public interface Log {
  boolean isDebugEnabled();
  boolean isTraceEnabled();
  void error(String s, Throwable e);
  void error(String s);
  void debug(String s);
  void trace(String s);
  void warn(String s);
}

這里定義了一些常用的方法

LogFactory的靜態(tài)代碼塊加載對應(yīng)是日志適配器：

日志工廠類LogFactory

    static {
        tryImplementation(LogFactory::useSlf4jLogging);
        tryImplementation(LogFactory::useCommonsLogging);
        tryImplementation(LogFactory::useLog4J2Logging);
        tryImplementation(LogFactory::useLog4JLogging);
        tryImplementation(LogFactory::useJdkLogging);
        tryImplementation(LogFactory::useNoLogging);
    }
    private static void tryImplementation(Runnable runnable) {
        if (logConstructor == null) {
            try {
                runnable.run();
            } catch (Throwable t) {
                // ignore
            }
        }
    }
    public static synchronized void useLog4JLogging() {
        setImplementation(org.apache.ibatis.logging.log4j.Log4jImpl.class);
    }

當(dāng)logConstructor用來記錄日志適配器的構(gòu)造方法，當(dāng)logConstructor不為空的時候就不再加載其他的日志適配器了。

分析一下setImplementation()方法：

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

獲取適配器的構(gòu)造方法
然后獲取這個適配器的實例，如果獲取成功，把它的構(gòu)造方法記錄在logConstructor中，否則就會拋出異常

下面就分析一下Log4jImpl類：

Log接口的實現(xiàn)類

public class Log4jImpl implements Log {
  private static final String FQCN = Log4jImpl.class.getName();
  private final Logger log;
  public Log4jImpl(String clazz) {
    log = Logger.getLogger(clazz);
  }

  @Override
  public boolean isDebugEnabled() {
    return log.isDebugEnabled();
  }

  @Override
  public boolean isTraceEnabled() {
    return log.isTraceEnabled();
  }

  @Override
  public void error(String s, Throwable e) {
    log.log(FQCN, Level.ERROR, s, e);
  }

  @Override
  public void error(String s) {
    log.log(FQCN, Level.ERROR, s, null);
  }

  @Override
  public void debug(String s) {
    log.log(FQCN, Level.DEBUG, s, null);
  }

  @Override
  public void trace(String s) {
    log.log(FQCN, Level.TRACE, s, null);
  }

  @Override
  public void warn(String s) {
    log.log(FQCN, Level.WARN, s, null);
  }

}

Log4jImpl是一個適配器類，它實現(xiàn)了Mybatis自定義的Log接口，它的成員變量Logger是log4j里的類，我們要做的是調(diào)用Log接口的方法就可以使用log4j，Logger是需要我們適配的類，因此我們在實現(xiàn)Log接口的方法的時候調(diào)用Logger中的方法來完成適配，這就是適配器模式的實現(xiàn)