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-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);
        }
    }

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.xml
        URL 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logback
            Configurator 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logback
                BasicConfigurator basicConfigurator = new BasicConfigurator();
                basicConfigurator.setContext(loggerContext);
                basicConfigurator.configure(loggerContext);
            }
        }
    }