public class LoggingApplicationListener implements GenericApplicationListener {

	private static final ConfigurationPropertyName LOGGING_LEVEL = ConfigurationPropertyName.of("logging.level");//默认日志级别level
	public static final String LOGGING_SYSTEM_BEAN_NAME = "springBootLoggingSystem";
private static final Map<String, List<String>> DEFAULT_GROUP_LOGGERS;
	static {//静态代码块表示可以配置输出的日志范围
		MultiValueMap<String, String> loggers = new LinkedMultiValueMap<>();
		loggers.add("web", "org.springframework.core.codec");
		loggers.add("web", "org.springframework.http");
		loggers.add("web", "org.springframework.web");
		loggers.add("web", "org.springframework.boot.actuate.endpoint.web");
		loggers.add("web", "org.springframework.boot.web.servlet.ServletContextInitializerBeans");
		loggers.add("sql", "org.springframework.jdbc.core");
		loggers.add("sql", "org.hibernate.SQL");
		loggers.add("sql", "org.jooq.tools.LoggerListener");
		DEFAULT_GROUP_LOGGERS = Collections.unmodifiableMap(loggers);
	}
	}
//进行一系列初始化环境加载操作，也就是把配置文件里配置的东西初始化
	protected void initialize(ConfigurableEnvironment environment, ClassLoader classLoader) {
		getLoggingSystemProperties(environment).apply();
		this.logFile = LogFile.get(environment);
		if (this.logFile != null) {
			this.logFile.applyToSystemProperties();
		}
		this.loggerGroups = new LoggerGroups(DEFAULT_GROUP_LOGGERS);
		initializeEarlyLoggingLevel(environment);
		initializeSystem(environment, this.loggingSystem, this.logFile);
		initializeFinalLoggingLevels(environment, this.loggingSystem);
		registerShutdownHookIfNecessary(environment, this.loggingSystem);
	}
	private void initializeSystem(ConfigurableEnvironment environment, LoggingSystem system, LogFile logFile) {
		String logConfig = StringUtils.trimWhitespace(environment.getProperty(CONFIG_PROPERTY));
		try {
			LoggingInitializationContext initializationContext = new LoggingInitializationContext(environment);
			if (ignoreLogConfig(logConfig)) {
				system.initialize(initializationContext, null, logFile);
			}
			else {
				system.initialize(initializationContext, logConfig, logFile);
			}
		}
		catch (Exception ex) {
			Throwable exceptionToReport = ex;
			while (exceptionToReport != null && !(exceptionToReport instanceof FileNotFoundException)) {
				exceptionToReport = exceptionToReport.getCause();
			}
			exceptionToReport = (exceptionToReport != null) ? exceptionToReport : ex;
			// NOTE: We can't use the logger here to report the problem
			System.err.println("Logging system failed to initialize using configuration from '" + logConfig + "'");
			exceptionToReport.printStackTrace(System.err);
			throw new IllegalStateException(ex);
		}
	}

	private boolean ignoreLogConfig(String logConfig) {
		return !StringUtils.hasLength(logConfig) || logConfig.startsWith("-D");
	}
//初始化日志级别
	private void initializeFinalLoggingLevels(ConfigurableEnvironment environment, LoggingSystem system) {
		bindLoggerGroups(environment);
		if (this.springBootLogging != null) {
			initializeSpringBootLogging(system, this.springBootLogging);
		}
		setLogLevels(system, environment);
	}

	private void bindLoggerGroups(ConfigurableEnvironment environment) {
		if (this.loggerGroups != null) {
			Binder binder = Binder.get(environment);
			binder.bind(LOGGING_GROUP, STRING_STRINGS_MAP).ifBound(this.loggerGroups::putAll);
		}
	}
	protected void initializeSpringBootLogging(LoggingSystem system, LogLevel springBootLogging) {
		BiConsumer<String, LogLevel> configurer = getLogLevelConfigurer(system);
		SPRING_BOOT_LOGGING_LOGGERS.getOrDefault(springBootLogging, Collections.emptyList())
				.forEach((name) -> configureLogLevel(name, springBootLogging, configurer));
	}

	/**
	 * Set logging levels based on relevant {@link Environment} properties.
	 * @param system the logging system
	 * @param environment the environment
	 */
	protected void setLogLevels(LoggingSystem system, ConfigurableEnvironment environment) {
		BiConsumer<String, LogLevel> customizer = getLogLevelConfigurer(system);
		Binder binder = Binder.get(environment);
		Map<String, LogLevel> levels = binder.bind(LOGGING_LEVEL, STRING_LOGLEVEL_MAP).orElseGet(Collections::emptyMap);
		levels.forEach((name, level) -> configureLogLevel(name, level, customizer));
	}

	private void configureLogLevel(String name, LogLevel level, BiConsumer<String, LogLevel> configurer) {
		if (this.loggerGroups != null) {
			LoggerGroup group = this.loggerGroups.get(name);
			if (group != null && group.hasMembers()) {
				group.configureLogLevel(level, configurer);
				return;
			}
		}
		configurer.accept(name, level);
	}

	private BiConsumer<String, LogLevel> getLogLevelConfigurer(LoggingSystem system) {
		return (name, level) -> {
			try {
				name = name.equalsIgnoreCase(LoggingSystem.ROOT_LOGGER_NAME) ? null : name;
				system.setLogLevel(name, level);
			}
			catch (RuntimeException ex) {
				this.logger.error(LogMessage.format("Cannot set level '%s' for '%s'", level, name));
			}
		};
	}

	private void registerShutdownHookIfNecessary(Environment environment, LoggingSystem loggingSystem) {
		boolean registerShutdownHook = environment.getProperty(REGISTER_SHUTDOWN_HOOK_PROPERTY, Boolean.class, false);
		if (registerShutdownHook) {
			Runnable shutdownHandler = loggingSystem.getShutdownHandler();
			if (shutdownHandler != null && shutdownHookRegistered.compareAndSet(false, true)) {
				registerShutdownHook(new Thread(shutdownHandler));
			}
		}
	}

	void registerShutdownHook(Thread shutdownHook) {
		Runtime.getRuntime().addShutdownHook(shutdownHook);
	}

	public void setOrder(int order) {
		this.order = order;
	}

	@Override
	public int getOrder() {
		return this.order;
	}

	/**
	 * Sets a custom logging level to be used for Spring Boot and related libraries.
	 * @param springBootLogging the logging level
	 */
	public void setSpringBootLogging(LogLevel springBootLogging) {
		this.springBootLogging = springBootLogging;
	}

	/**
	 * Sets if initialization arguments should be parsed for {@literal debug} and
	 * {@literal trace} properties (usually defined from {@literal --debug} or
	 * {@literal --trace} command line args). Defaults to {@code true}.
	 * @param parseArgs if arguments should be parsed
	 */
	public void setParseArgs(boolean parseArgs) {
		this.parseArgs = parseArgs;
	}

}

private static final LoggingSystemFactory SYSTEM_FACTORY = LoggingSystemFactory.fromSpringFactories();

/**
	 * Detect and return the logging system in use. Supports Logback and Java Logging.
	 * @param classLoader the classloader
	 * @return the logging system
	 */
	public static LoggingSystem get(ClassLoader classLoader) {
        //SYSTEMS中存放了SpringBoot中Logback，Log4j2，Java Util Logging几个日志框架适配器的路径，检查这些类适配器是否存在以判断这些日志框架是否引入
		String loggingSystemClassName = System.getProperty(SYSTEM_PROPERTY);
		if (StringUtils.hasLength(loggingSystemClassName)) {
			if (NONE.equals(loggingSystemClassName)) {
				return new NoOpLoggingSystem();
			}
			return get(classLoader, loggingSystemClassName);
		}
		LoggingSystem loggingSystem = SYSTEM_FACTORY.getLoggingSystem(classLoader);
		Assert.state(loggingSystem != null, "No suitable logging system located");
		return loggingSystem;
	}