The default error response from a Spring Boot REST endpoint looks something like this:

{
    "error": "Not Found", 
    "exception": "com.mycompany.rest.MyException", 
    "message": "A record with value [foo] was not found", 
    "path": "/foo", 
    "status": 404, 
    "timestamp": 1507117452642
}

If we want to include custom information into the error response it's easy. Just add an ErrorAttributes bean to the spring context.

The following code snippet checks if the thrown exception is a custom exception MyException. This exception type contains a code attribute that holds a unique error code. We want to include that code as a specific key in the error response.

@SpringBootApplication
@EnableResourceServer
public class MyApplication {

    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }

    @Bean
    public ErrorAttributes errorAttributes() {
        return new DefaultErrorAttributes() {
            @Override
            public Map getErrorAttributes(RequestAttributes requestAttributes, boolean includeStackTrace) {
                final Map<String, Object> errorAttributes = super.getErrorAttributes(requestAttributes, includeStackTrace);
                // Customize the default entries in errorAttributes by adding the error code.
                Throwable exception = this.getError(requestAttributes);
                if (exception instanceof MyException) {
                    errorAttributes.put("code", ((MyException) exception).getCode());
                }
                return errorAttributes;
            }
        };
    }
}

If a controller method throws MyException the response now looks like this:

{
    "code": "12345",
    "error": "Not Found", 
    "exception": "com.mycompany.rest.MyException", 
    "message": "A record with value [foo] was not found", 
    "path": "/foo", 
    "status": 404, 
    "timestamp": 1507117452642
}