Debugging a Silent Spring @ExceptionHandler Failure

@DisplayName("유저 삭제 테스트 실패 - 잘못된 요정")  
@Test  
void deleteUser_userNotFound() throws Exception {  
  // ================== given ==================  
  UUID userId = UUID.randomUUID();  
  doThrow(new UserNotFoundException(userId))  
      .when(userService)  
      .delete(userId);  
  
  // ================== when & then ==================  
  mockMvc.perform(delete("/api/users/" + userId))  
      .andExpect(status().isNotFound());  
}

• The core problem
  • A MockMvc test, designed to verify a 404 Not Found status for a delete operation on a non-existent user, was unexpectedly failing with a 500 Internal Server Error.
  • The issue wasn’t in the test logic or the controller. The root cause was a silent crash inside the @ExceptionHandler method itself, which prevented it from creating the correct 404 error response. This internal crash was triggered by a chain of seemingly unrelated issues.

Root cause

The fundamental issue was in our base custom exception, DiscodeitException. Its constructor received an error message but never passed it up to the parent RuntimeException class. The call to super(message) was missing.

Incorrect code:

// DiscodeitException.java
@Getter
public abstract class DiscodeitException extends RuntimeException {
    // ... other fields
    
    public DiscodeitException(String message, /*...other params...*/) {
        // PROBLEM: The message is never sent to the parent RuntimeException.
        // super(message); <-- THIS LINE WAS MISSING.
        // ...
    }
}

• All exceptions like UserException (which has child exceptions like UserNotFoundException) extends DiscodeitException
• In DiscodeitException the super was missing
  • Because of this, any attempt to call e.getMessage() on an instance of DiscodeitException (or its subclasses) would return null.

Handler Code

// GlobalExceptionHandler.java
@Slf4j  
@RestControllerAdvice  
public class GlobalExceptionHandler {  
  
  @ExceptionHandler(DiscodeitException.class)  
  public ResponseEntity<ErrorResponse> handleException(DiscodeitException e) {  
    log.warn("Custom exception handled [{}]: {}", e.getErrorCode(), e.getMessage()); // <<<<<<< HERE
    ErrorResponse errorResponse = new ErrorResponse(e);  
    return new ResponseEntity<>(errorResponse, e.getErrorCode().getStatus());  
  }
  ...
}

• So, when e.getMessage() returned null, the logging framework threw a NullPointerException. This new, unexpected exception crashed the handleException method mid-execution.

(UserNotFoundException and UserException that was being used, which is correct)

public class UserNotFoundException extends UserException {  
  
  public UserNotFoundException(UUID userId) {  
    super(  
        Instant.now(),  
        ErrorCode.USER_NOT_FOUND,  
        Map.of("userId", userId));  
  }  
}

public class UserException extends DiscodeitException {  
  
  public UserException(Instant timestamp, ErrorCode errorCode, Map<String, Object> details) {  
    super(errorCode.getMessage(), timestamp, errorCode, details);  
  }  
}

Corrected code

@Getter  
public abstract class DiscodeitException extends RuntimeException {  
  
  private final Instant timestamp;  
  private final ErrorCode errorCode;  
  private final Map<String, Object> details;  
  
  // The 'message' parameter is passed to the parent 'RuntimeException'  
  public DiscodeitException(String message, Instant timestamp, ErrorCode errorCode,Map<String, Object> details) {  
    super(message); // this is the fix  
    this.timestamp = timestamp;  
    this.errorCode = errorCode;  
    this.details = details != null ? Map.copyOf(details) : Map.of();  
  }  
}

• You need to add super(message)

GlobalHandler

@Slf4j  
@RestControllerAdvice  
public class GlobalExceptionHandler {  
  
  @ExceptionHandler(DiscodeitException.class)  
  public ResponseEntity<ErrorResponse> handleException(DiscodeitException e) {  
    log.warn("Custom exception handled [{}]: {}", e.getErrorCode(), e.getMessage());
    ErrorResponse errorResponse = new ErrorResponse(e);  
    return new ResponseEntity<>(errorResponse, e.getErrorCode().getStatus());  
  }
  ...
}

• Now when e.getMessage is called, we don’t get the error!

@Getter  
@AllArgsConstructor // <-- This annotation creates the constructor you need  
public class ErrorResponse {  
  
  private final Instant timestamp = Instant.now();  
  private final String code;  
  private final String message;  
  private final Map<String, Object> details;  
  private final String exceptionType; // 발생한 예외의 클래스 이름  
  private final int status; // HTTP 상태코드  
  
  // custom ErrorResponse  
  public ErrorResponse(DiscodeitException e) {  
    this.code = e.getErrorCode().getCode();  
    this.message = e.getErrorCode().getMessage();  
    this.details = e.getDetails();  
    this.exceptionType = e.getClass().getSimpleName();  
    this.status = e.getErrorCode().getStatus().value();  
  }  
  
}