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);}}}