Merge branch '4.0.x'

Closes gh-49384

core/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystem.java

@@ -123,11 +123,14 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 	/**
 	 * Create a new {@link Log4J2LoggingSystem} instance.
 	 * @param classLoader the class loader to use.
-	 * @param loggerContext the {@link LoggerContext} to use.
+	 * @throws IllegalArgumentException if the logger context is not a
+	 * {@link LoggerContext}.
 	 */
-	Log4J2LoggingSystem(ClassLoader classLoader, LoggerContext loggerContext) {
+	Log4J2LoggingSystem(ClassLoader classLoader) {
 		super(classLoader);
-		this.loggerContext = loggerContext;
+		org.apache.logging.log4j.spi.LoggerContext spiLoggerContext = LogManager.getContext(classLoader, false);
+		Assert.isInstanceOf(LoggerContext.class, spiLoggerContext);
+		this.loggerContext = (LoggerContext) spiLoggerContext;
 	}
 
 	@Override
@@ -535,10 +538,11 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 		@Override
 		public @Nullable LoggingSystem getLoggingSystem(ClassLoader classLoader) {
 			if (PRESENT) {
-				org.apache.logging.log4j.spi.LoggerContext spiLoggerContext = LogManager.getContext(classLoader, false);
-				Assert.state(spiLoggerContext instanceof LoggerContext, "");
-				if (spiLoggerContext instanceof LoggerContext coreLoggerContext) {
-					return new Log4J2LoggingSystem(classLoader, coreLoggerContext);
+				try {
+					return new Log4J2LoggingSystem(classLoader);
+				}
+				catch (IllegalStateException ex) {
+					// Continue
 				}
 			}
 			return null;

core/spring-boot/src/test/java/org/springframework/boot/logging/LoggingSystemTests.java

@@ -65,6 +65,18 @@ class LoggingSystemTests {
 		assertThat(loggingSystem).isInstanceOf(NoOpLoggingSystem.class);
 	}
 
+	@Test
+	void log4j2CanBeForcedUsingSystemProperty() {
+		System.setProperty(LoggingSystem.SYSTEM_PROPERTY, Log4J2LoggingSystem.class.getName());
+		assertThat(LoggingSystem.get(getClass().getClassLoader())).isInstanceOf(Log4J2LoggingSystem.class);
+	}
+
+	@Test
+	void julCanBeForcedUsingSystemProperty() {
+		System.setProperty(LoggingSystem.SYSTEM_PROPERTY, JavaLoggingSystem.class.getName());
+		assertThat(LoggingSystem.get(getClass().getClassLoader())).isInstanceOf(JavaLoggingSystem.class);
+	}
+
 	@Test
 	void getLoggerConfigurationIsUnsupported() {
 		assertThatExceptionOfType(UnsupportedOperationException.class)

core/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystemTests.java

@@ -125,6 +125,7 @@ class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 		this.loggingSystem.getConfiguration().stop();
 		this.loggingSystem.cleanUp();
 		PluginRegistry.getInstance().clear();
+		LogManager.getFactory().removeContext(this.loggingSystem.getLoggerContext());
 		clearRollingPolicySystemProperties();
 	}
 

core/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/TestLog4J2LoggingSystem.java

@@ -16,7 +16,6 @@
 
 package org.springframework.boot.logging.log4j2;
 
-import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.Configuration;
 import org.jspecify.annotations.Nullable;
 
@@ -26,7 +25,8 @@ class TestLog4J2LoggingSystem extends Log4J2LoggingSystem {
 
 	TestLog4J2LoggingSystem(String contextName) {
 		// Tests add resources to the thread context classloader
-		super(Thread.currentThread().getContextClassLoader(), new LoggerContext(contextName));
+		super(Thread.currentThread().getContextClassLoader());
+		getLoggerContext().setName(contextName);
 		getLoggerContext().start();
 	}