Unified Exception Handling in Spring Boot Using @ControllerAdvice, Assert, and Enum‑Based Business Errors

During Java development, developers often write many try { ... } catch { ... } finally { ... } blocks, which makes the code verbose and hard to read. The article first shows an "ugly" try‑catch style and a cleaner controller style, then introduces Spring 3.2's @ControllerAdvice and @ExceptionHandler annotations for centralized exception handling.

Using @ControllerAdvice , a single class can define methods that handle all exceptions thrown from any controller, eliminating the need to duplicate error‑handling code in each controller or to create a base controller class.

To further simplify validation, the article recommends replacing explicit if (obj == null) { throw new IllegalArgumentException(...); } checks with Spring's org.springframework.util.Assert style assertions. A custom Assert interface is shown:

public abstract class Assert {
    public static void notNull(Object object, String message) {
        if (object == null) {
            throw new IllegalArgumentException(message);
        }
    }
}

For business‑specific errors, the article proposes an enum that implements a BusinessExceptionAssert interface, providing both an error code and a message. Example:

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 enum supplies newException methods that create a BusinessException carrying the enum's code and formatted message, allowing callers to write concise validation such as:

ResponseEnum.LICENCE_NOT_FOUND.assertNotNull(licence);
ResponseEnum.BAD_LICENCE_TYPE.assertNotNull(licenceTypeEnum);

A complete UnifiedExceptionHandler class is presented, annotated with @ControllerAdvice and containing methods like handleBusinessException , handleBaseException , handleServletException , handleBindException , handleValidException , and a generic handleException . Each method logs the error, determines an appropriate error code (using enums such as CommonResponseEnum or ServletResponseEnum ), and returns a standardized ErrorResponse object.

The article also explains how to make 404 errors throw exceptions by setting spring.mvc.throw-exception-if-no-handler-found=true and disabling static resource mappings, enabling the unified handler to capture missing‑endpoint errors.

Finally, a unified response model is described: a base class containing code and message , a CommonResponse that adds a data field, and shortcut classes R and QR for simple success responses. The article demonstrates testing the handler with various scenarios (missing resources, invalid parameters, database errors) and shows screenshots of the resulting JSON payloads.