Propagates logback log levels to java.util.logging

Adds the LevelChangePropagator logback listener in order to propagate
Logback's log level changes to java.util.logging loggers.

Logback documentation :
http://logback.qos.ch/manual/configuration.html#LevelChangePropagator

Fixes gh-3924
Closes gh-3926

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

@@ -46,7 +46,7 @@ public abstract class Slf4JLoggingSystem extends AbstractLoggingSystem {
 
 	private void configureJdkLoggingBridgeHandler() {
 		try {
-			if (bridgeHandlerIsAvailable()) {
+			if (isBridgeHandlerAvailable()) {
 				removeJdkLoggingBridgeHandler();
 				SLF4JBridgeHandler.install();
 			}
@@ -56,13 +56,13 @@ public abstract class Slf4JLoggingSystem extends AbstractLoggingSystem {
 		}
 	}
 
-	private boolean bridgeHandlerIsAvailable() {
+	protected final boolean isBridgeHandlerAvailable() {
 		return ClassUtils.isPresent(BRIDGE_HANDLER, getClassLoader());
 	}
 
 	private void removeJdkLoggingBridgeHandler() {
 		try {
-			if (bridgeHandlerIsAvailable()) {
+			if (isBridgeHandlerAvailable()) {
 				try {
 					SLF4JBridgeHandler.removeHandlersForRootLogger();
 				}

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

@@ -37,6 +37,7 @@ import org.springframework.util.StringUtils;
 
 import ch.qos.logback.classic.Level;
 import ch.qos.logback.classic.LoggerContext;
+import ch.qos.logback.classic.jul.LevelChangePropagator;
 import ch.qos.logback.classic.turbo.TurboFilter;
 import ch.qos.logback.classic.util.ContextInitializer;
 import ch.qos.logback.core.spi.FilterReply;
@@ -99,8 +100,7 @@ public class LogbackLoggingSystem extends Slf4JLoggingSystem {
 	@Override
 	protected void loadDefaults(LogFile logFile) {
 		LoggerContext context = getLoggerContext();
-		context.stop();
+		stopAndReset(context);
-		context.reset();
 		LogbackConfigurator configurator = new LogbackConfigurator(context);
 		new DefaultLogbackConfiguration(logFile).apply(configurator);
 	}
@@ -112,8 +112,7 @@ public class LogbackLoggingSystem extends Slf4JLoggingSystem {
 			logFile.applyToSystemProperties();
 		}
 		LoggerContext context = getLoggerContext();
-		context.stop();
+		stopAndReset(context);
-		context.reset();
 		try {
 			URL url = ResourceUtils.getURL(location);
 			new ContextInitializer(context).configureByResource(url);
@@ -124,6 +123,21 @@ public class LogbackLoggingSystem extends Slf4JLoggingSystem {
 		}
 	}
 
+	private void stopAndReset(LoggerContext loggerContext) {
+		loggerContext.stop();
+		loggerContext.reset();
+		if (isBridgeHandlerAvailable()) {
+			addLevelChangePropagator(loggerContext);
+		}
+	}
+
+	private void addLevelChangePropagator(LoggerContext loggerContext) {
+		LevelChangePropagator levelChangePropagator = new LevelChangePropagator();
+		levelChangePropagator.setResetJUL(true);
+		levelChangePropagator.setContext(loggerContext);
+		loggerContext.addListener(levelChangePropagator);
+	}
+
 	@Override
 	protected void reinitialize() {
 		getLoggerContext().reset();

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

@@ -146,6 +146,18 @@ public class LogbackLoggingSystemTests extends AbstractLoggingSystemTests {
 		assertTrue("Wrong output:\n" + output, output.contains("Hello world"));
 	}
 
+	@Test
+	public void loggingLevelIsPropagatedToJulI() {
+		this.loggingSystem.beforeInitialize();
+		this.loggingSystem.initialize(null, null);
+		this.loggingSystem.setLogLevel(getClass().getName(), LogLevel.DEBUG);
+		java.util.logging.Logger julLogger = java.util.logging.Logger
+				.getLogger(getClass().getName());
+		julLogger.fine("Hello debug world");
+		String output = this.output.toString().trim();
+		assertTrue("Wrong output:\n" + output, output.contains("Hello debug world"));
+	}
+
 	@Test
 	public void jbossLoggingIsConfiguredToUseSlf4j() {
 		this.loggingSystem.beforeInitialize();