slf4j初始化线程安全历险记

最近发现项目中的日志用得不大规范，于是准备做些改造，在我们项目中用的是 slf4j1.5.10 的版本，顺便看了下 slf4j 的源码，看到 LoggerFactory 初始化的逻辑如下

  public static ILoggerFactory getILoggerFactory() {
    if (INITIALIZATION_STATE == UNINITIALIZED) {
      INITIALIZATION_STATE = ONGOING_INITILIZATION;
      performInitialization();
    }
    switch (INITIALIZATION_STATE) {
    case SUCCESSFUL_INITILIZATION:
      return getSingleton().getLoggerFactory();
    case FAILED_INITILIZATION:
      throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
    case ONGOING_INITILIZATION:
      // support re-entrant behavior.
      // See also http://bugzilla.slf4j.org/show_bug.cgi?id=106
      return TEMP_FACTORY;
    }
    throw new IllegalStateException("Unreachable code");
  }

有发现什么问题吗？再看看 INITIALIZATION_STATE 这个变量是怎么声明的，如下

  static int INITIALIZATION_STATE = UNINITIALIZED;

现在很明确了，如果在多线程环境下，多个线程调用 LoggerFactory.getLogger() 方法的时候初始化会出现问题，这简直有点颠覆世界观，说好的大神代码呢？怀着极度怀疑自己的心情翻看了最新的 slf4j1.7.21 代码，看到初始化逻辑如下:

     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");
    }

再看看 INITIALIZATION_STATE 变量的声明

        static volatile int INITIALIZATION_STATE = UNINITIALIZED;

顿时泪流满面，这才像传说中的大神代码啊