Make ROOT logger name case-insensitive when setting log level

Fixes gh-9693

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

@@ -341,17 +341,23 @@ public class LoggingApplicationListener implements GenericApplicationListener {
 	protected void setLogLevels(LoggingSystem system, Environment environment) {
 		Map<String, Object> levels = new RelaxedPropertyResolver(environment)
 				.getSubProperties("logging.level.");
+		boolean rootProcessed = false;
 		for (Entry<String, Object> entry : levels.entrySet()) {
-			setLogLevel(system, environment, entry.getKey(), entry.getValue().toString());
+			String name = entry.getKey();
+			if (name.equalsIgnoreCase(LoggingSystem.ROOT_LOGGER_NAME)) {
+				if (rootProcessed) {
+					return;
+				}
+				name = null;
+				rootProcessed = true;
+			}
+			setLogLevel(system, environment, name, entry.getValue().toString());
 		}
 	}
 
 	private void setLogLevel(LoggingSystem system, Environment environment, String name,
 			String level) {
 		try {
-			if (name.equalsIgnoreCase(LoggingSystem.ROOT_LOGGER_NAME)) {
-				name = null;
-			}
 			level = environment.resolvePlaceholders(level);
 			system.setLogLevel(name, coerceLogLevel(level));
 		}

spring-boot/src/test/java/org/springframework/boot/logging/LoggingApplicationListenerTests.java

@@ -18,6 +18,7 @@ package org.springframework.boot.logging;
 
 import java.io.File;
 import java.io.IOException;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
@@ -47,6 +48,8 @@ import org.springframework.context.ApplicationListener;
 import org.springframework.context.event.ContextClosedEvent;
 import org.springframework.context.event.SimpleApplicationEventMulticaster;
 import org.springframework.context.support.GenericApplicationContext;
+import org.springframework.core.env.MapPropertySource;
+import org.springframework.core.env.MutablePropertySources;
 import org.springframework.test.context.support.TestPropertySourceUtils;
 import org.springframework.test.util.ReflectionTestUtils;
 
@@ -481,6 +484,17 @@ public class LoggingApplicationListenerTests {
 		assertThat(System.getProperty("CONSOLE_LOG_PATTERN")).isEqualTo("console ${pid}");
 	}
 
+	@Test
+	public void lowPriorityPropertySourceShouldNotOverrideRootLoggerConfig() throws Exception {
+		MutablePropertySources propertySources = this.context.getEnvironment().getPropertySources();
+		propertySources.addFirst(new MapPropertySource("test1", Collections.<String, Object>singletonMap("logging.level.ROOT", "DEBUG")));
+		propertySources.addLast(new MapPropertySource("test2", Collections.<String, Object>singletonMap("logging.level.root", "WARN")));
+		this.initializer.initialize(this.context.getEnvironment(),
+				this.context.getClassLoader());
+		this.logger.debug("testatdebug");
+		assertThat(this.outputCapture.toString()).contains("testatdebug");
+	}
+
 	@Test
 	public void logFilePropertiesCanReferenceSystemProperties() {
 		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.context,