slf4j 运行原理

slf4j组件对外提供统一的日志对象访问接口，log4j，logback这类组件会实现这个接口中的相关标准规定,下面分为几步详细说下怎么实现。

1.先调用slf4j组件的方法 Logger logger = LoggerFactory.getLogger(App.class);

2.调用slf4j组件的 getILoggerFactory()方法开始获取具体的实现类log4j；

3.getILoggerFactory会调用bind方法，bind会再调用findPossibleStaticLoggerBinderPathSet()方法尝试获取实现了标准的类，loggerFactoryClassLoader.getResources("org/slf4j/impl/StaticLoggerBinder.class") 会去classpath查找这个类，如果有，说明项目引入了具体的日志组件，可以进行日志的记录，这是会返回具体的对象，否则就会提示没有找到相关的类；

4.获取到具体对象后(这里是Log4jLoggerFactory)，会调用该对象的getLogger方法，获取到具体的日志对象，至此，开始输出日志，程序结束。

代码

//Logger logger = LoggerFactory.getLogger(Object.class);   通过LoggerFactory去拿slf4j提供的一个Logger接口的具体实现
public static Logger getLogger(Class clazz) {
    Logger logger = getLogger(clazz.getName());
    if (DETECT_LOGGER_NAME_MISMATCH) {
        Class autoComputedCallingClass = Util.getCallingClass();
        if (autoComputedCallingClass != null && nonMatchingClasses(clazz, autoComputedCallingClass)) {
            Util.report(String.format("Detected logger name mismatch. Given name: \"%s\"; computed name: \"%s\".", logger.getName(),
                            autoComputedCallingClass.getName()));
            Util.report("See " + LOGGER_NAME_MISMATCH_URL + " for an explanation");
        }
    }
    return logger;
}
//LoggerFactory的bind()方法
private final static void bind() {
    try {
        SetstaticLoggerBinderPathSet = null;
        if (!isAndroid()) {
            staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
            reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
        }
        // the next line does the binding
        StaticLoggerBinder.getSingleton();
        INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
        reportActualBinding(staticLoggerBinderPathSet);
        fixSubstituteLoggers();
        replayEvents();
        // release all resources in SUBST_FACTORY
        SUBST_FACTORY.clear();
    } catch (NoClassDefFoundError ncde) {
        String msg = ncde.getMessage();
        if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
            INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
            Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
            Util.report("Defaulting to no-operation (NOP) logger implementation");
            Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
        } else {
            failedBinding(ncde);
            throw ncde;
        }
    } catch (java.lang.NoSuchMethodError nsme) {
        String msg = nsme.getMessage();
        if (msg != null && msg.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
            INITIALIZATION_STATE = FAILED_INITIALIZATION;
            Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
            Util.report("Your binding is version 1.5.5 or earlier.");
            Util.report("Upgrade your binding to version 1.6.x.");
        }
        throw nsme;
    } catch (Exception e) {
        failedBinding(e);
        throw new IllegalStateException("Unexpected initialization failure", e);
    }
}
//这个地方重点其实就是第12行的代码，getLogger的时候会去classpath下找STATIC_LOGGER_BINDER_PATH，
//STATIC_LOGGER_BINDER_PATH值为"org/slf4j/impl/StaticLoggerBinder.class"，即所有slf4j的实现，在提供的jar包路径下，
//一定是有"org/slf4j/impl/StaticLoggerBinder.class"存在的
static SetfindPossibleStaticLoggerBinderPathSet() {
    // use Set instead of list in order to deal with bug #138
    // LinkedHashSet appropriate here because it preserves insertion order
    // during iteration
    SetstaticLoggerBinderPathSet = new LinkedHashSet();
    try {
        ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
        Enumerationpaths;
        if (loggerFactoryClassLoader == null) {
            paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
        } else {
            paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
        }
        while (paths.hasMoreElements()) {
            URL path = paths.nextElement();
            staticLoggerBinderPathSet.add(path);
        }
    } catch (IOException ioe) {
        Util.report("Error getting resources from path", ioe);
    }
    return staticLoggerBinderPathSet;
}