Merge pull request #45212 from nosan

* gh-45212:
  Polish "Fix parameter replacement when message matches its code"
  Fix parameter replacement when message matches its code

Closes gh-45212

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/validation/MessageSourceMessageInterpolator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -34,6 +34,8 @@
  */
 class MessageSourceMessageInterpolator implements MessageInterpolator {
 
+	private static final String DEFAULT_MESSAGE = MessageSourceMessageInterpolator.class.getName();
+
 	private static final char PREFIX = '{';
 
 	private static final char SUFFIX = '}';
@@ -115,13 +117,11 @@ else if (buf.charAt(i) == SUFFIX) {
 
 	private String replaceParameter(String parameter, Locale locale, Set<String> visitedParameters) {
 		parameter = replaceParameters(parameter, locale, visitedParameters);
-		String value = this.messageSource.getMessage(parameter, null, null, locale);
-		return (value != null && !isUsingCodeAsDefaultMessage(value, parameter))
-				? replaceParameters(value, locale, visitedParameters) : null;
-	}
-
-	private boolean isUsingCodeAsDefaultMessage(String value, String parameter) {
-		return value.equals(parameter);
+		String value = this.messageSource.getMessage(parameter, null, DEFAULT_MESSAGE, locale);
+		if (value == null || value.equals(DEFAULT_MESSAGE)) {
+			return null;
+		}
+		return replaceParameters(value, locale, visitedParameters);
 	}
 
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/validation/MessageSourceMessageInterpolatorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -66,6 +66,19 @@ void interpolateWhenParametersAreUnknownUsingCodeAsDefaultShouldLeaveThemUnchang
 			.isEqualTo("{foo}{child}+{child}{bar}");
 	}
 
+	@Test
+	void interpolateShouldReplaceParameterThatReferencesAMessageThatMatchesItsCode() {
+		this.messageSource.addMessage("foo", Locale.getDefault(), "foo");
+		assertThat(this.interpolator.interpolate("{foo}", this.context)).isEqualTo("foo");
+	}
+
+	@Test
+	void interpolateUsingCodeAsDefaultShouldReplaceParameterThatReferencesAMessageThatMatchesItsCode() {
+		this.messageSource.setUseCodeAsDefaultMessage(true);
+		this.messageSource.addMessage("foo", Locale.getDefault(), "foo");
+		assertThat(this.interpolator.interpolate("{foo}", this.context)).isEqualTo("foo");
+	}
+
 	@Test
 	void interpolateWhenParametersAreNestedShouldFullyReplaceAllParameters() {
 		this.messageSource.addMessage("top", Locale.getDefault(), "{child}+{child}");