Tolerate parallel init of SLF4J

Closes gh-37477

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java

@@ -43,6 +43,7 @@
 import org.slf4j.LoggerFactory;
 import org.slf4j.Marker;
 import org.slf4j.bridge.SLF4JBridgeHandler;
+import org.slf4j.helpers.SubstituteLoggerFactory;
 
 import org.springframework.aot.AotDetector;
 import org.springframework.beans.factory.aot.BeanFactoryInitializationAotContribution;
@@ -384,7 +385,7 @@ private ch.qos.logback.classic.Logger getLogger(String name) {
 	}
 
 	private LoggerContext getLoggerContext() {
-		ILoggerFactory factory = LoggerFactory.getILoggerFactory();
+		ILoggerFactory factory = getLoggerFactory();
 		Assert.isInstanceOf(LoggerContext.class, factory,
 				() -> String.format(
 						"LoggerFactory is not a Logback LoggerContext but Logback is on "
@@ -396,6 +397,21 @@ private LoggerContext getLoggerContext() {
 		return (LoggerContext) factory;
 	}
 
+	private ILoggerFactory getLoggerFactory() {
+		ILoggerFactory factory = LoggerFactory.getILoggerFactory();
+		while (factory instanceof SubstituteLoggerFactory) {
+			try {
+				Thread.sleep(50);
+			}
+			catch (InterruptedException ex) {
+				Thread.currentThread().interrupt();
+				throw new IllegalStateException("Interrupted while waiting for non-subtitute logger factory", ex);
+			}
+			factory = LoggerFactory.getILoggerFactory();
+		}
+		return factory;
+	}
+
 	private Object getLocation(ILoggerFactory factory) {
 		try {
 			ProtectionDomain protectionDomain = factory.getClass().getProtectionDomain();

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/logback/LogbackLoggingSystemParallelInitializationTests.java

@@ -0,0 +1,76 @@
+/*
+ * 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.logging.logback;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import ch.qos.logback.classic.LoggerContext;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+import org.slf4j.LoggerFactory;
+
+import org.springframework.boot.logging.LoggingSystem;
+import org.springframework.boot.testsupport.classpath.ForkedClassPath;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for parallel initialization of {@link LogbackLoggingSystem} that are separate
+ * from {@link LogbackLoggingSystemTests}. This isolation allows them to have complete
+ * control over how and when the logging system is initialized.
+ *
+ * @author Andy Wilkinson
+ */
+class LogbackLoggingSystemParallelInitializationTests {
+
+	private final LoggingSystem loggingSystem = LoggingSystem
+		.get(LogbackLoggingSystemParallelInitializationTests.class.getClassLoader());
+
+	@AfterEach
+	void cleanUp() {
+		this.loggingSystem.cleanUp();
+		((LoggerContext) LoggerFactory.getILoggerFactory()).stop();
+	}
+
+	@Test
+	@ForkedClassPath
+	void noExceptionsAreThrownWhenBeforeInitializeIsCalledInParallel() {
+		List<Thread> threads = new ArrayList<>();
+		List<Throwable> exceptions = new CopyOnWriteArrayList<>();
+		for (int i = 0; i < 10; i++) {
+			Thread thread = new Thread(() -> this.loggingSystem.beforeInitialize());
+			thread.setUncaughtExceptionHandler((t, ex) -> exceptions.add(ex));
+			threads.add(thread);
+		}
+		threads.forEach(Thread::start);
+		threads.forEach(this::join);
+		assertThat(exceptions).isEmpty();
+	}
+
+	private void join(Thread thread) {
+		try {
+			thread.join();
+		}
+		catch (InterruptedException ex) {
+			Thread.currentThread().interrupt();
+			throw new RuntimeException(ex);
+		}
+	}
+
+}

src/checkstyle/checkstyle-suppressions.xml

@@ -10,6 +10,7 @@
 	<suppress files="LogbackInitializer\.java" checks="IllegalImport" />
 	<suppress files="LogbackLoggingSystem\.java" checks="IllegalImport" />
 	<suppress files="LogbackLoggingSystemTests\.java" checks="IllegalImport" />
+	<suppress files="LogbackLoggingSystemParallelInitializationTests\.java" checks="IllegalImport" />
 	<suppress files="LogbackConfigurationAotContributionTests\.java" checks="IllegalImport" />
 	<suppress files="MetricsAutoConfigurationMeterRegistryPostProcessorIntegrationTests\.java" checks="IllegalImport" message="LoggerFactory"/>
 	<suppress files="SpringApplicationTests\.java" checks="FinalClass" />