Fail more helpfully when schema or data location is a directory

Previously a schema or data locatio that was a directory would
result in an attempt being made to apply the directory listing as
SQL scripts. This would typically result in a hard to diagnose
failure due to the directory listing not being valid SQL.

This commit updates the initializer to ignore locations for
which the Resources is not readable. This works as Framework's
Resource abstraction does not consider directory resources to be
readable.

Closes gh-36386

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/sql/init/AbstractScriptDatabaseInitializer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2023 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.
@@ -121,7 +121,7 @@ public abstract class AbstractScriptDatabaseInitializer implements ResourceLoade
 				location = location.substring(OPTIONAL_LOCATION_PREFIX.length());
 			}
 			for (Resource resource : doGetResources(location, locationResolver)) {
-				if (resource.exists()) {
+				if (resource.isReadable()) {
 					resources.add(resource);
 				}
 				else if (!optional) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/sql/init/AbstractScriptDatabaseInitializerTests.java

@@ -44,6 +44,16 @@ public abstract class AbstractScriptDatabaseInitializerTests<T extends AbstractS
 		assertThat(numberOfEmbeddedRows("SELECT COUNT(*) FROM EXAMPLE")).isEqualTo(1);
 	}
 
+	@Test
+	void whenDatabaseIsInitializedWithDirectoryLocationsThenFailureIsHelpful() {
+		DatabaseInitializationSettings settings = new DatabaseInitializationSettings();
+		settings.setSchemaLocations(Arrays.asList("/org/springframework/boot/sql/init"));
+		settings.setDataLocations(Arrays.asList("/org/springframework/boot/sql/init"));
+		T initializer = createEmbeddedDatabaseInitializer(settings);
+		assertThatIllegalStateException().isThrownBy(initializer::initializeDatabase)
+			.withMessage("No schema scripts found at location '/org/springframework/boot/sql/init'");
+	}
+
 	@Test
 	void whenContinueOnErrorIsFalseThenInitializationFailsOnError() {
 		DatabaseInitializationSettings settings = new DatabaseInitializationSettings();