Spring tests: separate business rules, wiring and transport

Last updated: 1 Oct 20264 min read

tutorial

A test boundary selects which components are real and which are supplied fixtures, determining the failures that test can detect.

Download Spring source kit

This lesson uses the downloadable source kit: Java 21, Spring Boot 4.0.8 and its managed Spring Framework 7 dependencies. The version is pinned for repeatable builds.

Three tests answer three questions

The direct report test asks whether a supplied exporter produces the expected report. The context test asks whether the container can create and reuse the report bean. The MVC test asks whether a JSON request is converted, validated and translated into the promised response.

Starting the whole Boot application for every test adds unrelated failure modes to a small assertion. Testing everything through direct method calls misses wiring, request filters and serialization. Use both where each supplies distinct evidence.

Name the excluded behavior

H2 rollback tests establish local transaction behavior for the fixture schema. They do not check a production database’s lock timing, migrations or multi-instance recovery. A deterministic cache test does not establish distributed invalidation.

The source kit makes those limits visible. A failing test should identify a contract the service actually promises; avoid tests that simply assert every private field has the value used to initialize it.

Checked source

assertEquals("csv:receipt", new ContainerContracts.ReceiptReport(new ContainerContracts.CsvExporter()).render());
// Container identity, MVC conversion and real HTTP filters have separate tests.
assertThrows(DataIntegrityViolationException.class, ledger::batchThatFails);
assertEquals(0, ledger.count());

Test the boundary

Run mvn test in the source-kit directory. CoreBoundaryTest and ReceiptBoundaryTest checks the behavior described here. Java excerpts belong to the named source-kit classes; they are not independent source files unless the complete class is shown.

Costs and boundaries

Test cost grows with the boundary’s assembly and external resources. Keep focused tests fast enough to run repeatedly, and retain integration tests for contracts that focused fixtures cannot establish.