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-97
            return 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 bindings
                reportMultipleBindingAmbiguity(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_FACTORY
            SUBST_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-292
            if (!StatusUtil.contextHasStatusListener(defaultLoggerContext)) {
                StatusPrinter.printInCaseOfErrorsOrWarnings(defaultLoggerContext);
            }
            contextSelectorBinder.init(defaultLoggerContext, KEY);
            initialized = true;
        } catch (Exception t) { // see LOGBACK-1159
            Util.report("Failed to instantiate [" + LoggerContext.class.getName() + "]", t);
        }
    }