Add initial support for RestClient

Introduce initial support for Spring Framework's `RestClient`, in the
form of a `RestClientCustomizer` and `RestClientAutoConfiguration`.

See gh-36213

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/client/RestClientAutoConfiguration.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2012-2023 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.autoconfigure.web.client;
+
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.autoconfigure.AutoConfiguration;
+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.condition.ConditionalOnWebApplication;
+import org.springframework.boot.autoconfigure.condition.NoneNestedConditions;
+import org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration;
+import org.springframework.boot.web.client.RestClientCustomizer;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Conditional;
+import org.springframework.context.annotation.Scope;
+import org.springframework.web.client.RestClient;
+
+/**
+ * {@link EnableAutoConfiguration Auto-configuration} for {@link RestClient}.
+ * <p>
+ * This will produce a {@link RestClient.Builder RestClient.Builder} bean with the
+ * {@code prototype} scope, meaning each injection point will receive a newly cloned
+ * instance of the builder.
+ *
+ * @author Arjen Poutsma
+ * @since 3.2.0
+ */
+@AutoConfiguration(after = HttpMessageConvertersAutoConfiguration.class)
+@ConditionalOnClass(RestClient.class)
+@Conditional(RestClientAutoConfiguration.NotReactiveWebApplicationCondition.class)
+public class RestClientAutoConfiguration {
+
+	@Bean
+	@Scope("prototype")
+	@ConditionalOnMissingBean
+	public RestClient.Builder webClientBuilder(ObjectProvider<RestClientCustomizer> customizerProvider) {
+		RestClient.Builder builder = RestClient.builder();
+		customizerProvider.orderedStream().forEach((customizer) -> customizer.customize(builder));
+		return builder;
+	}
+
+	static class NotReactiveWebApplicationCondition extends NoneNestedConditions {
+
+		NotReactiveWebApplicationCondition() {
+			super(ConfigurationPhase.PARSE_CONFIGURATION);
+		}
+
+		@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
+		private static class ReactiveWebApplication {
+
+		}
+
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/client/RestClientAutoConfigurationTests.java

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+/*
+ * Copyright 2012-2023 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.autoconfigure.web.client;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.boot.web.client.RestClientCustomizer;
+import org.springframework.boot.web.codec.CodecCustomizer;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.web.client.RestClient;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.BDDMockito.then;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link RestClientAutoConfiguration}
+ *
+ * @author Arjen Poutsma
+ */
+class RestClientAutoConfigurationTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+		.withConfiguration(AutoConfigurations.of(RestClientAutoConfiguration.class));
+
+	@Test
+	void shouldCreateBuilder() {
+		this.contextRunner.run((context) -> {
+			RestClient.Builder builder = context.getBean(RestClient.Builder.class);
+			RestClient restClient = builder.build();
+			assertThat(restClient).isNotNull();
+		});
+	}
+
+	@Test
+	void restClientShouldApplyCustomizers() {
+		this.contextRunner.withUserConfiguration(RestClientCustomizerConfig.class).run((context) -> {
+			RestClient.Builder builder = context.getBean(RestClient.Builder.class);
+			RestClientCustomizer customizer = context.getBean("webClientCustomizer", RestClientCustomizer.class);
+			builder.build();
+			then(customizer).should().customize(any(RestClient.Builder.class));
+		});
+	}
+
+	@Test
+	void shouldGetPrototypeScopedBean() {
+		this.contextRunner.withUserConfiguration(RestClientCustomizerConfig.class).run((context) -> {
+			RestClient.Builder firstBuilder = context.getBean(RestClient.Builder.class);
+			RestClient.Builder secondBuilder = context.getBean(RestClient.Builder.class);
+			assertThat(firstBuilder).isNotEqualTo(secondBuilder);
+		});
+	}
+
+	@Test
+	void shouldNotCreateClientBuilderIfAlreadyPresent() {
+		this.contextRunner.withUserConfiguration(CustomRestClientBuilderConfig.class).run((context) -> {
+			RestClient.Builder builder = context.getBean(RestClient.Builder.class);
+			assertThat(builder).isInstanceOf(MyWebClientBuilder.class);
+		});
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class CodecConfiguration {
+
+		@Bean
+		CodecCustomizer myCodecCustomizer() {
+			return mock(CodecCustomizer.class);
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class RestClientCustomizerConfig {
+
+		@Bean
+		RestClientCustomizer webClientCustomizer() {
+			return mock(RestClientCustomizer.class);
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class CustomRestClientBuilderConfig {
+
+		@Bean
+		MyWebClientBuilder myWebClientBuilder() {
+			return mock(MyWebClientBuilder.class);
+		}
+
+	}
+
+	interface MyWebClientBuilder extends RestClient.Builder {
+
+	}
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/client/RestClientCustomizer.java

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+/*
+ * Copyright 2012-2023 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.web.client;
+
+import org.springframework.web.client.RestClient;
+
+/**
+ * Callback interface that can be used to customize a
+ * {@link org.springframework.web.client.RestClient.Builder RestClient.Builder}.
+ *
+ * @author Arjen Poutsma
+ * @since 3.2.0
+ */
+@FunctionalInterface
+public interface RestClientCustomizer {
+
+	/**
+	 * Callback to customize a {@link org.springframework.web.client.RestClient.Builder
+	 * RestClient.Builder} instance.
+	 * @param restClientBuilder the client builder to customize
+	 */
+	void customize(RestClient.Builder restClientBuilder);
+
+}