slf4j门面模式实现原理

在使用slf4j的时候，只需要通过门面类LoggerFactory类获取Logger

/**
   * Return a logger named according to the name parameter using the statically
   * bound {@link ILoggerFactory} instance.
   *
   * @param name The name of the logger.
   * @return logger
   */
  public static Logger getLogger(String name) {
    ILoggerFactory iLoggerFactory = getILoggerFactory();
    return iLoggerFactory.getLogger(name);
  }

  /**
   * Return a logger named corresponding to the class passed as parameter, using
   * the statically bound {@link ILoggerFactory} instance.
   *
   * @param clazz the returned logger will be named after clazz
   * @return logger
   */
  public static Logger getLogger(Class clazz) {
    return getLogger(clazz.getName());
  }

实际上，这种方式会得调用另外一个类

/**
   * Return the {@link ILoggerFactory} instance in use.
   * <p/>
   * <p/>
   * ILoggerFactory instance is bound with this class at compile time.
   *
   * @return the ILoggerFactory instance in use
   */
  public static ILoggerFactory getILoggerFactory() {
    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://bugzilla.slf4j.org/show_bug.cgi?id=106
        return TEMP_FACTORY;
    }
    throw new IllegalStateException("Unreachable code");
  }

注意看的是 return StaticLoggerBinder.getSingleton().getLoggerFactory();

实际上，它真正的方式是通过 org.slf4j.impl.StaticLoggerBinder
，但是，StaticLoggerBinder类在slf4j包中是不存在的。

这实际上就是slf4j的能自动识别实际上的logger日志的原理了。

它调用这个类，这个类，slf4j有一个大概的雏形，但是没有实现，（需要保证slf4j本身编译不报错）。也没有被打包成jar里面，而是由具体的实现该类的，比如log4j等自己实现这个类，并加入具体的jar。那么在实际运行时，就能调用到真实的日志包了。