Back off auto-config of Log4J2Metrics when context is backed by SLF4J

Closes gh-14883

spring-boot-project/spring-boot-actuator-autoconfigure/pom.xml

@@ -480,11 +480,6 @@
 			<artifactId>log4j-slf4j-impl</artifactId>
 			<scope>test</scope>
 		</dependency>
-		<dependency>
-			<groupId>org.apache.logging.log4j</groupId>
-			<artifactId>log4j-api</artifactId>
-			<scope>test</scope>
-		</dependency>
 		<dependency>
 			<groupId>org.aspectj</groupId>
 			<artifactId>aspectjrt</artifactId>

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/Log4J2MetricsAutoConfiguration.java

@@ -16,16 +16,25 @@
 
 package org.springframework.boot.actuate.autoconfigure.metrics;
 
+import java.util.Collections;
+
 import io.micrometer.core.instrument.MeterRegistry;
 import io.micrometer.core.instrument.binder.logging.Log4j2Metrics;
+import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.core.LoggerContext;
 
+import org.springframework.boot.actuate.autoconfigure.metrics.Log4J2MetricsAutoConfiguration.Log4JCoreLoggerContextCondition;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
+import org.springframework.boot.autoconfigure.condition.ConditionOutcome;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.SpringBootCondition;
 import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.ConditionContext;
+import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.type.AnnotatedTypeMetadata;
 
 /**
  * Auto-configuration for Log4J2 metrics.
@@ -35,14 +44,33 @@ import org.springframework.context.annotation.Configuration;
  */
 @Configuration
 @AutoConfigureAfter(MetricsAutoConfiguration.class)
-@ConditionalOnClass({ Log4j2Metrics.class, LoggerContext.class })
+@ConditionalOnClass({ Log4j2Metrics.class, LoggerContext.class, LogManager.class })
 @ConditionalOnBean(MeterRegistry.class)
+@Conditional(Log4JCoreLoggerContextCondition.class)
 public class Log4J2MetricsAutoConfiguration {
 
 	@Bean
 	@ConditionalOnMissingBean
 	public Log4j2Metrics log4j2Metrics() {
-		return new Log4j2Metrics();
+		return new Log4j2Metrics(Collections.emptyList(),
+				(LoggerContext) LogManager.getContext(false));
+	}
+
+	static class Log4JCoreLoggerContextCondition extends SpringBootCondition {
+
+		@Override
+		public ConditionOutcome getMatchOutcome(ConditionContext context,
+				AnnotatedTypeMetadata metadata) {
+			org.apache.logging.log4j.spi.LoggerContext loggerContext = LogManager
+					.getContext(false);
+			if (loggerContext instanceof LoggerContext) {
+				return ConditionOutcome.match(
+						"LoggerContext was an instance of org.apache.logging.log4j.spi.LoggerContext");
+			}
+			return ConditionOutcome.noMatch(
+					"Logger context was not an instance of org.apache.logging.log4j.spi.LoggerContext");
+		}
+
 	}
 
 }

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/Log4J2MetricsWithSlf4jLoggerContextAutoConfigurationTests.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2012-2018 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
+ *
+ *      http://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.metrics;
+
+import io.micrometer.core.instrument.binder.logging.Log4j2Metrics;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.actuate.autoconfigure.metrics.test.MetricsRun;
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.boot.testsupport.runner.classpath.ClassPathExclusions;
+import org.springframework.boot.testsupport.runner.classpath.ClassPathOverrides;
+import org.springframework.boot.testsupport.runner.classpath.ModifiedClassPathRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link Log4J2MetricsAutoConfiguration}.
+ *
+ * @author Andy Wilkinson
+ */
+@RunWith(ModifiedClassPathRunner.class)
+@ClassPathOverrides({ "org.apache.logging.log4j:log4j-to-slf4j:2.11.1",
+		"org.apache.logging.log4j:log4j-core:2.11.1" })
+@ClassPathExclusions("log4j-slf4j-impl-*.jar")
+public class Log4J2MetricsWithSlf4jLoggerContextAutoConfigurationTests {
+
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+			.with(MetricsRun.simple()).withConfiguration(
+					AutoConfigurations.of(Log4J2MetricsAutoConfiguration.class));
+
+	@Test
+	public void backsOffWhenLoggerContextIsBackedBySlf4j() {
+		this.contextRunner.run(
+				(context) -> assertThat(context).doesNotHaveBean(Log4j2Metrics.class));
+	}
+
+}