MockGen: The Complete Guide to Generating Mocks for Go Testing

MockGen vs Handwritten Mocks: When to Automate Mock Generation

Short verdict

• Automate with MockGen when interfaces are numerous, large, or require precise interaction verification.
• Handwrite fakes when interfaces are small, behavior is simple, or you want explicit, idiomatic test code without code generation.

When to choose MockGen (automate)

• Large/complex interfaces: Saves substantial boilerplate for many methods.
• Detailed interaction assertions: Need ordered calls, call counts, argument matchers, or strict expectation checks.
• Consistent, repeatable mocks: Team-wide standardization and CI-friendly generation.
• Faster iteration on tests: Regenerate mocks after interface changes instead of editing hand-written code.
• Tooling exists in your workflow: You accept generated files or use go:generate and keep mocks as _test or separate package files.

When to handwrite mocks (do not automate)

• Tiny, single-method interfaces: Writing a small fake is quicker and clearer.
• Highly custom behavior: Tests need bespoke logic, stateful scenarios, or complex side effects that are simpler to express by hand.
• Avoiding code generation: Policy or preference to keep repository free of generated code or extra tooling.
• To prevent coupling/complexity: Handwritten fakes can be simpler to reason about and debug.
• To avoid package/cyclic dependency issues: Placing minimal fakes in tests can sidestep generation placement problems.

Practical guidelines (decision checklist)

1. If you have >3 interfaces or interfaces with >4 methods — prefer MockGen.
2. If tests require verifying call order, counts, or argument matchers — prefer MockGen.
3. If a fake needs complex, stateful, scenario-specific behavior — handwrite it.
4. If your CI/build process prohibits codegen or your team dislikes generated files in VCS — handwrite or keep generated mocks out of repo.
5. If cyclic dependencies appear when placing generated mocks, prefer test-only hand-written fakes or adjust package placement with care.

Tips for using both

• Keep simple fakes hand-written and use MockGen for the rest.
• Generate mocks into_test.go files or a mocks package to avoid production-package pollution.
• Add go:generate lines so team members can regenerate easily.
• Treat generated mocks as disposable: prefer regenerating over manual edits.

Sources: gomock/mockgen community guides and Go testing best-practice articles.