在项目中引入一个二方包后在后台日志诡异的不见了，于是使用
mvn dependency:tree -l tree.txt输出依赖关系树，并定位到新引入的二方包部分,二方包引入了spring-boot-starter-logging其依赖了logback-classic与项目中的log4j产生了冲突，所以将前者排掉就项目就可以正常启动了，这个排包的过程不算难，那slf4j是如何实现绑定的呢？
这里使用到的slf4j-api版本是1.7.25，在老版本中是存在一些线程安全问题的，通常我们打日志的时候都会写一段这样的代码
private static final Logger logger = LoggerFactory.getLogger(XXX.class) 我们从LoggerFactory这个类开始分析源码,首先看到getLogger方法 public static Logger getLogger(Class<?> clazz) { Logger logger = getLogger(clazz.getName()); ..... } public static Logger getLogger(String name) { ILoggerFactory iLoggerFactory = getILoggerFactory(); return iLoggerFactory.getLogger(name); }从上面源码可以看到会调用getLogger(String name)这个方法，通过LoggerFactory最终获取Logger对象， 所以重点就是获取LoggerFactory.
我们重点看下 getILoggerFactory这个方法 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"); }从源码中可以看到INITIALIZATION_STATE是一个静态的volatile变量，在之前的版本中没有volatile修饰，在这个方法中主要就是调用了performInitialization方法完成初始化,在该方法中主要完成绑定工作并进行检查
private final static void performInitialization() { bind(); if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) { versionSanityCheck(); } } private final static void bind() { try { Set<URL> staticLoggerBinderPathSet = 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(); } ...其中有个重要的方法findPossibleStaticLoggerBinderPathSet 顾名思义就是找可能存在的StaticLoggerBinder路径
private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class"; static Set<URL> findPossibleStaticLoggerBinderPathSet() { // use Set instead of list in order to deal with bug #138 // LinkedHashSet appropriate here because it preserves insertion order // during iteration Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>(); try { ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader(); Enumeration<URL> paths; 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); } } ... return staticLoggerBinderPathSet; }其实就是找日志实现包中的的StaticLoggerBinder，如slf4j-log4j12中的，将这些类的类路径添加到上面的set中，接着通过reportMultipleBindingAmbiguity方法检查是不是存在多个日志实现绑定产生冲突, 即看一下binderPathSet中元素个数是不是大于1，很简单，因此当项目中logback和slf4j-log4j同时存在时会打印出多个"Found binding in …"
private static boolean isAmbiguousStaticLoggerBinderPathSet(Set<URL> binderPathSet) { return binderPathSet.size() > 1; } private static void reportMultipleBindingAmbiguity(Set<URL> binderPathSet) { if (isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) { Util.report("Class path contains multiple SLF4J bindings."); for (URL path : binderPathSet) { Util.report("Found binding in [" + path + "]"); } Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation."); } }回到bind方法，看到StaticLoggerBinder.getSingleton();，其实就是创建一个单例的StaticLoggerBinder对象，而这个对象中含有一个LoggerFactory，针对不同的日志框架有不同的实现
log4j private final ILoggerFactory loggerFactory; private StaticLoggerBinder() { loggerFactory = new Log4jLoggerFactory(); try { Level level = Level.TRACE; } catch (NoSuchFieldError nsfe) { Util .report("This version of SLF4J requires log4j version 1.2.12 or later. See also http://www.slf4j.org/codes.html#log4j_version"); } } logback-classic private boolean initialized = false; private LoggerContext defaultLoggerContext = new LoggerContext(); private final ContextSelectorStaticBinder contextSelectorBinder = ContextSelectorStaticBinder.getSingleton(); private StaticLoggerBinder() { defaultLoggerContext.setName(CoreConstants.DEFAULT_CONTEXT_NAME); }通过这种绑定的方式就可以实现LoggerFactory的获取，如引入了log4j，就会利用log4j实现的StaticLoggerBinder类来获取log4j的LoggerFactory,而LoggerFactory可以简单地理解为一个Map，key为loggerName,value为Logger对象
