Merge branch '3.1.x'

Closes gh-37571

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/config/StandardConfigDataLocationResolver.java

@@ -228,6 +228,9 @@ public class StandardConfigDataLocationResolver
 				return Collections.singleton(reference);
 			}
 		}
+		if (configDataLocation.isOptional()) {
+			return Collections.emptySet();
+		}
 		throw new IllegalStateException("File extension is not known to any PropertySourceLoader. "
 				+ "If the location is meant to reference a directory, it must end in '/' or File.separator");
 	}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/config/StandardConfigDataLocationResolverTests.java

@@ -43,6 +43,7 @@ import static org.mockito.Mockito.mock;
  *
  * @author Madhura Bhave
  * @author Phillip Webb
+ * @author Moritz Halbritter
  */
 class StandardConfigDataLocationResolverTests {
 
@@ -263,6 +264,26 @@ class StandardConfigDataLocationResolverTests {
 		assertThat(locations).isEmpty();
 	}
 
+	@Test
+	void resolveWhenOptionalAndLoaderIsUnknownShouldNotFail() {
+		ConfigDataLocation location = ConfigDataLocation.of("optional:some-unknown-loader:dummy.properties");
+		assertThatNoException().isThrownBy(() -> this.resolver.resolve(this.context, location));
+	}
+
+	@Test
+	void resolveWhenOptionalAndLoaderIsUnknownAndExtensionIsUnknownShouldNotFail() {
+		ConfigDataLocation location = ConfigDataLocation
+			.of("optional:some-unknown-loader:dummy.some-unknown-extension");
+		List<StandardConfigDataResource> locations = this.resolver.resolve(this.context, location);
+		assertThatNoException().isThrownBy(() -> this.resolver.resolve(this.context, location));
+	}
+
+	@Test
+	void resolveWhenOptionalAndExtensionIsUnknownShouldNotFail() {
+		ConfigDataLocation location = ConfigDataLocation.of("optional:file:dummy.some-unknown-extension");
+		assertThatNoException().isThrownBy(() -> this.resolver.resolve(this.context, location));
+	}
+
 	private String filePath(String... components) {
 		return "file [" + String.join(File.separator, components) + "]";
 	}