Rewrite Java/Spring codebases to Rust via the LEAP Protocol

To rewrite Java/Spring to Rust, we follow a spec-first, test-gated path: extract specs → generate tests → implement in parallel → roll out with parity. Using the LEAP Protocol, we first extract executable specifications from your Spring Boot services: OpenAPI from Springdoc or annotated controllers, MockMvc contracts, JPA/Hibernate entity mappings, Flyway/Liquibase schemas, and message schemas from Kafka/RabbitMQ (Avro/JSON/Protobuf). Next, we generate a comprehensive parity test suite that runs against both the existing Java app and the new Rust services, covering HTTP status/headers/bodies, serialization rules (Jackson to Serde), security (Spring Security to Axum/Actix middleware), transactions, and asynchronous workflows. We then implement the Rust services in parallel—typically Axum or Actix Web for HTTP, SQLx or Diesel for data access, Tokio for async, rdkafka for Kafka, Tonic for gRPC—guided strictly by the spec and gated by the tests. Finally, we roll out with traffic shadowing and canaries, keeping both stacks live until tests, logs (OpenTelemetry/Micrometer parity), and SLOs confirm equivalence. The result: a predictable, auditable migration from Java/Spring to Rust without surprises.

Why Rust for Java/Spring migrations

Predictable latency and lower memory footprint: no JVM warmup or GC pauses; Rust’s static binaries avoid GC-induced tail latency spikes common under high allocation in Spring apps.
Faster cold starts in containers/serverless: Rust’s small, self-contained binaries start quickly, unlike JVMs that require classloading and JIT warmup.
Strong compile-time guarantees: Rust’s ownership and type system catch data races and nullability issues (Option) at compile time, reducing concurrency bugs common in thread-pooled Spring services.
Async I/O without thread explosion: Tokio-based services handle high concurrency efficiently versus large thread pools typical of Spring MVC; works well behind Nginx/Envoy.
Safer, explicit data access: SQLx can check SQL at compile time; moving from Hibernate/JPA to explicit queries or Diesel reduces N+1 and lazy-loading surprises.

Our methodology

1) Extract executable specs from Spring - Derive OpenAPI from Springdoc or annotations on @RestController methods, including error responses and validation constraints (Bean Validation). - Capture DB schemas and migration history from Flyway/Liquibase; catalog JPA entities, relationships, and transactional boundaries (@Transactional propagation/isolation). - Enumerate messaging contracts (Kafka/RabbitMQ topics, headers, schema registry/Avro) and scheduled jobs (@Scheduled). - Record cross-cutting concerns: Spring Security rules (method/URL/role), filters/interceptors, Actuator endpoints, Micrometer metrics, property binding and profiles.

2) Generate parity tests and fixtures - Build black-box tests that hit both Java and Rust targets, verifying status codes, headers, bodies, content negotiation, and exact JSON field naming/casing. - Create stateful integration tests with Testcontainers (PostgreSQL/MySQL), ephemeral Kafka, and WireMock for upstream dependencies (RestTemplate/WebClient/Feign). - Encode transactional semantics, idempotency keys, and retry/backoff expectations; snapshot date/time/locale formats and BigDecimal rounding to match Rust’s chrono/decimal behavior.

3) Implement Rust services in parallel - HTTP/gRPC: Axum or Actix Web with tower-http for middleware, Tonic for gRPC; Serde for JSON; validator for request schemas; utoipa for OpenAPI parity. - Data: SQLx or Diesel with explicit queries and joins to replace Hibernate; careful handling of lazy-loading equivalents and fetch graphs. - Messaging and jobs: rdkafka for Kafka consumers/producers; cron-like schedulers (e.g., cronbacked tasks) to mirror @Scheduled; outbox patterns where needed. - Security and observability: tower-layers for authn/z, JWT/OAuth2 parity with Spring Security; OpenTelemetry tracing/metrics mapped from Micrometer; structured logging with tracing.

4) Roll out with verified parity - Shadow traffic and canaries behind your ingress/load balancer; compare responses and latencies in production-like environments. - Blue/green or service-by-service cutovers; feature flags for risky endpoints; SLO guards and rollback hooks in CI/CD. - Keep the parity suite as a living regression harness; once green across SLAs, decommission the Spring service.

Specific Java/Spring concerns we handle

JPA/Hibernate to explicit SQL: Replacing lazy loading and cascades with explicit joins and transactions; eliminating N+1 patterns with compile-checked queries in SQLx/Diesel.
@Transactional semantics: Matching propagation (REQUIRED, REQUIRES_NEW), isolation levels, and rollback rules; reproducing read-only hints and consistent snapshot behavior.
Spring Security parity: Translating method and URL security (pre/post annotations, role hierarchies) to middleware; preserving JWT/OAuth2 flows and CSRF/cors rules where applicable.
Data binding and validation: Mapping Jackson’s naming, null handling, and custom serializers to Serde; converting Bean Validation groups/messages to Rust validator equivalents.
Error handling and HTTP semantics: Mapping exceptions/ControllerAdvice to typed error responses; precise status codes, headers, and problem+json payloads.
Date/time, locale, and numbers: Aligning java.time, timezone defaults, and BigDecimal rounding/scale with Rust’s chrono and decimal crates to prevent financial drift.

Proof

sqlite-leap — five SQLite engines generated from one spec: https://github.com/safitudo/sqlite-leap
semver-leap — 5,632 passing tests validating spec conformance: https://github.com/safitudo/semver-leap
LEAP Protocol (spec-first, test-gated, agent-agnostic): https://github.com/safitudo/leap

Pricing & timeline

Most Java/Spring to Rust rewrites complete in 4–12 weeks. After a 3–5 day discovery (free), we provide a fixed-price proposal, typically $40k–$120k depending on endpoint count, database/messaging complexity, security/SLAs, and rollout requirements. Work is milestone-based (spec extraction, parity test suite, Rust implementation, rollout), with exit ramps after each gate.

Ready to de-risk your Java/Spring to Rust migration? Email hello@leapagentic.com with a link to your API docs or a repo, and we’ll return a spec map and fixed bid.

Leap Agentic is distinct from Legacyleap.ai and Impetus Leap AI.