Add Tomcat locale mapping for Japanese to preserve UTF-8 charset

See gh-30535

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactory.java

@@ -260,6 +260,7 @@ public class TomcatServletWebServerFactory extends AbstractServletWebServerFacto
 	private void resetDefaultLocaleMapping(TomcatEmbeddedContext context) {
 		context.addLocaleEncodingMappingParameter(Locale.ENGLISH.toString(), DEFAULT_CHARSET.displayName());
 		context.addLocaleEncodingMappingParameter(Locale.FRENCH.toString(), DEFAULT_CHARSET.displayName());
+		context.addLocaleEncodingMappingParameter(Locale.JAPANESE.toString(), DEFAULT_CHARSET.displayName());
 	}
 
 	private void addLocaleMappings(TomcatEmbeddedContext context) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactoryTests.java

@@ -411,6 +411,7 @@ class TomcatServletWebServerFactoryTests extends AbstractServletWebServerFactory
 		// override defaults, see org.apache.catalina.util.CharsetMapperDefault.properties
 		assertThat(getCharset(Locale.ENGLISH)).isEqualTo(StandardCharsets.UTF_8);
 		assertThat(getCharset(Locale.FRENCH)).isEqualTo(StandardCharsets.UTF_8);
+		assertThat(getCharset(Locale.JAPANESE)).isEqualTo(StandardCharsets.UTF_8);
 	}
 
 	@Test