我怎样才能检索JDBC ResultSet作为一个ArrayList？

问题描述

我在做一个查询来获取大量的ID（整数）。而是通过ResultSet迭代数百万次，将所有内容复制一个接一个到一个ArrayList的，是有一些方法来只检索一切，一个ArrayList

据我了解，ResultSet的应该是迭代，因为底层的实现可能会缓存的东西，但在我的情况下，我只需要所有的ID，立竿见影。我知道我可以设置FETCHSIZE大量，但后来我还有一个接一个检索的标识。

澄清：我想这样做的原因是性能。分析显示我这样做ResultSet.next（），ResultSet.getInt（）和ArrayList.add（）数百万次需要相当长一段时间。我估计，该数据库（我使用的是H2，它是用Java编写的）可能有数组或列表的地方在内存中，所以我在寻找一种方式把它复制到我，而不是直接通过ResultSet接口迭代

解决方案

使用Apache 库您可以轻松地返回ResultSet作为地图列表。

 公开名单查询（查询字符串）{
    列表结果= NULL;
    尝试{
        QueryRunner QRUN =新QueryRunner（）;
        结果=（列表）qrun.query（连接，查询，新MapListHandler（））;
    }赶上（例外前）{
        ex.printStackTrace（）;
    }
    返回结果;
}
 

I'm doing a query to retrieve a large amount of IDs (integers). Instead of iterating millions of times through the ResultSet and copying everything one-by-one to an ArrayList, is there some way to simply retrieve everything as an ArrayList?

I understand that ResultSet is supposed to be iterated because the underlying implementation may be caching stuff, but in my situation I just need all the IDs straight away. I know I can set the FetchSize to a large number, but then I still have to retrieve the IDs one-by-one.

Clarification: the reason I want to do this is performance. Profiling shows me that doing ResultSet.next(), ResultSet.getInt() and ArrayList.add() millions of times takes quite some time. I figure that the database (I'm using H2, which is written in Java) probably has the array or list somewhere in memory, so I'm looking for a way to have it copied to me directly instead of through the ResultSet iterating interface.

解决方案

Using the Apache DbUtils library you can easily return a ResultSet as a List of Maps.

public List query(String query) {
    List result = null;
    try {
        QueryRunner qrun = new QueryRunner();
        result = (List) qrun.query(connection, query, new MapListHandler());
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    return result;
}

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