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slf4j绑定logback源码解析

Logger log = LoggerFactory.getLogger(LogbackDemo.class);

如上述代码所示，在项目中通常会这样创建一个Logger对象去打印日志。
然后点进去，会走到LoggerFactory的getILoggerFactory()方法，如下代码所示。

	public static ILoggerFactory getILoggerFactory() {if (INITIALIZATION_STATE == UNINITIALIZED) {synchronized (LoggerFactory.class) {if (INITIALIZATION_STATE == UNINITIALIZED) {INITIALIZATION_STATE = ONGOING_INITIALIZATION;performInitialization();}}}switch (INITIALIZATION_STATE) {case SUCCESSFUL_INITIALIZATION:return StaticLoggerBinder.getSingleton().getLoggerFactory();case NOP_FALLBACK_INITIALIZATION:return NOP_FALLBACK_FACTORY;case FAILED_INITIALIZATION:throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);case ONGOING_INITIALIZATION:// support re-entrant behavior.// See also http://jira.qos.ch/browse/SLF4J-97return SUBST_FACTORY;}throw new IllegalStateException("Unreachable code");}

performInitialization()方法表示执行初始化，点进去会调用到LoggerFactory的bind()方法，如下代码所示。

			Set<URL> staticLoggerBinderPathSet = null;if (!isAndroid()) {// 查找org/slf4j/impl/StaticLoggerBinder.class这个类的路径staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();// 如果有多个则打印Class path contains multiple SLF4J bindingsreportMultipleBindingAmbiguity(staticLoggerBinderPathSet);}// StaticLoggerBinder类是各个日志框架提供的，比如logback，如下图所示StaticLoggerBinder.getSingleton();INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;reportActualBinding(staticLoggerBinderPathSet);fixSubstituteLoggers();replayEvents();// release all resources in SUBST_FACTORYSUBST_FACTORY.clear();

StaticLoggerBinder类加载时会执行初始化，如下代码所示。

	static {SINGLETON.init();}void init() {try {try {// 这里会完成logback的自动配置new ContextInitializer(defaultLoggerContext).autoConfig();} catch (JoranException je) {Util.report("Failed to auto configure default logger context", je);}// logback-292if (!StatusUtil.contextHasStatusListener(defaultLoggerContext)) {StatusPrinter.printInCaseOfErrorsOrWarnings(defaultLoggerContext);}contextSelectorBinder.init(defaultLoggerContext, KEY);initialized = true;} catch (Exception t) { // see LOGBACK-1159Util.report("Failed to instantiate [" + LoggerContext.class.getName() + "]", t);}}

logback源码解析

ContextInitializer.autoConfig()方法源码如下：

    public void autoConfig() throws JoranException {StatusListenerConfigHelper.installIfAsked(loggerContext);// 查找配置文件，优先级：系统属性logback.configurationFile > logback-test.xml > logback.xmlURL url = findURLOfDefaultConfigurationFile(true);if (url != null) {configureByResource(url);} else {// 使用java的spi机制，查找Configurator的实现，如果有，则自动配置logbackConfigurator c = EnvUtil.loadFromServiceLoader(Configurator.class);if (c != null) {try {c.setContext(loggerContext);c.configure(loggerContext);} catch (Exception e) {throw new LogbackException(String.format("Failed to initialize Configurator: %s using ServiceLoader", c != null ? c.getClass().getCanonicalName() : "null"), e);}} else {// 使用默认的BasicConfigurator来自动配置logbackBasicConfigurator basicConfigurator = new BasicConfigurator();basicConfigurator.setContext(loggerContext);basicConfigurator.configure(loggerContext);}}}

spring boot初始化logback流程

Spring Boot启动的时候，由org.springframework.boot.context.logging.LoggingApplicationListener根据情况初始化并使用。

在spring 初始化启动的过程中，会根据生命周期的不同阶段，发出对应的动作。这就是Spring ApplicationListener，设计基于观察者模式，而其中LoggingApplicationListener类便是负责logging日志框架的初始化操作。

LoggingApplicationListener被配置在spring-boot-x.x.x.jar的spring.factories文件中，spring启动的时候会去读取这个文件。

LoggingApplicationListener.onApplicationEvent()方法源码如下。

	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&& ((ContextClosedEvent) event).getApplicationContext().getParent() == null) {onContextClosedEvent();}else if (event instanceof ApplicationFailedEvent) {onApplicationFailedEvent();}}

第一个事件就是ApplicationStartingEvent了，点进去到LoggingSystem.get()方法。

因此，最终返回出去的是LogbackLoggingSystem对象，然后执行loggingSystem.beforeInitialize()方法，beforeInitialize会使用到logback的StaticLoggerBinder类，因此会读取logback.xml完成logback的初始化。

第二个事件是ApplicationEnvironmentPreparedEvent，会执行日志系统的初始化。

	protected void initialize(ConfigurableEnvironment environment, ClassLoader classLoader) {new LoggingSystemProperties(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);}// 主要看initializeSystem方法private void initializeSystem(ConfigurableEnvironment environment, LoggingSystem system, LogFile logFile) {LoggingInitializationContext initializationContext = new LoggingInitializationContext(environment);// 如果application.yml里指定了logging.config，则使用指定的文件初始化，否则按默认处理String logConfig = environment.getProperty(CONFIG_PROPERTY);if (ignoreLogConfig(logConfig)) {system.initialize(initializationContext, null, logFile);}else {try {ResourceUtils.getURL(logConfig).openStream().close();system.initialize(initializationContext, logConfig, logFile);}catch (Exception ex) {// NOTE: We can't use the logger here to report the problemSystem.err.println("Logging system failed to initialize using configuration from '" + logConfig + "'");ex.printStackTrace(System.err);throw new IllegalStateException(ex);}}}// 下面会走到LogbackLoggingSystem.initialize()方法public void initialize(LoggingInitializationContext initializationContext, String configLocation, LogFile logFile) {LoggerContext loggerContext = getLoggerContext();if (isAlreadyInitialized(loggerContext)) {return;}super.initialize(initializationContext, configLocation, logFile);loggerContext.getTurboFilterList().remove(FILTER);markAsInitialized(loggerContext);if (StringUtils.hasText(System.getProperty(CONFIGURATION_FILE_PROPERTY))) {getLogger(LogbackLoggingSystem.class.getName()).warn("Ignoring '" + CONFIGURATION_FILE_PROPERTY+ "' system property. Please use 'logging.config' instead.");}}// 再到AbstractLoggingSystem.initialize()方法public void initialize(LoggingInitializationContext initializationContext, String configLocation, LogFile logFile) {if (StringUtils.hasLength(configLocation)) {initializeWithSpecificConfig(initializationContext, configLocation, logFile);return;}initializeWithConventions(initializationContext, logFile);}// 再到initializeWithConventions方法private void initializeWithConventions(LoggingInitializationContext initializationContext, LogFile logFile) {// 查找logback自己的配置文件logback-test.xml、logback.xmlString config = getSelfInitializationConfig();if (config != null && logFile == null) {// logback在前面第一次使用StaticLoggerBinder时已经完成了初始化，这里会重新初始化一次reinitialize(initializationContext);return;}if (config == null) {// 获取logback-spring.xmlconfig = getSpringInitializationConfig();}if (config != null) {// 加载配置logback-spring.xmlloadConfiguration(initializationContext, config, logFile);return;}// 加载默认配置loadDefaults(initializationContext, logFile);}// reinitialize方法会走到LogbackLoggingSystem.configureByResourceUrl()方法，这里使用了SpringBootJoranConfiguratorprivate void configureByResourceUrl(LoggingInitializationContext initializationContext, LoggerContext loggerContext,URL url) throws JoranException {if (url.toString().endsWith("xml")) {JoranConfigurator configurator = new SpringBootJoranConfigurator(initializationContext);configurator.setContext(loggerContext);configurator.doConfigure(url);}else {// 否则使用logback的ContextInitializer进行初始化，这就不支持springProperty标签new ContextInitializer(loggerContext).configureByResource(url);}}// SpringBootJoranConfigurator添加了额外的规则，如springProperty标签class SpringBootJoranConfigurator extends JoranConfigurator {@Overridepublic void addInstanceRules(RuleStore rs) {super.addInstanceRules(rs);Environment environment = this.initializationContext.getEnvironment();rs.addRule(new ElementSelector("configuration/springProperty"), new SpringPropertyAction(environment));rs.addRule(new ElementSelector("*/springProfile"), new SpringProfileAction(environment));rs.addRule(new ElementSelector("*/springProfile/*"), new NOPAction());}}