Prevent exception when loading optional config data locations

Closes gh-35683

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

@@ -226,6 +226,9 @@ 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

@@ -43,18 +43,19 @@ import static org.mockito.Mockito.mock;
  *
  * @author Madhura Bhave
  * @author Phillip Webb
+ * @author Moritz Halbritter
  */
 class StandardConfigDataLocationResolverTests {
 
 	private StandardConfigDataLocationResolver resolver;
 
-	private ConfigDataLocationResolverContext context = mock(ConfigDataLocationResolverContext.class);
+	private final ConfigDataLocationResolverContext context = mock(ConfigDataLocationResolverContext.class);
 
 	private MockEnvironment environment;
 
 	private Binder environmentBinder;
 
-	private ResourceLoader resourceLoader = new DefaultResourceLoader();
+	private final ResourceLoader resourceLoader = new DefaultResourceLoader();
 
 	@BeforeEach
 	void setup() {
@@ -260,6 +261,26 @@ 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) + "]";
 	}