Improve mapping any Exception to ErrorResponse

Add protected, convenience method in ResponseEntityExceptionHandler
to create a ProblemDetail for any exception, along with a
MessageSource lookup for the "detail" field.

Closes gh-29384

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityExceptionHandler.java

@@ -292,6 +292,35 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 		return handleExceptionInternal(ex, null, headers, status, exchange);
 	}
 
+	/**
+	 * Convenience method to create a {@link ProblemDetail} for any exception
+	 * that doesn't implement {@link ErrorResponse}, also performing a
+	 * {@link MessageSource} lookup for the "detail" field.
+	 * @param ex the exception being handled
+	 * @param status the status to associate with the exception
+	 * @param defaultDetail default value for the "detail" field
+	 * @param detailMessageCode the code to use to look up the "detail" field
+	 * through a {@code MessageSource}, falling back on
+	 * {@link ErrorResponse#getDefaultDetailMessageCode(Class, String)}
+	 * @param detailMessageArguments the arguments to go with the detailMessageCode
+	 * @return the created {@code ProblemDetail} instance
+	 */
+	protected ProblemDetail createProblemDetail(
+			Exception ex, HttpStatusCode status, @Nullable HttpHeaders headers,
+			String defaultDetail, @Nullable String detailMessageCode, @Nullable Object[] detailMessageArguments,
+			ServerWebExchange exchange) {
+
+		ErrorResponse response = ErrorResponse.createFor(
+				ex, status, headers, defaultDetail, detailMessageCode, detailMessageArguments);
+
+		return response.updateAndGetBody(this.messageSource, getLocale(exchange));
+	}
+
+	private static Locale getLocale(ServerWebExchange exchange) {
+		Locale locale = exchange.getLocaleContext().getLocale();
+		return (locale != null ? locale : Locale.getDefault());
+	}
+
 	/**
 	 * Internal handler method that all others in this class delegate to, for
 	 * common handling, and for the creation of a {@link ResponseEntity}.
@@ -311,7 +340,7 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 	 * @return a {@code Mono} with the {@code ResponseEntity} for the response
 	 */
 	protected Mono<ResponseEntity<Object>> handleExceptionInternal(
-			Exception ex, @Nullable Object body, HttpHeaders headers, HttpStatusCode status,
+			Exception ex, @Nullable Object body, @Nullable HttpHeaders headers, HttpStatusCode status,
 			ServerWebExchange exchange) {
 
 		if (exchange.getResponse().isCommitted()) {
@@ -319,25 +348,12 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 		}
 
 		if (body == null && ex instanceof ErrorResponse errorResponse) {
-			body = resolveDetailViaMessageSource(errorResponse, exchange.getLocaleContext().getLocale());
+			body = errorResponse.updateAndGetBody(this.messageSource, getLocale(exchange));
 		}
 
 		return createResponseEntity(body, headers, status, exchange);
 	}
 
-	private ProblemDetail resolveDetailViaMessageSource(ErrorResponse response, @Nullable Locale locale) {
-		ProblemDetail body = response.getBody();
-		if (this.messageSource != null) {
-			locale = (locale != null ? locale : Locale.getDefault());
-			Object[] arguments = response.getDetailMessageArguments(this.messageSource, locale);
-			String detail = this.messageSource.getMessage(response.getDetailMessageCode(), arguments, null, locale);
-			if (detail != null) {
-				body.setDetail(detail);
-			}
-		}
-		return body;
-	}
-
 	/**
 	 * Create the {@link ResponseEntity} to use from the given body, headers,
 	 * and statusCode. Subclasses can override this method to inspect and possibly
@@ -351,7 +367,8 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 	 * @since 6.0
 	 */
 	protected Mono<ResponseEntity<Object>> createResponseEntity(
-			@Nullable Object body, HttpHeaders headers, HttpStatusCode status, ServerWebExchange exchange) {
+			@Nullable Object body, @Nullable HttpHeaders headers, HttpStatusCode status,
+			ServerWebExchange exchange) {
 
 		return Mono.just(new ResponseEntity<>(body, headers, status));
 	}

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityExceptionHandlerTests.java

@@ -59,7 +59,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  */
 public class ResponseEntityExceptionHandlerTests {
 
-	private final ResponseEntityExceptionHandler exceptionHandler = new GlobalExceptionHandler();
+	private final GlobalExceptionHandler exceptionHandler = new GlobalExceptionHandler();
 
 	private final MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/").build());
 
@@ -149,6 +149,29 @@ public class ResponseEntityExceptionHandlerTests {
 				"Content-Type application/json not supported. Supported: [application/atom+xml, application/xml]");
 	}
 
+	@Test
+	void customExceptionToProblemDetailViaMessageSource() {
+
+		Locale locale = Locale.UK;
+		LocaleContextHolder.setLocale(locale);
+
+		StaticMessageSource messageSource = new StaticMessageSource();
+		messageSource.addMessage(
+				"problemDetail." + IllegalStateException.class.getName(), locale,
+				"Invalid state: {0}");
+
+		this.exceptionHandler.setMessageSource(messageSource);
+
+		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/")
+				.acceptLanguageAsLocales(locale).build());
+
+		ResponseEntity<?> responseEntity =
+				this.exceptionHandler.handleException(new IllegalStateException(), exchange).block();
+
+		ProblemDetail body = (ProblemDetail) responseEntity.getBody();
+		assertThat(body.getDetail()).isEqualTo("Invalid state: A");
+	}
+
 
 	@SuppressWarnings("unchecked")
 	private ResponseEntity<ProblemDetail> testException(ErrorResponseException exception) {
@@ -247,6 +270,12 @@ public class ResponseEntityExceptionHandlerTests {
 
 			return handleAndSetTypeToExceptionName(ex, headers, status, exchange);
 		}
+
+		public Mono<ResponseEntity<Object>> handleException(IllegalStateException ex, ServerWebExchange exchange) {
+			HttpStatus status = HttpStatus.INTERNAL_SERVER_ERROR;
+			ProblemDetail body = createProblemDetail(ex, status, null, ex.getMessage(), null, new Object[] {"A"}, exchange);
+			return handleExceptionInternal(ex, body, null, status, exchange);
+		}
 	}
 
 }