Why Centralized Error Handling

Without a plan, every controller method has its own try/catch and its own error JSON. One endpoint returns {"error": "..."}, another {"message": "..."}, and an unexpected exception returns a 500 with a stack-trace-like body. Frontend and mobile developers hate this.

The Spring way: throw meaningful exceptions from the service layer, and handle them in one place.

ProductService.getById(42)
    throws ProductNotFoundException("Product 42 not found")
        |
        v
DispatcherServlet catches it
        |
        v
@RestControllerAdvice  ->  @ExceptionHandler(ProductNotFoundException.class)
        |
        v
404  application/problem+json  {"title":"Product not found","status":404,...}