Use instance field for ProblemDetail in ErrorResponse's

Closes gh-32644

spring-web/src/main/java/org/springframework/web/ErrorResponse.java

@@ -61,6 +61,11 @@ public interface ErrorResponse {
 	 * Return the body for the response, formatted as an RFC 7807
 	 * {@link ProblemDetail} whose {@link ProblemDetail#getStatus() status}
 	 * should match the response status.
+	 * <p><strong>Note:</strong> The returned {@code ProblemDetail} may be
+	 * updated before the response is rendered, e.g. via
+	 * {@link #updateAndGetBody(MessageSource, Locale)}. Therefore, implementing
+	 * methods should use an instance field, and should not re-create the
+	 * {@code ProblemDetail} on every call, nor use a static variable.
 	 */
 	ProblemDetail getBody();
 

spring-web/src/main/java/org/springframework/web/context/request/async/AsyncRequestTimeoutException.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,6 +37,9 @@ import org.springframework.web.ErrorResponse;
 @SuppressWarnings("serial")
 public class AsyncRequestTimeoutException extends RuntimeException implements ErrorResponse {
 
+	private final ProblemDetail body = ProblemDetail.forStatus(getStatusCode());
+
+
 	@Override
 	public HttpStatusCode getStatusCode() {
 		return HttpStatus.SERVICE_UNAVAILABLE;
@@ -44,7 +47,7 @@ public class AsyncRequestTimeoutException extends RuntimeException implements Er
 
 	@Override
 	public ProblemDetail getBody() {
-		return ProblemDetail.forStatus(getStatusCode());
+		return this.body;
 	}
 
 }