unit-test-exception-handler

Unit Testing ExceptionHandler and ControllerAdvice

Overview

This skill provides patterns for writing unit tests for Spring Boot exception handlers. It covers testing @ExceptionHandler methods in @ControllerAdvice classes using MockMvc, including HTTP status assertions, JSON response validation, field-level validation error testing, and mocking handler dependencies.

When to Use

• Writing unit tests for @ExceptionHandler methods
• Testing @ControllerAdvice global exception handling
• Validating REST API error response formatting
• Mocking exceptions in controller tests
• Testing field-level validation error responses
• Asserting custom error payloads and HTTP status codes

Instructions

1. Create a test controller that throws specific exceptions to trigger each @ExceptionHandler
2. Register ControllerAdvice via setControllerAdvice() on MockMvcBuilders.standaloneSetup()
3. Assert HTTP status codes with .andExpect(status().isXxx())
4. Verify error response fields using jsonPath("$.field") matchers
5. Test validation errors by sending invalid payloads and checking MethodArgumentNotValidException produces field-level details
6. Debug failures with .andDo(print()) — if handler not invoked, verify setControllerAdvice() is called and exception type matches

Examples

Exception Handler and Error DTO

@ControllerAdvice
public class GlobalExceptionHandler {

  @ExceptionHandler(ResourceNotFoundException.class)
  @ResponseStatus(HttpStatus.NOT_FOUND)
  public ErrorResponse handleNotFound(ResourceNotFoundException ex) {
    return new ErrorResponse(404, "Not Found", ex.getMessage());
  }

  @ExceptionHandler(ValidationException.class)
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  public ErrorResponse handleValidation(ValidationException ex) {
    return new ErrorResponse(400, "Bad Request", ex.getMessage());
  }

  @ExceptionHandler(MethodArgumentNotValidException.class)
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  public ValidationErrorResponse handleMethodArgumentNotValid(MethodArgumentNotValidException ex) {
    Map<String, String> errors = new HashMap<>();
    ex.getBindingResult().getFieldErrors().forEach(e -> errors.put(e.getField(), e.getDefaultMessage()));
    return new ValidationErrorResponse(400, "Validation Failed", errors);
  }
}

public record ErrorResponse(int status, String error, String message) {}
public record ValidationErrorResponse(int status, String error, Map<String, String> errors) {}