Allow optional directories without sub-directories

Update `StandardConfigDataLocationResolver` so that directory resources
are only required when the location is not optional.

Closes gh-26627
Co-authored-by: Phillip Webb <pwebb@vmware.com>

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

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -44,7 +44,18 @@ public class ConfigDataLocationNotFoundException extends ConfigDataNotFoundExcep
 	 * @param cause the exception cause
 	 */
 	public ConfigDataLocationNotFoundException(ConfigDataLocation location, Throwable cause) {
-		super(getMessage(location), cause);
+		this(location, getMessage(location), cause);
+	}
+
+	/**
+	 * Create a new {@link ConfigDataLocationNotFoundException} instance.
+	 * @param location the location that could not be found
+	 * @param message the exception message
+	 * @param cause the exception cause
+	 * @since 2.4.7
+	 */
+	public ConfigDataLocationNotFoundException(ConfigDataLocation location, String message, Throwable cause) {
+		super(message, cause);
 		Assert.notNull(location, "Location must not be null");
 		this.location = location;
 	}

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

@@ -42,6 +42,7 @@ import org.springframework.core.io.ResourceLoader;
 import org.springframework.core.io.support.SpringFactoriesLoader;
 import org.springframework.core.log.LogMessage;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 /**
@@ -264,10 +265,13 @@ public class StandardConfigDataLocationResolver
 	}
 
 	private Set<StandardConfigDataResource> resolvePatternEmptyDirectories(StandardConfigDataReference reference) {
-		Resource[] resources = this.resourceLoader.getResources(reference.getDirectory(), ResourceType.DIRECTORY);
-		Assert.state(resources.length > 0,
-				"No subdirectories found for mandatory directory location '" + reference.getDirectory() + "'.");
-		return Arrays.stream(resources).filter(Resource::exists)
+		Resource[] subdirectories = this.resourceLoader.getResources(reference.getDirectory(), ResourceType.DIRECTORY);
+		ConfigDataLocation location = reference.getConfigDataLocation();
+		if (location.isOptional() && ObjectUtils.isEmpty(subdirectories)) {
+			String message = String.format("Config data location '%s' contains no subdirectories", location);
+			throw new ConfigDataLocationNotFoundException(location, message, null);
+		}
+		return Arrays.stream(subdirectories).filter(Resource::exists)
 				.map((resource) -> new StandardConfigDataResource(reference, resource, true))
 				.collect(Collectors.toCollection(LinkedHashSet::new));
 	}

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

@@ -713,10 +713,9 @@ class ConfigDataEnvironmentPostProcessorIntegrationTests {
 
 	@Test
 	void runWhenMandatoryWildcardLocationHasNoSubdirectories() {
-		assertThatIllegalStateException().isThrownBy(
+		assertThatExceptionOfType(ConfigDataLocationNotFoundException.class).isThrownBy(
 				() -> this.application.run("--spring.config.location=file:src/test/resources/config/0-empty/*/"))
-				.withMessage(
-						"No subdirectories found for mandatory directory location 'file:src/test/resources/config/0-empty/*/'.");
+				.withMessage("Config data location 'file:src/test/resources/config/0-empty/*/' cannot be found");
 	}
 
 	@Test
@@ -725,6 +724,18 @@ class ConfigDataEnvironmentPostProcessorIntegrationTests {
 				.isThrownBy(() -> this.application.run("--spring.config.location=file:invalid/*/"));
 	}
 
+	@Test
+	void runWhenHasOptionalWildcardLocationThatDoesNotExistDoesNotThrow() {
+		assertThatNoException()
+				.isThrownBy(() -> this.application.run("--spring.config.location=optional:file:invalid/*/"));
+	}
+
+	@Test
+	void runWhenOptionalWildcardLocationHasNoSubdirectoriesDoesNotThrow() {
+		assertThatNoException().isThrownBy(() -> this.application
+				.run("--spring.config.location=optional:file:src/test/resources/config/0-empty/*/"));
+	}
+
 	@Test // gh-24990
 	void runWhenHasProfileSpecificFileWithActiveOnProfileProperty() {
 		ConfigurableApplicationContext context = this.application