Unified Exception Handling in Spring Boot Using @ControllerAdvice, Assert, and Enum-based Error Codes

In many Java backend projects a large amount of code is filled with try { ... } catch { ... } finally { ... } blocks, which makes the source hard to read and maintain. The article first compares an ugly try‑catch style with a more elegant controller implementation.

Spring 3.2 introduced @ControllerAdvice and @ExceptionHandler , which allow developers to define global exception handling logic that applies to all controllers without duplicating code in each class.

The article presents a custom Assert utility that wraps null‑checks and throws a specific exception when a condition fails. Example implementation: public abstract class Assert { public static void notNull(@Nullable Object object, String message) { if (object == null) { throw new IllegalArgumentException(message); } } }

To avoid creating many exception classes, an enum is used to hold error codes and messages. The enum implements a BusinessExceptionAssert interface that creates a BusinessException with the appropriate code and formatted message. Example enum snippet: public enum ResponseEnum implements BusinessExceptionAssert { BAD_LICENCE_TYPE(7001, "Bad licence type."), LICENCE_NOT_FOUND(7002, "Licence not found."); private int code; private String message; }

The unified exception handler class annotated with @ControllerAdvice defines several @ExceptionHandler methods: handling BusinessException , generic BaseException , servlet‑related exceptions, binding and validation errors, and a catch‑all Exception . Each method logs the error and returns a consistent ErrorResponse containing code and message . Example handler method: @ExceptionHandler(value = BusinessException.class) @ResponseBody public ErrorResponse handleBusinessException(BaseException e) { log.error(e.getMessage(), e); return new ErrorResponse(e.getResponseEnum().getCode(), getMessage(e)); }

For 404 handling, the default behavior is changed by setting spring.mvc.throw-exception-if-no-handler-found=true and disabling static resource mappings, so that a NoHandlerFoundException is thrown and can be processed by the unified handler.

In service code the enum‑based assert is used to validate inputs, e.g. ResponseEnum.LICENCE_NOT_FOUND.assertNotNull(licence); , which throws the corresponding exception without manual if checks.

Overall, the approach eliminates most repetitive try‑catch code, provides a single place for error response formatting, supports internationalized messages, and makes the backend codebase cleaner and more maintainable.