Avoid changing root logger level when setting level of unconfigured logger

Previously getLoggerConfig(loggerName) was used to retrieve the
LoggerConfig object on which the level was to be set. As described in the
method’s javadoc it will “remove tokens from the package name as
necessary or return the root LoggerConfig if no other matches were found”.
This is problematic as, if there’s no configuration for the logger whose
level is being configured, the level will be applied to a logger from an
outer package or to the root logger.

This commit updates Log4J2LoggingSystem to use the configuration’s map of
LoggerConfigs, rather than calling getLoggerConfig. In the event of the
level being set on an unconfigured logger, this will produce a null
LoggerConfig. When a null LoggerConfig is encountered, a new one is
created with the appropriate level. If the config already exists, its
level is set as it was before.

The code that was accessing the root logger using a magic null value
(which was then coerced into the root logger’s name (an empty string))
has also been updated to make it clearer that they are purposefully
dealing with the root logger.

Closes gh-3550

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

@@ -121,12 +121,12 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 	@Override
 	public void beforeInitialize() {
 		super.beforeInitialize();
-		getLoggerConfig(null).addFilter(FILTER);
+		getRootLoggerConfig().addFilter(FILTER);
 	}
 
 	@Override
 	public void initialize(String configLocation, LogFile logFile) {
-		getLoggerConfig(null).removeFilter(FILTER);
+		getRootLoggerConfig().removeFilter(FILTER);
 		super.initialize(configLocation, logFile);
 	}
 
@@ -164,15 +164,25 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 	}
 
 	@Override
-	public void setLogLevel(String loggerName, LogLevel level) {
-		getLoggerConfig(loggerName).setLevel(LEVELS.get(level));
+	public void setLogLevel(String loggerName, LogLevel logLevel) {
+		Level level = LEVELS.get(logLevel);
+		LoggerConfig loggerConfig = getLoggerConfig(loggerName);
+		if (loggerConfig == null) {
+			loggerConfig = new LoggerConfig(loggerName, level, true);
+			getLoggerContext().getConfiguration().addLogger(loggerName, loggerConfig);
+		}
+		else {
+			loggerConfig.setLevel(level);
+		}
 		getLoggerContext().updateLoggers();
 	}
 
+	private LoggerConfig getRootLoggerConfig() {
+		return getLoggerContext().getConfiguration().getLoggerConfig("");
+	}
+
 	private LoggerConfig getLoggerConfig(String loggerName) {
-		LoggerConfig loggerConfig = getLoggerContext().getConfiguration()
-				.getLoggerConfig(loggerName == null ? "" : loggerName);
-		return loggerConfig;
+		return getLoggerContext().getConfiguration().getLoggers().get(loggerName);
 	}
 
 	private LoggerContext getLoggerContext() {

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

@@ -34,9 +34,11 @@ import org.springframework.util.StringUtils;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
 
+import static org.hamcrest.CoreMatchers.containsString;
 import static org.hamcrest.Matchers.arrayContaining;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.not;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
@@ -115,6 +117,17 @@ public class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 				equalTo(1));
 	}
 
+	@Test
+	public void setLevelOfUnconfiguredLoggerDoesNotAffectRootConfiguration()
+			throws Exception {
+		this.loggingSystem.beforeInitialize();
+		this.loggingSystem.initialize(null, null);
+		LogManager.getRootLogger().debug("Hello");
+		this.loggingSystem.setLogLevel("foo.bar.baz", LogLevel.DEBUG);
+		LogManager.getRootLogger().debug("Hello");
+		assertThat(this.output.toString(), not(containsString("Hello")));
+	}
+
 	@Test
 	@Ignore("Fails on Bamboo")
 	public void loggingThatUsesJulIsCaptured() {