Make sure Liquibase works with only spring-jdbc on the classpath

This commit improves the Liquibase auto-configuration to only rely on
spring-jdbc when a `DataSource` should be created on-the-fly for the
purpose of its initialization.

Previously, a connection pool implementation must be added as well, now
we're fallbacking on `SimpleDriverDataSource` if necessary. This
improves the database initialization use case with R2DBC.

Closes gh-20715

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/liquibase/LiquibaseAutoConfiguration.java

@@ -50,6 +50,7 @@ import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
 import org.springframework.jdbc.core.JdbcOperations;
 import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
+import org.springframework.jdbc.datasource.SimpleDriverDataSource;
 import org.springframework.orm.jpa.AbstractEntityManagerFactoryBean;
 import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;
 
@@ -145,7 +146,13 @@ public class LiquibaseAutoConfiguration {
 			String url = getProperty(this.properties::getUrl, dataSourceProperties::determineUrl);
 			String user = getProperty(this.properties::getUser, dataSourceProperties::determineUsername);
 			String password = getProperty(this.properties::getPassword, dataSourceProperties::determinePassword);
-			return DataSourceBuilder.create().url(url).username(user).password(password).build();
+			return DataSourceBuilder.create().type(determineDataSourceType()).url(url).username(user).password(password)
+					.build();
+		}
+
+		private Class<? extends DataSource> determineDataSourceType() {
+			Class<? extends DataSource> type = DataSourceBuilder.findType(null);
+			return (type != null) ? type : SimpleDriverDataSource.class;
 		}
 
 		private String getProperty(Supplier<String> property, Supplier<String> defaultValue) {

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/liquibase/LiquibaseAutoConfigurationTests.java

@@ -45,6 +45,7 @@ import org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfigurat
 import org.springframework.boot.context.event.ApplicationStartingEvent;
 import org.springframework.boot.jdbc.DataSourceBuilder;
 import org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener;
+import org.springframework.boot.test.context.FilteredClassLoader;
 import org.springframework.boot.test.context.assertj.AssertableApplicationContext;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.test.context.runner.ContextConsumer;
@@ -54,6 +55,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Primary;
 import org.springframework.jdbc.core.JdbcTemplate;
+import org.springframework.jdbc.datasource.SimpleDriverDataSource;
 import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -80,7 +82,7 @@ class LiquibaseAutoConfigurationTests {
 				.onApplicationEvent(new ApplicationStartingEvent(new SpringApplication(Object.class), new String[0]));
 	}
 
-	private ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
 			.withConfiguration(AutoConfigurations.of(LiquibaseAutoConfiguration.class))
 			.withPropertyValues("spring.datasource.generate-unique-name=true");
 
@@ -99,6 +101,18 @@ class LiquibaseAutoConfigurationTests {
 				}));
 	}
 
+	@Test
+	void createsDataSourceWhenSpringJdbcOnlyAvailableWithNoDataSourceBeanAndLiquibaseUrl() {
+		this.contextRunner.withPropertyValues("spring.liquibase.url:jdbc:hsqldb:mem:liquibase")
+				.withClassLoader(
+						new FilteredClassLoader("org.apache.tomcat", "com.zaxxer.hikari", "org.apache.commons.dbcp2"))
+				.run(assertLiquibase((liquibase) -> {
+					DataSource dataSource = liquibase.getDataSource();
+					assertThat(dataSource).isInstanceOf(SimpleDriverDataSource.class);
+					assertThat(((SimpleDriverDataSource) dataSource).getUrl()).isEqualTo("jdbc:hsqldb:mem:liquibase");
+				}));
+	}
+
 	@Test
 	void defaultSpringLiquibase() {
 		this.contextRunner.withUserConfiguration(EmbeddedDataSourceConfiguration.class)