Testing gRPC Services in Java

The key difference from REST testing is that MockMvc doesn't work for gRPC — it speaks HTTP/1.1 with JSON, and gRPC needs HTTP/2 with protobuf. Test at three levels with the right tools for each.

Unit tests — fastest and simplest:

• Instantiate the service directly as a plain Java object, pass a mocked StreamObserver, and assert that onNext was called with the expected response and onCompleted was called; no Spring, no server, no network
• Use ArgumentCaptor to inspect the response object's fields; these run in under 10 milliseconds and give fast feedback on business logic

Integration tests — full gRPC pipeline without TCP:

• Use InProcessServerBuilder from the grpc-testing library to start a real gRPC server inside the JVM with no TCP socket; this exercises full protobuf serialization and the interceptor chain while staying fast and deterministic
• GrpcCleanupRule handles channel and server shutdown so you don't leak threads or channels across test runs

Spring Boot tests — full context, no port binding:

• Use @SpringBootTest with grpc.server.port=-1 and in-process-name to route everything through in-process transport; the full Spring context boots — @GrpcService discovery, interceptors, health service — but without binding a real port, eliminating CI port conflicts

Error path tests — non-negotiable:

• Assert that the stub throws StatusRuntimeException with the correct status code for every error scenario — NOT_FOUND for missing entities, INVALID_ARGUMENT for bad input, UNAUTHENTICATED for missing tokens
• Untested error paths are the first thing that breaks when real clients hit edge cases; the test assertThrows(StatusRuntimeException.class, () -> stub.callMethod(...)).getStatus().getCode() pattern is straightforward

I recommend unit tests for every service method, integration tests for every interceptor behavior, and at least one error path test per distinct Status.Code the service can return.