mybatis 与 日志

如上图所示，mybatis默认支持7种日志记录的方式，也可以自己实现Log接口，然后将实现类通过LogFactory注入到日志工厂中。

LogFactory是日志模块的入口，外层通过getLog获取Log对象，然后调用Log接口方法进行日志的记录，代码示例：

private static final Log log = LogFactory.getLog(BaseExecutor.class);
...
@Override
  public void close(boolean forceRollback) {
    try {
      try {
        rollback(forceRollback);
      } finally {
        if (transaction != null) {
          transaction.close();
        }
      }
    } catch (SQLException e) {
      // Ignore.  There's nothing that can be done at this point.
      log.warn("Unexpected exception on closing transaction.  Cause: " + e);
    } finally {
      transaction = null;
      deferredLoads = null;
      localCache = null;
      localOutputParameterCache = null;
      closed = true;
    }
  }

LogFactory的初始化过程：

  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 static void tryImplementation(Runnable runnable) {
    if (logConstructor == null) {
      try {
        runnable.run();
      } catch (Throwable t) {
        // ignore
      }
    }
  }

 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);
    }
  }

按照顺序依次去尝试着实例化各个实例（tryImplementation方法），slf4j -> common-logging -> log4j2 -> log4j -> jdk -> nologging，当某个Log实例化成功后，就会停止下面的初始化。

实例化是setImplementation方法做的，当所有日志都没有做配置或者缺少包的话就会失败，所以默认的日志是NoLoggingImpl。

我们可以在mybatis配置文件中指定具体的配置类或者自定义配置类。

我们可以在代码中调用LogFactory的方法来手动指定配置类，但是由于mybatis在初始化配置的时候将组建的各个配置都初始化好存放到configuration对象中，包括log。所以当mybatis初始化完成之后再去调用LogFactory的方法更改log对于configuration是没有影响的，所以，我们应该在SqlSessionFactoryBuilder开始之前对LogFactory操作，下面是使用代码配置成控制台log：

public class TestLog {

    public static void main(String[] args) throws IOException {
        Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
        LogFactory.useStdOutLogging();
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(reader);
        SqlSession session = factory.openSession();
        List<Gscope> result = session.selectList("getGscopes");
        session.close();
        System.out.println(result);
    }
}

logging包还有个重要的组件，负责对JDBC对象拦截和日志记录：

其中，基类BaseJdbcLogger实现了一些工具类，并封装了column，它的4个子类ConnectionLogger，StatementLogger,PreparedStatementLogger和ResultLogger实现了InvacationHandler接口，作为JDBC对象Connection，Statement,ResultSet的代理，其中，StatementLogger和PreparedStatementLogger在ConnectionLogger中拦截statement方法并返回代理对象，ResultLogger在StatementLogger和PreparedStatementLogger中拦截execute并返回代理对象，

而ConnectionLogger是在Executor中创建的代理：

BaseExecutor.java

  protected Connection getConnection(Log statementLog) throws SQLException {
    Connection connection = transaction.getConnection();
    if (statementLog.isDebugEnabled()) {
      return ConnectionLogger.newInstance(connection, statementLog, queryStack);
    } else {
      return connection;
    }
  }

上例中的控制台打印的log如下

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Opening JDBC Connection
Tue Jan 03 23:54:04 CST 2017 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
Created connection 1635546341.
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@617c74e5]
==>  Preparing: select * from gscope;
==> Parameters:
<==    Columns: sid, id, code, name, pinyin, first_pinyin, level, parent_id, alias, orderby, lng, lat, show_in_web
<==        Row: 1, 22, TJ , 天津, tianji, tj, 1, 0, 天津, 1, 0, 0, 1
...省略...
<==      Total: 133
Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@617c74e5]
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@617c74e5]
Returned connection 1635546341 to pool.