从源码分析 SpringBoot 的 LoggingSystem → 它是如何绑定日志组件的

@Override
public void onApplicationEvent(ApplicationEvent event) {
	if (event instanceof ApplicationStartingEvent) {
		onApplicationStartingEvent((ApplicationStartingEvent) event);
	}
	else if (event instanceof ApplicationEnvironmentPreparedEvent) {
		onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
	}
	else if (event instanceof ApplicationPreparedEvent) {
		onApplicationPreparedEvent((ApplicationPreparedEvent) event);
	}
	else if (event instanceof ContextClosedEvent) {
		onContextClosedEvent((ContextClosedEvent) event);
	}
	else if (event instanceof ApplicationFailedEvent) {
		onApplicationFailedEvent();
	}
}

private void onApplicationStartingEvent(ApplicationStartingEvent event) {
	this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
	this.loggingSystem.beforeInitialize();
}

public static LoggingSystem get(ClassLoader classLoader) {
	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;
}

/**
 * {@link LoggingSystem} that does nothing.
 */
static class NoOpLoggingSystem extends LoggingSystem {

	@Override
	public void beforeInitialize() {

	}

	@Override
	public void setLogLevel(String loggerName, LogLevel level) {

	}

	@Override
	public List<LoggerConfiguration> getLoggerConfigurations() {
		return Collections.emptyList();
	}

	@Override
	public LoggerConfiguration getLoggerConfiguration(String loggerName) {
		return null;
	}

}

@Override
public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
	List<LoggingSystemFactory> delegates = (this.delegates != null) ? this.delegates.apply(classLoader) : null;
	if (delegates != null) {
		for (LoggingSystemFactory delegate : delegates) {
			LoggingSystem loggingSystem = delegate.getLoggingSystem(classLoader);
			if (loggingSystem != null) {
				return loggingSystem;
			}
		}
	}
	return null;
}

/**
 * Return a {@link LoggingSystemFactory} backed by {@code spring.factories}.
 * @return a {@link LoggingSystemFactory} instance
 */
static LoggingSystemFactory fromSpringFactories() {
	return new DelegatingLoggingSystemFactory(
			(classLoader) -> SpringFactoriesLoader.loadFactories(LoggingSystemFactory.class, classLoader));
}

private static final boolean PRESENT = ClassUtils.isPresent("ch.qos.logback.classic.LoggerContext",
		Factory.class.getClassLoader());

@Override
public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
	if (PRESENT) {
		return new LogbackLoggingSystem(classLoader);
	}
	return null;
}

@Override
public void beforeInitialize() {
	LoggerContext loggerContext = getLoggerContext();
	if (isAlreadyInitialized(loggerContext)) {
		return;
	}
	super.beforeInitialize();
	loggerContext.getTurboFilterList().add(FILTER);
}

private LoggerContext getLoggerContext() {
	ILoggerFactory factory = StaticLoggerBinder.getSingleton().getLoggerFactory();
	Assert.isInstanceOf(LoggerContext.class, factory,
			() -> String.format(
					"LoggerFactory is not a Logback LoggerContext but Logback is on "
							+ "the classpath. Either remove Logback or the competing "
							+ "implementation (%s loaded from %s). If you are using "
							+ "WebLogic you will need to add 'org.slf4j' to "
							+ "prefer-application-packages in WEB-INF/weblogic.xml",
					factory.getClass(), getLocation(factory)));
	return (LoggerContext) factory;
}

private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
	SpringApplication springApplication = event.getSpringApplication();
	if (this.loggingSystem == null) {
		this.loggingSystem = LoggingSystem.get(springApplication.getClassLoader());
	}
	initialize(event.getEnvironment(), springApplication.getClassLoader());
}

protected void initialize(ConfigurableEnvironment environment, ClassLoader classLoader) {
	getLoggingSystemProperties(environment).apply();
	this.logFile = LogFile.get(environment);
	if (this.logFile != null) {
		this.logFile.applyToSystemProperties();
	}
	// 日志分组，暂不关注
	this.loggerGroups = new LoggerGroups(DEFAULT_GROUP_LOGGERS);
	// 设置早期日志级别，主要debug和trace之间的抉择
	initializeEarlyLoggingLevel(environment);
	// 初始化日志系统
	initializeSystem(environment, this.loggingSystem, this.logFile);
	// 设置最终日志级别
	initializeFinalLoggingLevels(environment, this.loggingSystem);
	registerShutdownHookIfNecessary(environment, this.loggingSystem);
}

private void initializeWithConventions(LoggingInitializationContext initializationContext, LogFile logFile) {
	String config = getSelfInitializationConfig();
	if (config != null && logFile == null) {
		// self initialization has occurred, reinitialize in case of property changes
		reinitialize(initializationContext);
		return;
	}
	if (config == null) {
		config = getSpringInitializationConfig();
	}
	if (config != null) {
		loadConfiguration(initializationContext, config, logFile);
		return;
	}
	loadDefaults(initializationContext, logFile);
}

protected String[] getSpringConfigLocations() {
	String[] locations = getStandardConfigLocations();
	for (int i = 0; i < locations.length; i++) {
		String extension = StringUtils.getFilenameExtension(locations[i]);
		locations[i] = locations[i].substring(0, locations[i].length() - extension.length() - 1) + "-spring."
				+ extension;
	}
	return locations;
}

private void onApplicationPreparedEvent(ApplicationPreparedEvent event) {
	ConfigurableApplicationContext applicationContext = event.getApplicationContext();
	ConfigurableListableBeanFactory beanFactory = applicationContext.getBeanFactory();
	if (!beanFactory.containsBean(LOGGING_SYSTEM_BEAN_NAME)) {
		beanFactory.registerSingleton(LOGGING_SYSTEM_BEAN_NAME, this.loggingSystem);
	}
	if (this.logFile != null && !beanFactory.containsBean(LOG_FILE_BEAN_NAME)) {
		beanFactory.registerSingleton(LOG_FILE_BEAN_NAME, this.logFile);
	}
	if (this.loggerGroups != null && !beanFactory.containsBean(LOGGER_GROUPS_BEAN_NAME)) {
		beanFactory.registerSingleton(LOGGER_GROUPS_BEAN_NAME, this.loggerGroups);
	}
	if (!beanFactory.containsBean(LOGGING_LIFECYCLE_BEAN_NAME) && applicationContext.getParent() == null) {
		beanFactory.registerSingleton(LOGGING_LIFECYCLE_BEAN_NAME, new Lifecycle());
	}
}

public static Log createLog(String name) {
	switch (logApi) {
		case LOG4J:
			return Log4jAdapter.createLog(name);
		case SLF4J_LAL:
			return Slf4jAdapter.createLocationAwareLog(name);
		case SLF4J:
			return Slf4jAdapter.createLog(name);
		default:
			// Defensively use lazy-initializing adapter class here as well since the
			// java.logging module is not present by default on JDK 9. We are requiring
			// its presence if neither Log4j nor SLF4J is available; however, in the
			// case of Log4j or SLF4J, we are trying to prevent early initialization
			// of the JavaUtilLog adapter - e.g. by a JVM in debug mode - when eagerly
			// trying to parse the bytecode for all the cases of this switch clause.
			return JavaUtilAdapter.createLog(name);
	}
}

private static final String LOG4J_SPI = "org.apache.logging.log4j.spi.ExtendedLogger";

private static final String LOG4J_SLF4J_PROVIDER = "org.apache.logging.slf4j.SLF4JProvider";

private static final String SLF4J_SPI = "org.slf4j.spi.LocationAwareLogger";

private static final String SLF4J_API = "org.slf4j.Logger";

private static final LogApi logApi;

static {
	if (isPresent(LOG4J_SPI)) {
		if (isPresent(LOG4J_SLF4J_PROVIDER) && isPresent(SLF4J_SPI)) {
			// log4j-to-slf4j bridge -> we'll rather go with the SLF4J SPI;
			// however, we still prefer Log4j over the plain SLF4J API since
			// the latter does not have location awareness support.
			logApi = LogApi.SLF4J_LAL;
		}
		else {
			// Use Log4j 2.x directly, including location awareness support
			logApi = LogApi.LOG4J;
		}
	}
	else if (isPresent(SLF4J_SPI)) {
		// Full SLF4J SPI including location awareness support
		logApi = LogApi.SLF4J_LAL;
	}
	else if (isPresent(SLF4J_API)) {
		// Minimal SLF4J API without location awareness support
		logApi = LogApi.SLF4J;
	}
	else {
		// java.util.logging as default
		logApi = LogApi.JUL;
	}
}