Guard ZipkinTracingAutoConfiguration against missing Encoding bean

This commit harmonizes the check for the presence of an Encoding bean.
Also, as this bean may be provided by another auto-configuration, this
commit orders them explicitly.

This means the dependency to spring-boot-zipkin has to elevate from a
test-only dependency to (at least) an optional dependency to validate
the links between the two auto-configurations.

Closes gh-46295

module/spring-boot-tracing/build.gradle

@@ -37,6 +37,7 @@ dependencies {
 	optional(project(":core:spring-boot-testcontainers"))
 	optional(project(":module:spring-boot-actuator-autoconfigure"))
 	optional(project(":module:spring-boot-metrics"))
+	optional(project(":module:spring-boot-zipkin"))
 	optional("io.micrometer:micrometer-core")
 	optional("io.micrometer:micrometer-tracing-bridge-brave")
 	optional("io.micrometer:micrometer-tracing-bridge-otel")
@@ -55,7 +56,6 @@ dependencies {
 	testImplementation(project(":core:spring-boot-test"))
 	testImplementation(project(":module:spring-boot-opentelemetry"))
 	testImplementation(project(":test-support:spring-boot-test-support"))
-	testImplementation(project(":module:spring-boot-zipkin"))
 	testImplementation("com.squareup.okhttp3:mockwebserver")
 	testImplementation("io.micrometer:micrometer-registry-prometheus")
 	testImplementation("io.opentelemetry:opentelemetry-exporter-common")

module/spring-boot-tracing/src/main/java/org/springframework/boot/tracing/autoconfigure/zipkin/ZipkinTracingAutoConfiguration.java

@@ -50,7 +50,7 @@ import org.springframework.context.annotation.Import;
  * @author Phillip Webb
  * @since 4.0.0
  */
-@AutoConfiguration
+@AutoConfiguration(afterName = "org.springframework.boot.zipkin.autoconfigure.ZipkinAutoConfiguration")
 @ConditionalOnClass(Encoding.class)
 @Import({ BraveConfiguration.class, OpenTelemetryConfiguration.class })
 public class ZipkinTracingAutoConfiguration {
@@ -60,6 +60,7 @@ public class ZipkinTracingAutoConfiguration {
 	static class BraveConfiguration {
 
 		@Bean
+		@ConditionalOnBean(Encoding.class)
 		@ConditionalOnMissingBean(value = MutableSpan.class, parameterizedContainer = BytesEncoder.class)
 		BytesEncoder<MutableSpan> mutableSpanBytesEncoder(Encoding encoding,
 				ObjectProvider<Tag<Throwable>> throwableTagProvider) {

module/spring-boot-tracing/src/test/java/org/springframework/boot/tracing/autoconfigure/zipkin/ZipkinTracingAutoConfigurationTests.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2012-present 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.tracing.autoconfigure.zipkin;
+
+import io.opentelemetry.exporter.zipkin.ZipkinSpanExporter;
+import io.opentelemetry.sdk.trace.export.SpanExporter;
+import org.junit.jupiter.api.Test;
+import zipkin2.reporter.BytesEncoder;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.boot.zipkin.autoconfigure.ZipkinAutoConfiguration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ZipkinTracingAutoConfiguration}.
+ *
+ * @author Stephane Nicoll
+ */
+class ZipkinTracingAutoConfigurationTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+		.withConfiguration(AutoConfigurations.of(ZipkinTracingAutoConfiguration.class));
+
+	@Test
+	void shouldNotSupplyBeansIfInfrastructureIsNotAvailable() {
+		this.contextRunner.run((context) -> assertThat(context).doesNotHaveBean(BytesEncoder.class)
+			.doesNotHaveBean(SpanExporter.class)
+			.doesNotHaveBean(ZipkinSpanExporter.class));
+	}
+
+	@Test
+	void shouldSupplyBeansIfInfrastructureIsAvailable() {
+		this.contextRunner.withConfiguration(AutoConfigurations.of(ZipkinAutoConfiguration.class)).run((context) -> {
+			assertThat(context).hasSingleBean(SpanExporter.class);
+			assertThat(context).hasSingleBean(ZipkinSpanExporter.class);
+		});
+	}
+
+	@Test
+	void shouldNotSupplyBeansIfTracingIsDisabled() {
+		this.contextRunner.withPropertyValues("management.tracing.enabled=false")
+			.withConfiguration(AutoConfigurations.of(ZipkinAutoConfiguration.class))
+			.run((context) -> {
+				assertThat(context).doesNotHaveBean(SpanExporter.class);
+				assertThat(context).doesNotHaveBean(ZipkinSpanExporter.class);
+			});
+	}
+
+}