xml文件示例如下：

<select id="selectMultiObject" parameterType="java.lang.String" resultType="java.util.HashMap"
            statementType="PREPARED">
       ${parameter}
</select>


<select id="selectObject" parameterType="java.lang.String" resultType="java.util.HashMap" statementType="STATEMENT">
       ${parameter}
</select>

\org\apache\ibatis\executor\statement\RoutingStatementHandler.java源码如下：    

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

  switch (ms.getStatementType()) {
    case STATEMENT:
      delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
      break;
    case PREPARED:
      delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
      break;
    case CALLABLE:
      delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
      break;
    default:
      throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
  }

}

由xml配置文件和源码文件可知，根据statementType来判断执行哪个方法。

取值说明： 
1、STATEMENT:直接操作sql，不进行预编译，获取数据：$—Statement ，如果这个时候使用占位符的话，就会报错，执行sql异常，因为不能翻译占位符，如select * from student limit ? ,不能正确执行，应该换成PREPARED这种就好了
2、PREPARED:预处理，参数，进行预编译，获取数据：#—–PreparedStatement:默认 
3、CALLABLE:执行存储过程————CallableStatement