaml, j2py.toml, [tool.j2py] in pyproject.toml, or j2py_config.py. Projects may set default llm_provider, llm_base_url, and model values there, while CLI flags override them for one command. See docs/CONFIGURATION.md for the schema.

Programmatic callers can use the Python API described in docs/API.md; supported imports and result models are listed in docs/API_REFERENCE.md.

Quality gates

make check         # ruff + mypy strict + pytest (excludes behavior, live_llm, target_translation)
make test-cov      # pytest with enforced line and branch coverage floors
make test-behavior # Java/Python stdout/stderr/exit-code equivalence (requires JDK)
make equivalence-report # verified fixture surface + library-wide denominator report
make test-targets  # future strict-xfail roadmap targets
make release-check # pre-release gate: release-test + dist-check (3.11+ in CI publish workflow)

Benchmark corpus

Translation quality is measured against a multi-library corpus: pinned checkouts of Spring Framework, Guava, Apache Commons Lang, Jackson, and Caffeine, plus small curated construct fixtures under tests/fixtures/corpus/. These libraries are open-source stress tests for the deterministic rule layer — not product scope or target runtime. Corpus-derived fast fixtures that should not affect committed baselines live under tests/fixtures/java/targets/ instead.

Corpus scores are breadth and regression signals, not enterprise-readiness claims. In particular, spring-dense measures Java constructs in Spring Framework sources; it does not mean Spring Boot, Hibernate, or Jakarta application semantics are ported. See docs/POSITIONING.md and docs/CORPUS_SCOREBOARD.md for how to read these metrics.

make corpus-list-presets              # show all pinned presets
make corpus-clone-all                 # one-time: clone all checkouts into .corpus/
make corpus-guava-dense-check         # Guava collect/base vs baseline
make corpus-commons-lang-dense-check  # Commons Lang utilities vs baseline
make corpus-jackson-dense-check       # Jackson databind vs baseline
make corpus-caffeine-dense-check      # Caffeine cache code vs baseline
make corpus-spring-dense-check        # Spring dense preset + construct fixtures
make corpus-spring-app-dense-check    # Spring app-layer samples (REST, JPA, @Transactional)
make corpus-petclinic-dense-check     # Spring PetClinic reference application
make test-spring-smoke                # optional translate -> sidecar -> wire -> FastAPI smoke
make corpus-hotspots                  # rank gaps across all committed baselines

Presets and baselines live in scripts/corpus/corpus_presets.py and tests/fixtures/corpus/. In git worktrees, set J2PY_CORPUS_ROOT to your main checkout so scripts reuse $J2PY_CORPUS_ROOT/.corpus/. Regenerate a baseline with make corpus-<name>-update-baseline only after comparison shows no regressions.

See docs/CORPUS_SCOREBOARD.md, docs/TRANSLATION_TARGETS.md, and the full documentation index, which is split into User, Developer, and Repo Hygiene docs plus source-framework-specific guides.

On-demand live LLM evaluation and harvest (excluded from make check):

make test-llm-e2e              # Anthropic live probes; requires ANTHROPIC_API_KEY
make test-llm-gemini-e2e       # Gemini live probe; requires GEMINI_API_KEY
make harvest-promote-dry        # triage + draft pattern-family issues; no LLM
make harvest-promote            # queue → Gemini batch → triage → draft issues
make harvest-promote-issues     # same + gh issue create
make harvest-queue REFRESH=1    # rebuild Tier-A queue from corpus-reports/
make harvest-pipeline           # local probe harvest → triage → FUTURE_TARGETS drafts
make harvest-gemini             # batch Gemini harvest from .j2py/harvest/queue.txt
make harvest-triage             # summarize local .j2py/harvest/records.jsonl
# promote vars: LIMIT=2 ISSUES=3; harvest-gemini: OFFSET=0 LIMIT=10 SLEEP=6 FILE_LIST=...

Worktrees: set J2PY_CORPUS_ROOT to the main checkout so .env, queue, cache, and .j2py/harvest/ resolve correctly. See docs/LLM_HARVEST.md for queue tiers, content cache, state files, and the harvest-promote agent skill.

Adding translation rules

For the detailed contributor workflow, start with docs/developer/RULE_AUTHORING.md, docs/developer/TRANSLATION_INTERNALS.md, and docs/developer/VALIDATION_GATES.md. The short version is:

1. Add or update a Java/Python fixture pair under tests/fixtures/.
2. Implement the smallest deterministic rule in j2py/translate/.
3. Graduate the behavior into normal tests once it passes.
4. Run make check and relevant corpus checks, such as make corpus-guava-dense-check for generics/collections or make corpus-spring-dense-check when construct-mix behavior may shift.
5. Update a corpus baseline only when comparison shows no regressions.

Material translation policy changes should get an ADR under docs/decisions/.

Beta release notes

j2py-converter is published as a beta package. Expect incomplete construct coverage, diagnostics for unsupported regions, known multi-file import limitations, and manual review on production-scale codebases. See docs/releases/0.8.0/RELEASE_NOTES.md for the current release notes, docs/RELEASING.md for the release checklist, and CHANGELOG.md for version history.

License

MIT. See LICENSE.