Break cycles between Zipkin senders and HTTP client observation

Previously, RestTemplateBuilder and WebClient.Builder beans were used
to create the HTTP client for sending out spans. Those same beans are
also instrumented for observability which results in a cycle.

This commit breaks the cycle by not using the application-web
builders to create the RestTemplate and WebClient's used by the Zipkin
senders. Instead, builders are created inline, with new callbacks
being introduced to allow the user to customize these Zipkin-specific
builders.

See gh-32528

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/zipkin/ZipkinConfigurations.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.actuate.autoconfigure.tracing.zipkin;
 
+import java.util.List;
+
 import io.opentelemetry.exporter.zipkin.ZipkinSpanExporter;
 import zipkin2.Span;
 import zipkin2.codec.BytesEncoder;
@@ -73,12 +75,12 @@ class ZipkinConfigurations {
 
 		@Bean
 		@ConditionalOnMissingBean(Sender.class)
-		@ConditionalOnBean(RestTemplateBuilder.class)
 		ZipkinRestTemplateSender restTemplateSender(ZipkinProperties properties,
-				RestTemplateBuilder restTemplateBuilder) {
-			RestTemplate restTemplate = restTemplateBuilder.setConnectTimeout(properties.getConnectTimeout())
-					.setReadTimeout(properties.getReadTimeout()).build();
-			return new ZipkinRestTemplateSender(properties.getEndpoint(), restTemplate);
+				List<ZipkinRestTemplateBuilderCustomizer> customizers) {
+			RestTemplateBuilder restTemplateBuilder = new RestTemplateBuilder()
+					.setConnectTimeout(properties.getConnectTimeout()).setReadTimeout(properties.getReadTimeout());
+			customizers.forEach((c) -> c.customize(restTemplateBuilder));
+			return new ZipkinRestTemplateSender(properties.getEndpoint(), restTemplateBuilder.build());
 		}
 
 	}
@@ -90,10 +92,11 @@ class ZipkinConfigurations {
 
 		@Bean
 		@ConditionalOnMissingBean(Sender.class)
-		@ConditionalOnBean(WebClient.Builder.class)
-		ZipkinWebClientSender webClientSender(ZipkinProperties properties, WebClient.Builder webClientBuilder) {
-			WebClient webClient = webClientBuilder.build();
-			return new ZipkinWebClientSender(properties.getEndpoint(), webClient);
+		ZipkinWebClientSender webClientSender(ZipkinProperties properties,
+				List<ZipkinWebClientBuilderCustomizer> customizers) {
+			WebClient.Builder builder = WebClient.builder();
+			customizers.forEach((c) -> c.customize(builder));
+			return new ZipkinWebClientSender(properties.getEndpoint(), builder.build());
 		}
 
 	}

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/tracing/zipkin/ZipkinConfigurationsSenderConfigurationTests.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.actuate.autoconfigure.tracing.zipkin;
 
 import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentMatchers;
 import zipkin2.reporter.Sender;
 import zipkin2.reporter.urlconnection.URLConnectionSender;
 
@@ -26,12 +27,12 @@ import org.springframework.boot.test.context.FilteredClassLoader;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
 import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
-import org.springframework.boot.web.client.RestTemplateBuilder;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.web.reactive.function.client.WebClient;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.then;
 import static org.mockito.Mockito.mock;
 
 /**
@@ -66,6 +67,8 @@ class ZipkinConfigurationsSenderConfigurationTests {
 					assertThat(context).doesNotHaveBean(URLConnectionSender.class);
 					assertThat(context).hasSingleBean(Sender.class);
 					assertThat(context).hasSingleBean(ZipkinWebClientSender.class);
+					then(context.getBean(ZipkinWebClientBuilderCustomizer.class)).should()
+							.customize(ArgumentMatchers.any());
 				});
 	}
 
@@ -90,9 +93,9 @@ class ZipkinConfigurationsSenderConfigurationTests {
 	}
 
 	@Test
-	void willUseRestTemplateInNonWebApplicationIfUrlConnectionSenderIsNotAvailable() {
+	void willUseRestTemplateInNonWebApplicationIfUrlConnectionSenderAndWebclientAreNotAvailable() {
 		this.contextRunner.withUserConfiguration(RestTemplateConfiguration.class)
-				.withClassLoader(new FilteredClassLoader("zipkin2.reporter.urlconnection")).run((context) -> {
+				.withClassLoader(new FilteredClassLoader(URLConnectionSender.class, WebClient.class)).run((context) -> {
 					assertThat(context).doesNotHaveBean(URLConnectionSender.class);
 					assertThat(context).hasSingleBean(Sender.class);
 					assertThat(context).hasSingleBean(ZipkinRestTemplateSender.class);
@@ -100,9 +103,9 @@ class ZipkinConfigurationsSenderConfigurationTests {
 	}
 
 	@Test
-	void willUseRestTemplateInServletWebApplicationIfUrlConnectionSenderIsNotAvailable() {
+	void willUseRestTemplateInServletWebApplicationIfUrlConnectionSenderAndWebClientNotAvailable() {
 		this.servletContextRunner.withUserConfiguration(RestTemplateConfiguration.class)
-				.withClassLoader(new FilteredClassLoader("zipkin2.reporter.urlconnection")).run((context) -> {
+				.withClassLoader(new FilteredClassLoader(URLConnectionSender.class, WebClient.class)).run((context) -> {
 					assertThat(context).doesNotHaveBean(URLConnectionSender.class);
 					assertThat(context).hasSingleBean(Sender.class);
 					assertThat(context).hasSingleBean(ZipkinRestTemplateSender.class);
@@ -110,9 +113,9 @@ class ZipkinConfigurationsSenderConfigurationTests {
 	}
 
 	@Test
-	void willUseRestTemplateInReactiveWebApplicationIfUrlConnectionSenderIsNotAvailable() {
+	void willUseRestTemplateInReactiveWebApplicationIfUrlConnectionSenderAndWebClientAreNotAvailable() {
 		this.reactiveContextRunner.withUserConfiguration(RestTemplateConfiguration.class)
-				.withClassLoader(new FilteredClassLoader("zipkin2.reporter.urlconnection")).run((context) -> {
+				.withClassLoader(new FilteredClassLoader(URLConnectionSender.class, WebClient.class)).run((context) -> {
 					assertThat(context).doesNotHaveBean(URLConnectionSender.class);
 					assertThat(context).hasSingleBean(Sender.class);
 					assertThat(context).hasSingleBean(ZipkinRestTemplateSender.class);
@@ -140,8 +143,8 @@ class ZipkinConfigurationsSenderConfigurationTests {
 	private static class RestTemplateConfiguration {
 
 		@Bean
-		RestTemplateBuilder restTemplateBuilder() {
-			return new RestTemplateBuilder();
+		ZipkinRestTemplateBuilderCustomizer restTemplateBuilder() {
+			return mock(ZipkinRestTemplateBuilderCustomizer.class);
 		}
 
 	}
@@ -150,8 +153,8 @@ class ZipkinConfigurationsSenderConfigurationTests {
 	private static class WebClientConfiguration {
 
 		@Bean
-		WebClient.Builder webClientBuilder() {
-			return WebClient.builder();
+		ZipkinWebClientBuilderCustomizer webClientBuilder() {
+			return mock(ZipkinWebClientBuilderCustomizer.class);
 		}
 
 	}