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Mybaits 如何实现打印sql语句

mybatis本身没有提供日志的实现，引入的是第三方组件。mybatis支持多个第三方日志插件，优先级由低到高为slf4J、commonsLoging、Log4J2、Log4J和JdkLog。

mybatis中有一个LogFactory，获取log的工厂类，在工程类中可以回去对应的日志实现。分析工程类，可以发现mybatis如何来选择log

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);
  }
}
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关于logConstructor的加载如下

static {
  tryImplementation(LogFactory::useSlf4jLogging);
  tryImplementation(LogFactory::useCommonsLogging);
  tryImplementation(LogFactory::useLog4J2Logging);
  tryImplementation(LogFactory::useLog4JLogging);
  tryImplementation(LogFactory::useJdkLogging);
  tryImplementation(LogFactory::useNoLogging);
}
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private static void tryImplementation(Runnable runnable) {
  if (logConstructor == null) {
    try {
      runnable.run();
    } catch (Throwable t) {
      // ignore
    }
  }
}
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在 tryImplementation ，中会设置mybatis使用的log类型。把引用的log设置到logConstructor中后，后续其他类型的log也不会再加载。所以在mybatis中优先级由低到高为slf4J、commonsLoging、Log4J2、Log4J和JdkLog。感觉也是属于SPI的一种实现方式，不同的是各种类型的第三方日志，无法形成一个统一的接口。故此，mybatis为了解决这一问题，使用了适配器模式。

适配器的实现一般是让适配器实现或者继承目标，并且内部持有一个适配者的引用。这样调用目标对象方法，实际上是调用适配者的方法。

mybatis 又是如何把这log，用起来的。根据mybatis的习惯，应该会使用代理模式，来打印这个日志。举例查询的语句查看，根据MapperProxy,查到最后查询的语句

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
  Statement stmt;
  Connection connection = getConnection(statementLog);
  stmt = handler.prepare(connection, transaction.getTimeout());
  handler.parameterize(stmt);
  return stmt;
}

protected Connection getConnection(Log statementLog) throws SQLException {
  Connection connection = transaction.getConnection();
  if (statementLog.isDebugEnabled()) {
    return ConnectionLogger.newInstance(connection, statementLog, queryStack);
  } else {
    return connection;
  }
}
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到此处可以看到mybatis在获取连接的时候，会根据日志的打印级别来判断是否会创建一个代理类。到这里就基本可以猜到，在代理类中，mybatis会去打印这个sql的语句

public static Connection newInstance(Connection conn, Log statementLog, int queryStack) {
  InvocationHandler handler = new ConnectionLogger(conn, statementLog, queryStack);
  ClassLoader cl = Connection.class.getClassLoader();
  return (Connection) Proxy.newProxyInstance(cl, new Class[]{Connection.class}, handler);
}
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用 ConnectionLogger 来举例，看到里面的invoke的方法

public Object invoke(Object proxy, Method method, Object[] params)
    throws Throwable {
  try {
    if (Object.class.equals(method.getDeclaringClass())) {
      return method.invoke(this, params);
    }
    if ("prepareStatement".equals(method.getName())) {
      if (isDebugEnabled()) {
        debug(" Preparing: " + removeBreakingWhitespace((String) params[0]), true);
      }
      PreparedStatement stmt = (PreparedStatement) method.invoke(connection, params);
      stmt = PreparedStatementLogger.newInstance(stmt, statementLog, queryStack);
      return stmt;
    } else if ("prepareCall".equals(method.getName())) {
      if (isDebugEnabled()) {
        debug(" Preparing: " + removeBreakingWhitespace((String) params[0]), true);
      }
      PreparedStatement stmt = (PreparedStatement) method.invoke(connection, params);
      stmt = PreparedStatementLogger.newInstance(stmt, statementLog, queryStack);
      return stmt;
    } else if ("createStatement".equals(method.getName())) {
      Statement stmt = (Statement) method.invoke(connection, params);
      stmt = StatementLogger.newInstance(stmt, statementLog, queryStack);
      return stmt;
    } else {
      return method.invoke(connection, params);
    }
  } catch (Throwable t) {
    throw ExceptionUtil.unwrapThrowable(t);
  }
}
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可以看到，mybatis在里面还可以更具情况创建代理类。代理类又一次被代理，这也是mybatis喜欢的编程方式，比如插件也是代理类再次被代理，来实现多个插件并行。

原文 https://juejin.im/post/5f0dbfed6fb9a07ec07b4888

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