Support Elasticsearch RestClientBuilder auto-configuration without RestHighLevelClient

Prior to this commit, Spring Boot would only auto-configure
the `RestHighLevelClient` and `RestClientBuilder` if the `RestHighLevelClient`
was present. This was done in 1d73d4e.

This commit brings back the exposing of the `RestClient` bean in
Spring Boot when exposing the `RestHighLevelClient` or
when the `RestHighLevelClient` is not present.
It allows for using the Spring Boot auto configuration and its customizers
of the `RestClientBuilder` in a similar way as it is done for the `RestTeamplateBuilder`
and the `WebClient.Builder`.
Now the presence of the `org.elasticsearch.client:elasticsearch-rest-high-level-client` is optional.
This opens the door for potentially adding support to the new
[Elasticsearch Java Client](https://github.com/elastic/elasticsearch-java) that is based on the same `RestClient`

Author: filiphr

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/elasticsearch/ElasticsearchRestClientAutoConfiguration.java

@@ -17,12 +17,11 @@
 package org.springframework.boot.autoconfigure.elasticsearch;
 
 import org.elasticsearch.client.RestClient;
-import org.elasticsearch.client.RestHighLevelClient;
 
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
-import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchRestClientConfigurations.RestClientBuilderConfiguration;
+import org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchRestClientConfigurations.RestClientConfiguration;
 import org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchRestClientConfigurations.RestClientSnifferConfiguration;
 import org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchRestClientConfigurations.RestHighLevelClientConfiguration;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
@@ -38,11 +37,11 @@
  */
 @SuppressWarnings("deprecation")
 @Configuration(proxyBeanMethods = false)
-@ConditionalOnClass(RestHighLevelClient.class)
-@ConditionalOnMissingBean(RestClient.class)
+@ConditionalOnClass(RestClient.class)
 @EnableConfigurationProperties({ ElasticsearchProperties.class, ElasticsearchRestClientProperties.class,
 		DeprecatedElasticsearchRestClientProperties.class })
 @Import({ RestClientBuilderConfiguration.class, RestHighLevelClientConfiguration.class,
+		RestClientConfiguration.class,
 		RestClientSnifferConfiguration.class })
 public class ElasticsearchRestClientAutoConfiguration {
 

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/elasticsearch/ElasticsearchRestClientConfigurations.java

@@ -37,6 +37,7 @@
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnSingleCandidate;
 import org.springframework.boot.context.properties.PropertyMapper;
 import org.springframework.context.annotation.Bean;
@@ -112,14 +113,35 @@ private HttpHost createHttpHost(URI uri) {
 	}
 
 	@Configuration(proxyBeanMethods = false)
-	@ConditionalOnMissingBean(RestHighLevelClient.class)
+	@ConditionalOnClass(RestHighLevelClient.class)
+	@ConditionalOnMissingBean({
+			RestHighLevelClient.class,
+			RestClient.class
+	})
 	static class RestHighLevelClientConfiguration {
 
 		@Bean
 		RestHighLevelClient elasticsearchRestHighLevelClient(RestClientBuilder restClientBuilder) {
 			return new RestHighLevelClient(restClientBuilder);
 		}
 
+		@Bean
+		RestClient elasticsearchRestClient(RestHighLevelClient restHighLevelClient) {
+			return restHighLevelClient.getLowLevelClient();
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	@ConditionalOnMissingClass("org.elasticsearch.client.RestHighLevelClient")
+	@ConditionalOnMissingBean(RestClient.class)
+	static class RestClientConfiguration {
+
+		@Bean
+		RestClient elasticsearchRestClient(RestClientBuilder restClientBuilder) {
+			return restClientBuilder.build();
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/elasticsearch/ElasticsearchRestClientAutoConfigurationTests.java

@@ -55,16 +55,21 @@
  * @author Brian Clozel
  * @author Vedran Pavic
  * @author Evgeniy Cheban
+ * @author Filip Hrisafov
  */
 class ElasticsearchRestClientAutoConfigurationTests {
 
 	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
 			.withConfiguration(AutoConfigurations.of(ElasticsearchRestClientAutoConfiguration.class));
 
 	@Test
-	void configureShouldOnlyCreateHighLevelRestClient() {
-		this.contextRunner.run((context) -> assertThat(context).doesNotHaveBean(RestClient.class)
-				.hasSingleBean(RestHighLevelClient.class));
+	void configureShouldCreateHighLevelAndLowLevelRestClient() {
+		this.contextRunner.run((context) -> {
+			assertThat(context).hasSingleBean(RestClient.class)
+							.hasSingleBean(RestHighLevelClient.class);
+			assertThat(context.getBean(RestClient.class))
+					.isEqualTo(context.getBean(RestHighLevelClient.class).getLowLevelClient());
+		});
 	}
 
 	@Test
@@ -74,10 +79,19 @@ void configureWhenCustomRestClientShouldBackOff() {
 						.hasSingleBean(RestClient.class).hasBean("customRestClient"));
 	}
 
+	@Test
+	void configureWhenCustomRestHighLevelClientIsNotPresent() {
+		this.contextRunner.withClassLoader(new FilteredClassLoader(RestHighLevelClient.class))
+				.run((context) -> assertThat(context).doesNotHaveBean(RestHighLevelClient.class)
+						.hasSingleBean(RestClient.class)
+						.hasSingleBean(RestClientBuilder.class));
+	}
+
 	@Test
 	void configureWhenCustomRestHighLevelClientShouldBackOff() {
 		this.contextRunner.withUserConfiguration(CustomRestHighLevelClientConfiguration.class)
-				.run((context) -> assertThat(context).hasSingleBean(RestHighLevelClient.class));
+				.run((context) -> assertThat(context).hasSingleBean(RestHighLevelClient.class)
+						.doesNotHaveBean(RestClient.class));
 	}
 
 	@Test