Add auto-configuration for a no-op tracer

This auto-configuration ensures, if Micrometer Tracing is on the
classpath, that there is always a tracer. It backs off if there is
already a tracer, for example contributed by the Brave or the Otel
auto-configurations, which are run before.

See gh-38568

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

@@ -62,7 +62,7 @@ import org.springframework.core.env.Environment;
  * @author Jonatan Ivanov
  * @since 3.0.0
  */
-@AutoConfiguration(before = MicrometerTracingAutoConfiguration.class)
+@AutoConfiguration(before = { MicrometerTracingAutoConfiguration.class, NoopTracerAutoConfiguration.class })
 @ConditionalOnClass({ Tracer.class, BraveTracer.class })
 @EnableConfigurationProperties(TracingProperties.class)
 @Import({ BravePropagationConfigurations.PropagationWithoutBaggage.class,

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

@@ -0,0 +1,44 @@
+/*
+ * 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.actuate.autoconfigure.tracing;
+
+import io.micrometer.tracing.Tracer;
+
+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.context.annotation.Bean;
+
+/**
+ * {@link EnableAutoConfiguration Auto-configuration} for a no-op implementation of
+ * {@link Tracer}.
+ *
+ * @author Moritz Halbritter
+ * @since 3.2.1
+ */
+@AutoConfiguration(before = MicrometerTracingAutoConfiguration.class)
+@ConditionalOnClass(Tracer.class)
+@ConditionalOnMissingBean(Tracer.class)
+public class NoopTracerAutoConfiguration {
+
+	@Bean
+	Tracer noopTracer() {
+		return Tracer.NOOP;
+	}
+
+}

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

@@ -66,7 +66,8 @@ import org.springframework.context.annotation.Import;
  * @author Yanming Zhou
  * @since 3.0.0
  */
-@AutoConfiguration(value = "openTelemetryTracingAutoConfiguration", before = MicrometerTracingAutoConfiguration.class)
+@AutoConfiguration(value = "openTelemetryTracingAutoConfiguration",
+		before = { MicrometerTracingAutoConfiguration.class, NoopTracerAutoConfiguration.class })
 @ConditionalOnClass({ OtelTracer.class, SdkTracerProvider.class, OpenTelemetry.class })
 @EnableConfigurationProperties(TracingProperties.class)
 @Import({ OpenTelemetryPropagationConfigurations.PropagationWithoutBaggage.class,

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/resources/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports

@@ -104,6 +104,7 @@ org.springframework.boot.actuate.autoconfigure.startup.StartupEndpointAutoConfig
 org.springframework.boot.actuate.autoconfigure.system.DiskSpaceHealthContributorAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.BraveAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.MicrometerTracingAutoConfiguration
+org.springframework.boot.actuate.autoconfigure.tracing.NoopTracerAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.OpenTelemetryAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.otlp.OtlpAutoConfiguration
 org.springframework.boot.actuate.autoconfigure.tracing.prometheus.PrometheusExemplarsAutoConfiguration

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

@@ -0,0 +1,77 @@
+/*
+ * 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.actuate.autoconfigure.tracing;
+
+import io.micrometer.tracing.Tracer;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.FilteredClassLoader;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link NoopTracerAutoConfiguration}.
+ *
+ * @author Moritz Halbritter
+ */
+class NoopTracerAutoConfigurationTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+		.withConfiguration(AutoConfigurations.of(NoopTracerAutoConfiguration.class));
+
+	@Test
+	void shouldSupplyNoopTracer() {
+		this.contextRunner.run((context) -> {
+			assertThat(context).hasSingleBean(Tracer.class);
+			Tracer tracer = context.getBean(Tracer.class);
+			assertThat(tracer).isEqualTo(Tracer.NOOP);
+		});
+	}
+
+	@Test
+	void shouldBackOffOnCustomTracer() {
+		this.contextRunner.withUserConfiguration(CustomTracerConfiguration.class).run((context) -> {
+			assertThat(context).hasSingleBean(Tracer.class);
+			assertThat(context).hasBean("customTracer");
+			Tracer tracer = context.getBean(Tracer.class);
+			assertThat(tracer).isNotEqualTo(Tracer.NOOP);
+		});
+	}
+
+	@Test
+	void shouldBackOffIfMicrometerTracingIsMissing() {
+		this.contextRunner.withClassLoader(new FilteredClassLoader("io.micrometer.tracing"))
+			.run((context) -> assertThat(context).doesNotHaveBean(Tracer.class));
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	private static class CustomTracerConfiguration {
+
+		@Bean
+		Tracer customTracer() {
+			return mock(Tracer.class);
+		}
+
+	}
+
+}