Mybatis内部模块之---日志模块

首先日志模块采用的是适配器模式：因为日志厂商，没有专门针对Mybatis的专门日志模块。所以Mybatis要引入外部的日志模块，于是才用了该设计模式

2.日志模块所在位置

org.apache.ibatis.logging

 

例：

//jdkLog的适配器，实现的是Log接口（Mybatie自己规定的日志需要有的能力）
public class Jdk14LoggingImpl implements Log {

  //真正提供日志能力的jdk的日志类（这就是引入的外部实现类）
  private final Logger log;

  public Jdk14LoggingImpl(String clazz) {
    log = Logger.getLogger(clazz);
  }

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

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

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

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

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

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

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

}

3.Mybatis又是怎样加载到Mybatis框架里面的

下面我们看看 LogFactory 这个类，这里通过静态代码块，实现了日志框架的加载顺序

public final class LogFactory {

  /**
   * Marker to be used by logging implementations that support markers
   */
  public static final String MARKER = "MYBATIS";

  //被选定的第三方日志组件适配器的构造方法
  private static Constructor<? extends Log> logConstructor;

  //自动扫描日志实现，并且第三方日志插件加载优先级如下：slf4J → commonsLoging → Log4J2 → Log4J → JdkLog
  static {
    tryImplementation(LogFactory::useSlf4jLogging);
    tryImplementation(LogFactory::useCommonsLogging);
    tryImplementation(LogFactory::useLog4J2Logging);
    tryImplementation(LogFactory::useLog4JLogging);
    tryImplementation(LogFactory::useJdkLogging);
    tryImplementation(LogFactory::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(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 useLog4JLogging() {
    setImplementation(org.apache.ibatis.logging.log4j.Log4jImpl.class);
  }

  public static synchronized void useLog4J2Logging() {
    setImplementation(org.apache.ibatis.logging.log4j2.Log4j2Impl.class);
  }

  public static synchronized void useJdkLogging() {
    setImplementation(org.apache.ibatis.logging.jdk14.Jdk14LoggingImpl.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) {//当构造方法不为空才执行方法
      try {
        runnable.run();
      } catch (Throwable t) {
        // ignore
      }
    }
  }
  //通过指定的log类来初始化构造方法
  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);
    }
  }

}

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