Document how to initialize a database with R2DBC

This commit adds a section to the reference guide on how to initialize
a database using R2DBC. 2 smoke tests are also added to validate this
behaviour with Flyway and Liquibase.

Closes gh-20742

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

@@ -1937,7 +1937,7 @@ It is a Hibernate feature (and has nothing to do with Spring).
 
 
 [[howto-initialize-a-database-using-spring-jdbc]]
-=== Initialize a Database
+=== Initialize a Database using basic SQL scripts
 Spring Boot can automatically create the schema (DDL scripts) of your `DataSource` and initialize it (DML scripts).
 It loads SQL from the standard root classpath locations: `schema.sql` and `data.sql`, respectively.
 In addition, Spring Boot processes the `schema-$\{platform}.sql` and `data-$\{platform}.sql` files (if present), where `platform` is the value of `spring.datasource.platform`.
@@ -1965,6 +1965,24 @@ Make sure to disable `spring.jpa.hibernate.ddl-auto` if you use `schema.sql`.
 
 
 
+[[howto-initialize-a-database-using-r2dc]]
+=== Initialize a Database Using R2DBC
+If you are using R2DBC, the regular `DataSource` auto-configuration backs off so none of the options described above can be used.
+
+If you are using Spring Data R2DBC, you can initialize the database on startup using simple SQL scripts as shown in the following example:
+
+[source,java,indent=0]
+----
+include::{code-examples}/r2dbc/R2dbcDatabaseInitializationExample.java[tag=configuration]
+----
+
+Alternatively, you can configure either <<howto-execute-flyway-database-migrations-on-startup,Flyway>> or <<howto-execute-liquibase-database-migrations-on-startup,Liquibase>> to configure a `DataSource` for you for the duration of the migration.
+Both these libraries offer properties to set the `url`, `username` and `password` of the database to migrate.
+
+NOTE: When choosing this option, `org.springframework:spring-jdbc` is still a required dependency.
+
+
+
 [[howto-initialize-a-spring-batch-database]]
 === Initialize a Spring Batch Database
 If you use Spring Batch, it comes pre-packaged with SQL initialization scripts for most popular database platforms.

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/r2dbc/R2dbcDatabaseInitializationExample.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2012-2020 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.r2dbc;
+
+import io.r2dbc.spi.ConnectionFactory;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.io.DefaultResourceLoader;
+import org.springframework.core.io.Resource;
+import org.springframework.core.io.ResourceLoader;
+import org.springframework.data.r2dbc.connectionfactory.init.ResourceDatabasePopulator;
+
+/**
+ * Example configuration for initializing a database using R2DBC.
+ *
+ * @author Stephane Nicoll
+ */
+public class R2dbcDatabaseInitializationExample {
+
+	// tag::configuration[]
+	@Configuration(proxyBeanMethods = false)
+	static class DatabaseInitializationConfiguration {
+
+		@Autowired
+		void initializeDatabase(ConnectionFactory connectionFactory) {
+			ResourceLoader resourceLoader = new DefaultResourceLoader();
+			Resource[] scripts = new Resource[] { resourceLoader.getResource("classpath:schema.sql"),
+					resourceLoader.getResource("classpath:data.sql") };
+			new ResourceDatabasePopulator(scripts).execute(connectionFactory).block();
+		}
+
+	}
+	// end::configuration[]
+
+}