MyBatis 源碼篇-日誌模塊1

在 Java 開發中常用的日誌框架有 Log4j、Log4j2、Apache Common Log、java.util.logging、slf4j 等，這些日誌框架對外提供的接口各不相同。本章詳細描述 MyBatis 是如何通過適配器的方式集成和複用這些第三方框架的。

日誌適配器

MyBatis 的日誌模塊位於 org.apache.ibatis.logging 包中，該模塊中 Log 接口定義了日誌模塊的功能，然後分別爲不同的日誌框架定義不同的日誌適配器，這些日誌適配器都繼承 Log 接口，LogFactory 工廠負責創建對應的日誌框架適配器。

下面來看 jdk14 日誌適配器模式的類圖：

在 LogFactory 類加載時會執行其靜態代碼快，按照順序加載並實例化對應日誌框架的適配器，然後用 logConstructor 字段記錄當前使用的第三方日誌框架的適配器。

tryImplementation() 方法使用 try cache 捕獲了加載日誌框架過程中產生的異常信息，且在 cache 中沒做任何操作，所以不會有異常信息拋出，正常執行。如果沒有引入任何日誌框架，會使用 useJdkLogging ，這是 JDK 自帶的日誌工具。

public final class LogFactory {

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

  // 記錄當前使用的第三方日誌框架的適配器的構造方法
  private static Constructor<? extends Log> logConstructor;

  // 嘗試加載每種日誌框架，調用順序是：
  // 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);
  }

  // 嘗試加載日誌框架
  private static void tryImplementation(Runnable runnable) {
    if (logConstructor == null) {
      try {
        runnable.run();
      } catch (Throwable t) {
        // 加載異常被忽略了
        // ignore
      }
    }
  }

  private static void setImplementation(Class<? extends Log> implClass) {
    try {
      //獲取指定適配器的構造方法
      Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
      // 實例化適配器
      Log log = candidate.newInstance(LogFactory.class.getName());
      // 輸出日誌
      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 日誌框架

SLF4J 只是日誌框架統一的接口定義（參考：http://www.slf4j.org/manual.html），還需要引入實現，pom.xml 文件增加如下內容：

<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 類加載的時候，其靜態代碼塊會將 logConstructor 初始化爲 SLF4J 適配器的構造函數。

通過 simplelogger.properties 屬性文件配置日誌級別爲 debug，屬性文件的命名是固定的，參考 org.slf4j.impl.SimpleLoggerConfiguration 類：

執行一個簡單的查詢操作，會輸出如下日誌信息：

[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 類實現了 org.apache.ibatis.logging.Log 接口，並封裝了 SLF4J 的日誌接口，Log 接口的功能全部通過調用 SLF4J 的日誌接口實現。

Slf4jImpl 構造函數中實現了 SLF4J 的版本區分，根據不同的版本使用不同的接口實現日誌功能，源碼如下：

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);
  }

}