MyBatis 源码篇-日志模块1

在 Java 开发中常用的日志框架有 Log4j、Log4j2、Apache Common Log、java.util.logging、slf4j 等，这些日志框架对外提供的接口各不相同。本章详细描述 MyBatis 是如何通过适配器的方式集成和复用这些第三方框架的。

日志适配器

MyBatis 的日志模块位于 org.apache.ibatis.logging 包中，该模块中 Log 接口定义了日志模块的功能，然后分别为不同的日志框架定义不同的日志适配器，这些日志适配器都继承 Log 接口，LogFactory 工厂负责创建对应的日志框架适配器。

下面来看 jdk14 日志适配器模式的类图：

在 LogFactory 类加载时会执行其静态代码快，按照顺序加载并实例化对应日志框架的适配器，然后用 logConstructor 字段记录当前使用的第三方日志框架的适配器。

tryImplementation() 方法使用 try cache 捕获了加载日志框架过程中产生的异常信息，且在 cache 中没做任何操作，所以不会有异常信息抛出，正常执行。如果没有引入任何日志框架，会使用 useJdkLogging ，这是 JDK 自带的日志工具。

public final class LogFactory {

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

  // 记录当前使用的第三方日志框架的适配器的构造方法
  private static Constructor<? extends Log> logConstructor;

  // 尝试加载每种日志框架，调用顺序是：
  // useSlf4jLogging --> useCommonsLogging --> useLog4J2Logging --> 
  // useLog4JLogging --> useJdkLogging --> useNoLogging
  static {
    tryImplementation(new Runnable() {
      @Override
      public void run() {
        useSlf4jLogging();
      }
    });
    tryImplementation(new Runnable() {
      @Override
      public void run() {
        useCommonsLogging();
      }
    });
    tryImplementation(new Runnable() {
      @Override
      public void run() {
        useLog4J2Logging();
      }
    });
    tryImplementation(new Runnable() {
      @Override
      public void run() {
        useLog4JLogging();
      }
    });
    tryImplementation(new Runnable() {
      @Override
      public void run() {
        useJdkLogging();
      }
    });
    tryImplementation(new Runnable() {
      @Override
      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(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);
  }

  // 尝试加载日志框架
  private static void tryImplementation(Runnable runnable) {
    if (logConstructor == null) {
      try {
        runnable.run();
      } catch (Throwable t) {
        // 加载异常被忽略了
        // ignore
      }
    }
  }

  private static void setImplementation(Class<? extends Log> implClass) {
    try {
      //获取指定适配器的构造方法
      Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
      // 实例化适配器
      Log log = candidate.newInstance(LogFactory.class.getName());
      // 输出日志
      if (log.isDebugEnabled()) {
        log.debug("Logging initialized using '" + implClass + "' adapter.");
      }
      // 初始化logConstructor字段
      logConstructor = candidate;
    } catch (Throwable t) {
      throw new LogException("Error setting Log implementation.  Cause: " + t, t);
    }
  }

}

引入 SLF4J 日志框架

SLF4J 只是日志框架统一的接口定义（参考：http://www.slf4j.org/manual.html），还需要引入实现，pom.xml 文件增加如下内容：

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.7.28</version>
</dependency>

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-simple</artifactId>
    <version>1.7.28</version>
    <scope>test</scope>
</dependency>

在 LogFactory 类加载的时候，其静态代码块会将 logConstructor 初始化为 SLF4J 适配器的构造函数。

通过 simplelogger.properties 属性文件配置日志级别为 debug，属性文件的命名是固定的，参考 org.slf4j.impl.SimpleLoggerConfiguration 类：

执行一个简单的查询操作，会输出如下日志信息：

[main] DEBUG org.apache.ibatis.logging.LogFactory - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - PooledDataSource forcefully closed/removed all connections.
[main] DEBUG org.apache.ibatis.transaction.jdbc.JdbcTransaction - Opening JDBC Connection
[main] DEBUG org.apache.ibatis.datasource.pooled.PooledDataSource - Created connection 1426329391.
[main] DEBUG com.yjw.mybatis.dao.StudentMapper.selectByPrimaryKey - ==>  Preparing: select id, name, sex, selfcard_no, note from t_student where id = ? 
[main] DEBUG com.yjw.mybatis.dao.StudentMapper.selectByPrimaryKey - ==> Parameters: 1(Long)
[main] DEBUG com.yjw.mybatis.dao.StudentMapper.selectByPrimaryKey - <==      Total: 1

Slf4jImpl 类实现了 org.apache.ibatis.logging.Log 接口，并封装了 SLF4J 的日志接口，Log 接口的功能全部通过调用 SLF4J 的日志接口实现。

Slf4jImpl 构造函数中实现了 SLF4J 的版本区分，根据不同的版本使用不同的接口实现日志功能，源码如下：

public class Slf4jImpl implements Log {

  private Log log;

  public Slf4jImpl(String clazz) {
    Logger logger = LoggerFactory.getLogger(clazz);

    if (logger instanceof LocationAwareLogger) {
      try {
        // check for slf4j >= 1.6 method signature
        logger.getClass().getMethod("log", Marker.class, String.class, int.class, String.class, Object[].class, Throwable.class);
        log = new Slf4jLocationAwareLoggerImpl((LocationAwareLogger) logger);
        return;
      } catch (SecurityException e) {
        // fail-back to Slf4jLoggerImpl
      } catch (NoSuchMethodException e) {
        // fail-back to Slf4jLoggerImpl
      }
    }

    // Logger is not LocationAwareLogger or slf4j version < 1.6
    log = new Slf4jLoggerImpl(logger);
  }

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

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

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

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

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

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

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

}

MyBatis 源码篇

MyBatis 源码篇-整体架构

MyBatis 源码篇-SQL 执行的流程

MyBatis 源码篇-资源加载

MyBatis 源码篇-日志模块1

MyBatis 源码篇-日志模块2

MyBatis 源码篇-插件模块

MyBatis 源码篇-DataSource

MyBatis 源码篇-Transaction

MyBatis 源码篇-MyBatis-Spring 剖析