Spring + MyBaits 日志初始化两遍的问题

偶然发现一个问题，记录一下以备查询。

问题：系统启动时发现日志初始化了两次

14:28:04.798 [main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
14:28:04.970 [main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.

相关环境文件

mybatis-config.xml片段：

<settings>
  <setting name="logImpl" value="SLF4J" />      <!-- 日志实现包 -->
</settings>

applicitionContext.xml片段：

<!-- 配置SqlSessionFactoryBean -->
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="dataSource" ref="driverManagerDataSource" />
    <property name="configLocation" value="classpath:mybatis-config.xml" />
</bean>

既然是LogFactory打印的日志，先从LogFactory开始看，

public final class LogFactory {
private static Constructor<? extends Log> logConstructor;

  static {                   // 加载时按顺序尝试日志实现，这是第一条日志的输出
    tryImplementation(new Runnable() {
      public void run() {
        useSlf4jLogging();
      }
    });
    tryImplementation(new Runnable() {
      public void run() {
        useCommonsLogging();
      }
    });
　　// 忽略部分代码
    tryImplementation(new Runnable() {
      public void run() {
        useNoLogging();
      }
    });
  }

  private LogFactory() {
    // disable construction
  }

  public static Log getLog(Class<?> aClass) {
    return getLog(aClass.getName());
  }

  public static Log getLog(String logger) {
    try {
      return logConstructor.newInstance(new Object[] { logger });
    } catch (Throwable t) {
      throw new LogException("Error creating logger for logger " + logger + ".  Cause: " + t, t);
    }
  }

  public static synchronized void useCustomLogging(Class<? extends Log> clazz) {
    setImplementation(clazz);
  }

  public static synchronized void useSlf4jLogging() {
    setImplementation(org.apache.ibatis.logging.slf4j.Slf4jImpl.class);
  }

  public static synchronized void useCommonsLogging() {
    setImplementation(org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl.class);
  }
public static synchronized void useStdOutLogging() {
    setImplementation(org.apache.ibatis.logging.stdout.StdOutImpl.class);
  }

  public static synchronized void useNoLogging() {
    setImplementation(org.apache.ibatis.logging.nologging.NoLoggingImpl.class);
  }

  private static void tryImplementation(Runnable runnable) {
    if (logConstructor == null) {   // 当加载日志实现成功一次后，这里logConstructor已经不为null
      try {
        runnable.run();
      } catch (Throwable t) {
        // ignore
      }
    }
  }

  private static void setImplementation(Class<? extends Log> implClass) {
    try {
      Constructor<? extends Log> candidate = implClass.getConstructor(new Class[] { String.class });
      Log log = candidate.newInstance(new Object[] { LogFactory.class.getName() });
      log.debug("Logging initialized using '" + implClass + "' adapter.");
      logConstructor = candidate;
    } catch (Throwable t) {
      throw new LogException("Error setting Log implementation.  Cause: " + t, t);
    }
  }
}

可以看出在spring容器启动时，已经加载了一个“org.apache.ibatis.logging.slf4j.Slf4jImpl”；

那么第二条日志是什么时候输出呢？

容器在构造SqlSessionFactoryBean时，需要mybatis-config.xml，因为有<setting name="logImpl" value="SLF4J" />这一条配置，

所以在解析到这一条时调用了org.apache.ibatis.session.Configuration这个类又执行了一次初始化，

顺序SqlSessionFactoryBean -> SqlSessionFactoryBuilder -> XMLConfigBuilder -> Configuration

最终Configuration中：

  public void setLogImpl(Class<?> logImpl) {
    if (logImpl != null) {
      this.logImpl = (Class<? extends Log>) logImpl;
      LogFactory.useCustomLogging(this.logImpl);
    }
  }

由此也看明白了配置日志实现相当于手动执行org.apache.ibatis.logging.LogFactory.useSlf4jLogging()，mybatis文档中也写到了：

如果要手动调用LogFactory.use***Logging()方法，请在调用所有其他MyBatis方法前调用它。另外，只有在相应日志实现存在 的前提下，调用对应的方法才是有意义的，否则MyBatis一概忽略。如你环境中并不存在Log4J，你却调用了 相应的方法，MyBatis就会忽略这一调用，代之默认的查找顺序查找日志实现。