Improve Log4J log level reset support

Update `Log4J2LoggingSystem` so that call to `setLevel` with a `null`
level with remove the logger if it was previously configured by a
`LoggingSystem` call.

To track which loggers have been configured by us, and which have been
configure directly by the user, a custom `LoggerConfig` subclass is
used. We'll only remove `LevelSetLoggerConfig` classes, for any others
we'll call `setLevel(null)` on the config.

Prior to this commit, it was impossible to set then reset a logger
level using the actuator endpoint. This is because Log4J doesn't provide
a way to get the actual configured level. If the `setLevel(null)` has
been applied, then `getLevel()` will return the value of the parent
logger or a default value of `ERROR`.

Fixes gh-24298

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

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -214,16 +214,37 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 
 	@Override
 	public void setLogLevel(String loggerName, LogLevel logLevel) {
-		Level level = LEVELS.convertSystemToNative(logLevel);
+		setLogLevel(loggerName, LEVELS.convertSystemToNative(logLevel));
+	}
+
+	private void setLogLevel(String loggerName, Level level) {
 		LoggerConfig logger = getLogger(loggerName);
+		if (level == null) {
+			clearLogLevel(loggerName, logger);
+		}
+		else {
+			setLogLevel(loggerName, logger, level);
+		}
+		getLoggerContext().updateLoggers();
+	}
+
+	private void clearLogLevel(String loggerName, LoggerConfig logger) {
+		if (logger instanceof LevelSetLoggerConfig) {
+			getLoggerContext().getConfiguration().removeLogger(loggerName);
+		}
+		else {
+			logger.setLevel(null);
+		}
+	}
+
+	private void setLogLevel(String loggerName, LoggerConfig logger, Level level) {
 		if (logger == null) {
-			logger = new LoggerConfig(loggerName, level, true);
-			getLoggerContext().getConfiguration().addLogger(loggerName, logger);
+			getLoggerContext().getConfiguration().addLogger(loggerName,
+					new LevelSetLoggerConfig(loggerName, level, true));
 		}
 		else {
 			logger.setLevel(level);
 		}
-		getLoggerContext().updateLoggers();
 	}
 
 	@Override
@@ -348,4 +369,15 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 
 	}
 
+	/**
+	 * {@link LoggerConfig} used when the user has set a specific {@link Level}.
+	 */
+	private static class LevelSetLoggerConfig extends LoggerConfig {
+
+		LevelSetLoggerConfig(String name, Level level, boolean additive) {
+			super(name, level, additive);
+		}
+
+	}
+
 }

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

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,6 @@ import java.util.EnumSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.logging.Level;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
 import org.apache.commons.logging.Log;
@@ -34,6 +33,7 @@ import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.Configuration;
+import org.apache.logging.log4j.core.config.LoggerConfig;
 import org.apache.logging.log4j.core.config.Reconfigurable;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -241,7 +241,7 @@ class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 		this.loggingSystem.beforeInitialize();
 		this.loggingSystem.initialize(null, null, null);
 		java.util.logging.Logger julLogger = java.util.logging.Logger.getLogger(getClass().getName());
-		julLogger.setLevel(Level.INFO);
+		julLogger.setLevel(java.util.logging.Level.INFO);
 		julLogger.severe("Hello world");
 		assertThat(output).contains("Hello world");
 	}
@@ -338,6 +338,38 @@ class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 		verify(listener, times(4)).propertyChange(any(PropertyChangeEvent.class));
 	}
 
+	@Test
+	void getLoggingConfigurationWithResetLevelReturnsNull() {
+		this.loggingSystem.beforeInitialize();
+		this.loggingSystem.initialize(null, null, null);
+		this.loggingSystem.setLogLevel("com.example", LogLevel.WARN);
+		this.loggingSystem.setLogLevel("com.example.test", LogLevel.DEBUG);
+		LoggerConfiguration configuration = this.loggingSystem.getLoggerConfiguration("com.example.test");
+		assertThat(configuration)
+				.isEqualTo(new LoggerConfiguration("com.example.test", LogLevel.DEBUG, LogLevel.DEBUG));
+		this.loggingSystem.setLogLevel("com.example.test", null);
+		LoggerConfiguration updatedConfiguration = this.loggingSystem.getLoggerConfiguration("com.example.test");
+		assertThat(updatedConfiguration).isNull();
+	}
+
+	@Test
+	void getLoggingConfigurationWithResetLevelWhenAlreadyConfiguredReturnsParentConfiguredLevel() {
+		LoggerContext loggerContext = (LoggerContext) LogManager.getContext(false);
+		this.loggingSystem.beforeInitialize();
+		this.loggingSystem.initialize(null, null, null);
+		loggerContext.getConfiguration().addLogger("com.example.test",
+				new LoggerConfig("com.example.test", org.apache.logging.log4j.Level.INFO, false));
+		this.loggingSystem.setLogLevel("com.example", LogLevel.WARN);
+		this.loggingSystem.setLogLevel("com.example.test", LogLevel.DEBUG);
+		LoggerConfiguration configuration = this.loggingSystem.getLoggerConfiguration("com.example.test");
+		assertThat(configuration)
+				.isEqualTo(new LoggerConfiguration("com.example.test", LogLevel.DEBUG, LogLevel.DEBUG));
+		this.loggingSystem.setLogLevel("com.example.test", null);
+		LoggerConfiguration updatedConfiguration = this.loggingSystem.getLoggerConfiguration("com.example.test");
+		assertThat(updatedConfiguration)
+				.isEqualTo(new LoggerConfiguration("com.example.test", LogLevel.WARN, LogLevel.WARN));
+	}
+
 	private String getRelativeClasspathLocation(String fileName) {
 		String defaultPath = ClassUtils.getPackageName(getClass());
 		defaultPath = defaultPath.replace('.', '/');