Support extensionless file imports

Update `ResourceConfigDataLocationResolver` so that it can import files
that have no file extension.

Closes gh-22280

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -698,6 +698,21 @@ If you want to support your own locations, see the `ConfigDataLocationResolver`
 
 
 
+==== Importing Extensionless Files
+Some cloud platforms cannot add a file extension to volume mounted files.
+To import these extensionless files, you need to give Spring Boot a hint so that it knows how to load them.
+You can do this by putting an extension hint in square brackets.
+
+For example, suppose you have a `/etc/config/myconfig` file that you wish to import as yaml.
+You can import it from your `application.properties` using the following:
+
+[source,properties,indent=0]
+----
+	spring.config.import=file:/etc/config/myconfig[.yaml]
+----
+
+
+
 [[boot-features-external-config-files-configtree]]
 ==== Using Configuration Trees
 When running applications on a cloud platform (such as Kubernetes) you often need to read config values that the platform supplies.

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

@@ -25,6 +25,7 @@ import java.util.Comparator;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
 import org.apache.commons.logging.Log;
@@ -63,6 +64,8 @@ class ResourceConfigDataLocationResolver implements ConfigDataLocationResolver<R
 
 	private static final Pattern URL_PREFIX = Pattern.compile("^([a-zA-Z][a-zA-Z0-9*]*?:)(.*$)");
 
+	private static final Pattern EXTENSION_HINT_PATTERN = Pattern.compile("^(.*)\\[(\\.\\w+)\\](?!\\[)$");
+
 	private static final String NO_PROFILE = null;
 
 	private final Log logger;
@@ -183,11 +186,17 @@ class ResourceConfigDataLocationResolver implements ConfigDataLocationResolver<R
 	}
 
 	private Set<Resolvable> getResolvablesForFile(String fileLocation, boolean optional, String profile) {
+		Matcher extensionHintMatcher = EXTENSION_HINT_PATTERN.matcher(fileLocation);
+		boolean extensionHintLocation = extensionHintMatcher.matches();
+		if (extensionHintLocation) {
+			fileLocation = extensionHintMatcher.group(1) + extensionHintMatcher.group(2);
+		}
 		for (PropertySourceLoader loader : this.propertySourceLoaders) {
 			String extension = getLoadableFileExtension(loader, fileLocation);
 			if (extension != null) {
 				String root = fileLocation.substring(0, fileLocation.length() - extension.length() - 1);
-				return Collections.singleton(new Resolvable(null, root, optional, profile, extension, loader));
+				return Collections.singleton(new Resolvable(null, root, optional, profile,
+						(!extensionHintLocation) ? extension : null, loader));
 			}
 		}
 		throw new IllegalStateException("File extension is not known to any PropertySourceLoader. "
@@ -343,7 +352,7 @@ class ResourceConfigDataLocationResolver implements ConfigDataLocationResolver<R
 				PropertySourceLoader loader) {
 			String profileSuffix = (StringUtils.hasText(profile)) ? "-" + profile : "";
 			this.directory = directory;
-			this.resourceLocation = rootLocation + profileSuffix + "." + extension;
+			this.resourceLocation = rootLocation + profileSuffix + ((extension != null) ? "." + extension : "");
 			this.optional = optional;
 			this.profile = profile;
 			this.loader = loader;

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

@@ -26,6 +26,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.boot.context.properties.bind.Binder;
 import org.springframework.boot.env.PropertiesPropertySourceLoader;
 import org.springframework.boot.logging.DeferredLog;
+import org.springframework.core.env.PropertySource;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.DefaultResourceLoader;
 import org.springframework.core.io.ResourceLoader;
@@ -206,6 +207,17 @@ public class ResourceConfigDataLocationResolverTests {
 				.satisfies((ex) -> assertThat(ex.getCause()).hasMessageStartingWith("File extension is not known"));
 	}
 
+	@Test
+	void resolveWhenLocationUsesOptionalExtensionSyntaxResolves() throws Exception {
+		String location = "classpath:/application-props-no-extension[.properties]";
+		List<ResourceConfigDataLocation> locations = this.resolver.resolve(this.context, location, true);
+		assertThat(locations.size()).isEqualTo(1);
+		ResourceConfigDataLocation resolved = locations.get(0);
+		assertThat(resolved.getResource().getFilename()).endsWith("application-props-no-extension");
+		PropertySource<?> propertySource = resolved.load().get(0);
+		assertThat(propertySource.getProperty("withnotext")).isEqualTo("test");
+	}
+
 	@Test
 	void resolveProfileSpecificReturnsProfileSpecificFiles() {
 		String location = "classpath:/configdata/properties/";

spring-boot-project/spring-boot/src/test/resources/application-props-no-extension

@@ -0,0 +1 @@
+withnotext=test