Rewriting Java/Spring codebases to TypeScript with spec-first parity

How do I rewrite Java/Spring to TypeScript? We do it spec-first. Leap Agentic applies the LEAP Protocol (spec-first, test-gated, agent-agnostic): we extract executable specs from your Spring Boot app (controllers, validation, security, persistence, and messaging), generate cross-language tests, implement the TypeScript target in parallel, and roll out with parity gates. Concretely: we lift your OpenAPI via springdoc-openapi, capture Bean Validation constraints, infer JPA/Hibernate models and query behavior, and encode authz/authn and side-effects into a language-neutral LEAP spec. From that, we auto-generate Jest/Supertest API tests, data-conversion tests (e.g., BigDecimal, ZonedDateTime), contract tests for Kafka/RabbitMQ (AsyncAPI/Pact), and invariants around transactions and idempotency. While your Java service runs, we implement a NestJS or Fastify service in TypeScript (TypeORM or Prisma, class-validator/Zod), mapping Spring MVC annotations to decorators, services to providers, and repositories to query layers. We deploy behind feature flags, shadow live traffic, and dual-write where needed until tests pass at parity. Then we switch traffic, retire Java components incrementally, and keep the suite as a living safety net.

Why TypeScript for Java/Spring migrations

Faster cold starts and smaller containers than typical JVM services, especially helpful for serverless and autoscaled microservices.
Native async I/O and event loop in Node.js improve throughput on I/O-bound endpoints; efficient backpressure with Fastify/undici.
NestJS decorators and DI feel familiar to Spring (e.g., @Controller, @Injectable, Guards/Interceptors ≈ Filters/Aspects) for smoother mapping.
Shared types across frontend and backend reduce serialization mismatches; OpenAPI/JSON Schema can drive single-source generated types.
Access to edge and serverless runtimes (Cloudflare Workers, Vercel Edge, Deno/Bun) where the JVM isn’t a fit.

Our methodology

1) Extract executable specs from Spring - Generate OpenAPI via springdoc-openapi; normalize query/body params, status codes, error shapes. - Scrape javax.validation/Jakarta Bean Validation constraints and method-level security (e.g., @PreAuthorize) into the spec. - Introspect JPA/Hibernate entities and common HQL/Criteria queries; record transactional boundaries (@Transactional) and isolation. - Capture messaging contracts (Kafka/RabbitMQ) with AsyncAPI; snapshot example payloads and headers.

2) Generate cross-language tests - Translate existing JUnit/MockMvc behavior into Jest + Supertest API assertions; add property-based tests for edge cases. - Synthesize Pact-style consumer/provider contracts; create schema checks for DTOs and enums. - Build data-conversion tests for BigDecimal, Instant/ZonedDateTime, Optional, and nullability rules. - Record anonymized fixtures from production to ensure realistic payloads; define parity budgets and acceptance thresholds.

3) Implement the TypeScript target in parallel - Build with NestJS (Fastify adapter) or Fastify directly; map @RestController to @Controller, Filters to ExceptionFilters, AOP to Interceptors. - Choose Prisma or TypeORM depending on migration/mapping needs; provide explicit SQL where lazy-loading once hid joins. - Recreate validation with class-validator/Zod; implement Guards for role/permission checks; Passport.js or nestjs/passport for OAuth2/JWT. - Replace schedulers (@Scheduled) with BullMQ/cron; map JMS/Kafka listeners to kafkajs/rabbitmq clients. - Instrument with OpenTelemetry; wire AsyncLocalStorage for correlation IDs and MDC-equivalent logging.

4) Progressive rollout with parity gates - Stand up the TypeScript service behind a flag; mirror traffic (shadow) with Envoy/NGINX; compare responses and side-effects. - Dual-write where safe; run migration scripts with retryable, idempotent patterns. - Block promotion until the cross-language test suite passes; cut over endpoint-by-endpoint.

5) Cutover and hardening - Finalize error mapping (ConstraintViolationException → 400 series), rate limits, and backoff strategies. - Document runbooks and SLOs; hand over the LEAP spec and test suite as ongoing guardrails.

Specific Java/Spring concerns we handle

Transaction boundaries and isolation: faithfully mapping @Transactional semantics, connection pooling, and propagation to Prisma/TypeORM or explicit SQL transactions.
Lazy vs eager loading in Hibernate: eliminating hidden N+1s with explicit joins/selects; reconciling dirty checking vs explicit updates.
Spring Security to NestJS: reproducing OAuth2/JWT flows with Passport.js, translating @PreAuthorize/@Secured to Guards and role-based policies.
Checked exceptions and global error handling: converting exception hierarchies to HTTP responses with NestJS ExceptionFilters; consistent problem details.
ThreadLocal context (SecurityContextHolder, MDC): replacing with AsyncLocalStorage and OpenTelemetry context propagation to preserve tracing and user context.

Proof

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

Pricing & timeline

Timeline: 4–12 weeks depending on service size, database scope, and messaging complexity.
Commercials: fixed-price proposal after a short discovery; typical engagements range from $40k to $120k.
Deliverables: LEAP spec, full cross-language test suite, TypeScript service (NestJS/Fastify with Prisma/TypeORM), migration/runbooks, and observability wired to your stack.
Drivers: number of endpoints, data models, transactional complexity, and non-HTTP surfaces (Kafka/RabbitMQ/schedulers).

Ready to move your Spring Boot service to TypeScript safely and quickly? Email hello@leapagentic.com to request a free migration assessment and fixed-price plan.

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