unit-test-exception-handler_skill

Overview

This skill provides pragmatic patterns for unit testing Spring @ExceptionHandler methods and @ControllerAdvice classes using MockMvc. It focuses on verifying exception-to-error-response transformations, HTTP status codes, and error payload shape without running full integration tests. The goal is fast, maintainable tests that validate error formatting, validation details, and side effects like logging or localization.

How this skill works

Create lightweight test controllers that deliberately throw exceptions and register your ControllerAdvice with MockMvcBuilders.standaloneSetup(...).setControllerAdvice(...). Build targeted tests that perform MockMvc requests and assert status codes and JSON response fields. You can inject mocked services into the handler to verify localization, logging, or other contextual behavior while keeping tests small and focused.

When to use it

• Validating that @ExceptionHandler methods map exceptions to correct HTTP status codes
• Checking error response shape and required fields (timestamp, status, error, message)
• Testing validation error handling (MethodArgumentNotValidException) and field-level messages
• Verifying logging, localization, or other side effects triggered by exception handlers
• Writing fast unit tests that avoid full application context or integration test overhead

Best practices

• Use simple test controllers that throw specific exceptions to exercise each handler
• Register your ControllerAdvice explicitly with MockMvc standaloneSetup to isolate behavior
• Assert both HTTP status and JSON payload fields, including nested validation errors
• Mock dependent services (message sources, logging) to verify side effects without external systems
• Test each exception type, plus a generic fallback exception handler, to avoid gaps

Example use cases

• Unit test that ResourceNotFoundException returns 404 and expected error message
• Verify MethodArgumentNotValidException produces an errors map with per-field messages
• Assert DuplicateResourceException maps to 409 Conflict and contains conflict details
• Mock a MessageService to ensure business exceptions return localized messages
• Confirm LoggingService.logException is invoked when a BusinessException is handled