Allow indexed access of `flyway.locations`

This commit allows to use the `flyway.locations` in an indexed fashion
(i.e. typically in YAML configuration).

See gh-4973

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayAutoConfiguration.java

@@ -114,6 +114,8 @@ public class FlywayAutoConfiguration {
 			else {
 				flyway.setDataSource(this.dataSource);
 			}
+			//Explicitly set locations because the getter doesn't return a mutable value
+			flyway.setLocations(this.properties.getLocations().toArray(new String[0]));
 			return flyway;
 		}
 

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayProperties.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.autoconfigure.flyway;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
@@ -39,7 +40,7 @@ public class FlywayProperties {
 	/**
 	 * Locations of migrations scripts.
 	 */
-	private List<String> locations = Arrays.asList("db/migration");
+	private List<String> locations = new ArrayList<String>(Arrays.asList("db/migration"));
 
 	/**
 	 * Check that migration scripts location exists.

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/flyway/FlywayAutoConfigurationTests.java

@@ -104,8 +104,8 @@ public class FlywayAutoConfigurationTests {
 
 	@Test
 	public void overrideLocations() throws Exception {
-		EnvironmentTestUtils.addEnvironment(this.context,
-				"flyway.locations:classpath:db/changelog,classpath:db/migration");
+		EnvironmentTestUtils.addEnvironment(this.context, "flyway.locations[0]:classpath:db/changelog",
+				"flyway.locations[1]:classpath:db/migration");
 		registerAndRefresh(EmbeddedDataSourceConfiguration.class,
 				FlywayAutoConfiguration.class,
 				PropertyPlaceholderAutoConfiguration.class);