Use Log4jBridgeHandler to route JUL-based logging into Log4j 2

Since version 2.15.0 `log4j-jul` contains a `Log4jBridgeHandler`,
that forwards JUL to Log4j 2.x and synchronizes the logger levels of
the two frameworks.

This commmit adds support for the `Log4jBridgeHandler` and sets it as
the bridge handler for the Log4j 2.x stack, replacing the existing
JUL to SLF4J bridge that was used previously.

See gh-30003

spring-boot-project/spring-boot-starters/spring-boot-starter-log4j2/build.gradle

@@ -8,5 +8,4 @@ dependencies {
 	api("org.apache.logging.log4j:log4j-slf4j-impl")
 	api("org.apache.logging.log4j:log4j-core")
 	api("org.apache.logging.log4j:log4j-jul")
-	api("org.slf4j:jul-to-slf4j")
 }

spring-boot-project/spring-boot/build.gradle

@@ -52,6 +52,7 @@ dependencies {
 	optional("org.apache.httpcomponents.client5:httpclient5")
 	optional("org.apache.logging.log4j:log4j-api")
 	optional("org.apache.logging.log4j:log4j-core")
+	optional("org.apache.logging.log4j:log4j-jul")
 	optional("org.apache.tomcat.embed:tomcat-embed-core")
 	optional("org.apache.tomcat.embed:tomcat-embed-jasper")
 	optional("org.apache.tomcat:tomcat-jdbc")

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/Slf4JLoggingSystem.java

@@ -87,7 +87,7 @@ public abstract class Slf4JLoggingSystem extends AbstractLoggingSystem {
 		return ClassUtils.isPresent(BRIDGE_HANDLER, getClassLoader());
 	}
 
-	private boolean isJulUsingASingleConsoleHandlerAtMost() {
+	protected boolean isJulUsingASingleConsoleHandlerAtMost() {
 		Logger rootLogger = LogManager.getLogManager().getLogger("");
 		Handler[] handlers = rootLogger.getHandlers();
 		return handlers.length == 0 || (handlers.length == 1 && handlers[0] instanceof ConsoleHandler);
@@ -103,7 +103,7 @@ public abstract class Slf4JLoggingSystem extends AbstractLoggingSystem {
 		}
 	}
 
-	private void removeDefaultRootHandler() {
+	protected void removeDefaultRootHandler() {
 		try {
 			Logger rootLogger = LogManager.getLogManager().getLogger("");
 			Handler[] handlers = rootLogger.getHandlers();

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

@@ -25,6 +25,7 @@ import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.logging.Handler;
 import java.util.stream.Collectors;
 
 import org.apache.logging.log4j.Level;
@@ -42,6 +43,7 @@ import org.apache.logging.log4j.core.config.LoggerConfig;
 import org.apache.logging.log4j.core.config.composite.CompositeConfiguration;
 import org.apache.logging.log4j.core.filter.AbstractFilter;
 import org.apache.logging.log4j.core.util.NameUtil;
+import org.apache.logging.log4j.jul.Log4jBridgeHandler;
 import org.apache.logging.log4j.message.Message;
 
 import org.springframework.boot.context.properties.bind.BindResult;
@@ -75,6 +77,10 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 
 	private static final String FILE_PROTOCOL = "file";
 
+	private static final String LOG4J_BRIDGE_HANDLER = "org.apache.logging.log4j.jul.Log4jBridgeHandler";
+
+	private static final String LOG4J_LOG_MANAGER = "org.apache.logging.log4j.jul.LogManager";
+
 	private static final LogLevels<Level> LEVELS = new LogLevels<>();
 
 	static {
@@ -155,10 +161,51 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 		if (isAlreadyInitialized(loggerContext)) {
 			return;
 		}
-		super.beforeInitialize();
+		if (!configureJdkLoggingBridgeHandler()) {
+			super.beforeInitialize();
+		}
 		loggerContext.getConfiguration().addFilter(FILTER);
 	}
 
+	private boolean configureJdkLoggingBridgeHandler() {
+		try {
+			if (isJulUsingASingleConsoleHandlerAtMost() && !isLog4jLogManagerInstalled()) {
+				if (isLog4jBridgeHandlerAvailable()) {
+					removeDefaultRootHandler();
+					installLog4jBridgeHandler();
+					return true;
+				}
+			}
+		}
+		catch (Throwable ex) {
+			// Ignore. No java.util.logging bridge is installed.
+		}
+		return false;
+	}
+
+	private boolean isLog4jLogManagerInstalled() {
+		final String logManagerClassName = java.util.logging.LogManager.getLogManager().getClass().getName();
+		return LOG4J_LOG_MANAGER.equals(logManagerClassName);
+	}
+
+	private boolean isLog4jBridgeHandlerAvailable() {
+		return ClassUtils.isPresent(LOG4J_BRIDGE_HANDLER, getClassLoader());
+	}
+
+	private void installLog4jBridgeHandler() {
+		Log4jBridgeHandler.install(false, null, true);
+	}
+
+	private void removeLog4jBridgeHandler() {
+		java.util.logging.Logger rootLogger = java.util.logging.LogManager.getLogManager().getLogger("");
+		for (final Handler handler : rootLogger.getHandlers()) {
+			if (handler instanceof Log4jBridgeHandler) {
+				handler.close();
+				rootLogger.removeHandler(handler);
+			}
+		}
+	}
+
 	@Override
 	public void initialize(LoggingInitializationContext initializationContext, String configLocation, LogFile logFile) {
 		LoggerContext loggerContext = getLoggerContext();
@@ -366,6 +413,9 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 
 	@Override
 	public void cleanUp() {
+		if (isLog4jBridgeHandlerAvailable()) {
+			removeLog4jBridgeHandler();
+		}
 		super.cleanUp();
 		LoggerContext loggerContext = getLoggerContext();
 		markAsUninitialized(loggerContext);

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

@@ -25,6 +25,8 @@ import java.util.EnumSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.logging.Handler;
+import java.util.logging.Level;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
 import org.apache.commons.logging.Log;
@@ -37,10 +39,10 @@ import org.apache.logging.log4j.core.config.LoggerConfig;
 import org.apache.logging.log4j.core.config.Reconfigurable;
 import org.apache.logging.log4j.core.config.composite.CompositeConfiguration;
 import org.apache.logging.log4j.core.util.ShutdownCallbackRegistry;
+import org.apache.logging.log4j.jul.Log4jBridgeHandler;
 import org.apache.logging.log4j.util.PropertiesUtil;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 
@@ -247,7 +249,6 @@ class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 	}
 
 	@Test
-	@Disabled("Uses Logback unintentionally")
 	void loggingThatUsesJulIsCaptured(CapturedOutput output) {
 		this.loggingSystem.beforeInitialize();
 		this.loggingSystem.initialize(this.initializationContext, null, null);
@@ -381,6 +382,22 @@ class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 				.isEqualTo(new LoggerConfiguration("com.example.test", LogLevel.WARN, LogLevel.WARN));
 	}
 
+	@Test
+	void log4jLevelsArePropagatedToJul() {
+		this.loggingSystem.beforeInitialize();
+		java.util.logging.Logger rootLogger = java.util.logging.Logger.getLogger("");
+		// check if Log4jBridgeHandler is used
+		Handler[] handlers = rootLogger.getHandlers();
+		assertThat(handlers.length).isEqualTo(1);
+		assertThat(handlers[0]).isInstanceOf(Log4jBridgeHandler.class);
+
+		this.loggingSystem.initialize(this.initializationContext, null, null);
+		java.util.logging.Logger logger = java.util.logging.Logger.getLogger(Log4J2LoggingSystemTests.class.getName());
+		assertThat(logger.getLevel()).isNull();
+		this.loggingSystem.setLogLevel(Log4J2LoggingSystemTests.class.getName(), LogLevel.DEBUG);
+		assertThat(logger.getLevel()).isEqualTo(Level.FINE);
+	}
+
 	@Test
 	void shutdownHookIsDisabled() {
 		assertThat(