Add RestClient.Builder#messageConverters(List)

This commit also introduces the validation of the RestClient
message converters for both variants.

Closes spring-projectsgh-33536

spring-web/src/main/java/org/springframework/web/client/DefaultRestClientBuilder.java

@@ -19,6 +19,7 @@
 import java.net.URI;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -63,6 +64,7 @@
  *
  * @author Arjen Poutsma
  * @author Hyoungjune Kim
+ * @author Sebastien Deleuze
  * @since 6.1
  */
 final class DefaultRestClientBuilder implements RestClient.Builder {
@@ -358,6 +360,14 @@ public RestClient.Builder requestFactory(ClientHttpRequestFactory requestFactory
 	@Override
 	public RestClient.Builder messageConverters(Consumer<List<HttpMessageConverter<?>>> configurer) {
 		configurer.accept(initMessageConverters());
+		validateConverters(this.messageConverters);
+		return this;
+	}
+
+	@Override
+	public RestClient.Builder messageConverters(List<HttpMessageConverter<?>> messageConverters) {
+		validateConverters(messageConverters);
+		this.messageConverters = Collections.unmodifiableList(messageConverters);
 		return this;
 	}
 
@@ -413,6 +423,11 @@ else if (jsonbPresent) {
 		return this.messageConverters;
 	}
 
+	private void validateConverters(@Nullable List<HttpMessageConverter<?>> messageConverters) {
+		Assert.notEmpty(messageConverters, "At least one HttpMessageConverter is required");
+		Assert.noNullElements(messageConverters, "The HttpMessageConverter list must not contain null elements");
+	}
+
 
 	@Override
 	public RestClient.Builder clone() {

spring-web/src/main/java/org/springframework/web/client/RestClient.java

@@ -73,6 +73,7 @@
  * </ul>
  *
  * @author Arjen Poutsma
+ * @author Sebastien Deleuze
  * @since 6.1
  */
 public interface RestClient {
@@ -399,11 +400,22 @@ Builder defaultStatusHandler(Predicate<HttpStatusCode> statusPredicate,
 
 		/**
 		 * Configure the message converters for the {@code RestClient} to use.
-		 * @param configurer the configurer to apply
+		 * @param configurer the configurer to apply on the list of default
+		 * {@link HttpMessageConverter} pre-initialized
 		 * @return this builder
+		 * @see #messageConverters(List)
 		 */
 		Builder messageConverters(Consumer<List<HttpMessageConverter<?>>> configurer);
 
+		/**
+		 * Set the message converters for the {@code RestClient} to use.
+		 * @param messageConverters the list of {@link HttpMessageConverter} to use
+		 * @return this builder
+		 * @since 6.2
+		 * @see #messageConverters(Consumer)
+		 */
+		Builder messageConverters(List<HttpMessageConverter<?>> messageConverters);
+
 		/**
 		 * Configure the {@link io.micrometer.observation.ObservationRegistry} to use
 		 * for recording HTTP client observations.

spring-web/src/test/java/org/springframework/web/client/RestClientBuilderTests.java

@@ -18,8 +18,11 @@
 
 import java.lang.reflect.Field;
 import java.net.URI;
+import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
+import org.assertj.core.api.InstanceOfAssertFactories;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.client.ClientHttpRequestInitializer;
@@ -32,11 +35,13 @@
 import org.springframework.web.util.DefaultUriBuilderFactory;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.assertj.core.api.Assertions.fail;
 
 
 /**
  * @author Arjen Poutsma
+ * @author Sebastien Deleuze
  */
 public class RestClientBuilderTests {
 
@@ -103,6 +108,35 @@ void defaultUri() {
 		assertThat(fieldValue("baseUrl", defaultBuilder)).isEqualTo(baseUrl.toString());
 	}
 
+	@Test
+	void messageConvertersList() {
+		StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
+		RestClient.Builder builder = RestClient.builder();
+		builder.messageConverters(List.of(stringConverter));
+
+		assertThat(builder).isInstanceOf(DefaultRestClientBuilder.class);
+		DefaultRestClientBuilder defaultBuilder = (DefaultRestClientBuilder) builder;
+
+		assertThat(fieldValue("messageConverters", defaultBuilder))
+				.asInstanceOf(InstanceOfAssertFactories.LIST)
+				.containsExactly(stringConverter);
+	}
+
+	@Test
+	void messageConvertersListEmpty() {
+		RestClient.Builder builder = RestClient.builder();
+		List<HttpMessageConverter<?>> converters = Collections.emptyList();
+		assertThatIllegalArgumentException().isThrownBy(() -> builder.messageConverters(converters));
+	}
+
+	@Test
+	void messageConvertersListWithNullElement() {
+		RestClient.Builder builder = RestClient.builder();
+		List<HttpMessageConverter<?>> converters = new ArrayList<>();
+		converters.add(null);
+		assertThatIllegalArgumentException().isThrownBy(() -> builder.messageConverters(converters));
+	}
+
 	@Nullable
 	private static Object fieldValue(String name, DefaultRestClientBuilder instance) {
 		try {