Make FlywayMigrationStrategy an interface

Change FlywayMigrationStrategy from a class to an interface.

Fixes gh-3217

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

@@ -75,6 +75,9 @@ public class FlywayAutoConfiguration {
 		@FlywayDataSource
 		private DataSource flywayDataSource;
 
+		@Autowired(required = false)
+		private FlywayMigrationStrategy migrationStrategy;
+
 		@PostConstruct
 		public void checkLocationExists() {
 			if (this.properties.isCheckLocation()) {
@@ -96,12 +99,6 @@ public class FlywayAutoConfiguration {
 			return false;
 		}
 
-		@Bean
-		@ConditionalOnMissingBean
-		public FlywayMigrationStrategy flywayMigrationStrategy() {
-			return new FlywayMigrationStrategy();
-		}
-
 		@Bean
 		@ConfigurationProperties(prefix = "flyway")
 		public Flyway flyway() {
@@ -121,9 +118,8 @@ public class FlywayAutoConfiguration {
 		}
 
 		@Bean
-		public FlywayMigrationInitializer flywayInitializer(Flyway flyway,
-				FlywayMigrationStrategy migrationStrategy) {
-			return new FlywayMigrationInitializer(flyway, migrationStrategy);
+		public FlywayMigrationInitializer flywayInitializer(Flyway flyway) {
+			return new FlywayMigrationInitializer(flyway, this.migrationStrategy);
 
 		}
 
@@ -163,7 +159,12 @@ public class FlywayAutoConfiguration {
 
 		@Override
 		public void afterPropertiesSet() throws Exception {
-			this.migrationStrategy.migrate(this.flyway);
+			if (this.migrationStrategy != null) {
+				this.migrationStrategy.migrate(this.flyway);
+			}
+			else {
+				this.flyway.migrate();
+			}
 		}
 
 	}

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

@@ -25,10 +25,12 @@ import org.flywaydb.core.Flyway;
  * @author Andreas Ahlenstorf
  * @author Phillip Webb
  */
-public class FlywayMigrationStrategy {
+public interface FlywayMigrationStrategy {
 
-	public void migrate(Flyway flyway) {
-		flyway.migrate();
-	}
+	/**
+	 * Trigger flyway migration.
+	 * @param flyway the flyway instance
+	 */
+	void migrate(Flyway flyway);
 
 }

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

@@ -180,7 +180,7 @@ public class FlywayAutoConfigurationTests {
 	}
 
 	@Component
-	protected static class MockFlywayMigrationStrategy extends FlywayMigrationStrategy {
+	protected static class MockFlywayMigrationStrategy implements FlywayMigrationStrategy {
 
 		private boolean called = false;