在SimpleExecutor中，执行SQL时调用preareStatement()方法来对statement进行初始化及参数设置。

   private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    //初始化
    stmt = handler.prepare(connection);
    //参数设置`
    handler.parameterize(stmt);
    return stmt;
  }

这里PreparedStatementHandler为例。详细分析这两个过程。

Statement初始化

这是BaseStatementHandler.prepare()方法

public Statement prepare(Connection connection) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      //通过connection得到一个statement
      statement = instantiateStatement(connection);
      //设置执行超时时间
      setStatementTimeout(statement);
      setFetchSize(statement);
      return statement;
    } catch (SQLException e) {
      closeStatement(statement);
      throw e;
    } catch (Exception e) {
      closeStatement(statement);
      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
    }
  }

再看PreparedStatementHandler.instantiateStatement()方法

protected Statement instantiateStatement(Connection connection) throws SQLException {
    //被执行的SQL
    String sql = boundSql.getSql();
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      if (keyColumnNames == null) {
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
        return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() != null) {
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      //直接使用jdbc的方式获取了一个PreparedStatement对象
      return connection.prepareStatement(sql);
    }
  }

Statement参数设置

如下是PreparedStatementHandler.parameterize()方法

public void parameterize(Statement statement) throws SQLException {
    //直接调用了ParameterHandler的方法设置
    parameterHandler.setParameters((PreparedStatement) statement);
  }

DefaultParamterHandler.parameterize()方法

public void setParameters(PreparedStatement ps) throws SQLException {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    //取出sql中的参数映射列表
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            //主要通过MetaObject对象从参数出取数据，MetaObject前面已经详细分析过！
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            //根据参数名称获取值
            value = metaObject.getValue(propertyName);
          }
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) jdbcType = configuration.getJdbcTypeForNull();
          //调用对应的typeHandler设置参数
          typeHandler.setParameter(ps, i + 1, value, jdbcType);
        }
      }
    }
  }

TypeHandler主要有两个功能：

设置sql执行时的参数
从结果集中取数据

public interface TypeHandler<T> {
  //设置参数
  void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;
  //取数据
  T getResult(ResultSet rs, String columnName) throws SQLException;
  //取数据
  T getResult(ResultSet rs, int columnIndex) throws SQLException;
  //取数据
  T getResult(CallableStatement cs, int columnIndex) throws SQLException;

}

来看看类关系图就更清楚啦

这些都Mybatis内置的TypeHandler，我们也可以自定义一个！处置枚举类型可能很有用。具体的TypeHandler不做讨论。

小结

分析到这里，如果是执行update/insert/delete语句，那么整个过程基本上已经完成。如果是执行select语句，还有重要的两步：结果集映射及缓存！

Mybatis3源码分析(15)-Sql解析执行-Statement初始化和参数设置

Statement初始化

Statement参数设置

小结

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