流数据到速度模板的飞行

Question

有人能让我知道，如果有可能以某种方式打开速度Excel模板，并流到它的部分更大的数据量在飞行中？

假设我想在一个循环中读取外部资源ArrayLists中的数据。一个列表，每个迭代有10,000个项目。在简单的迭代中，我想把列表推到Velocity Excel模板中，跳到下一个迭代时忘掉它。在数据处理结束时，我将对Velocity上下文和模板与所有数据进行最后的合并。

null

Answer 1

下面是我测试过的解决方案，在我的情况下效果很好。我能够生成Excel表的一个巨大的amout数据直接从数据库加载。

Connection conn = getDs().getConnection();
// such a configuration of prepared statement is mandatory for large amount of data
PreparedStatement ps = conn.prepareStatement(MY_QUERY, ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY, ResultSet.CLOSE_CURSORS_AT_COMMIT);

// load huge amount of data per 50000 items
ps.setFetchSize(50000);
ResultSet rs = ps.executeQuery();

// calculate count of returned data earlier
final CountWrapper countWrapper = new CountWrapper(countCalculatedEarlier);
CustomResultSetIterator<MyObject> rowsIter = new CustomResultSetIterator<MyObject>(rs, new org.apache.commons.dbutils.BasicRowProcessor(), MyObject.class) {

    private Long iterCount = 0L;

    @Override
    public boolean hasNext() {
        // you can't call isLast method on defined as forward only cursor of result set, hence there is a homegrown calculation whether last item is reached or not
        return iterCount < countWrapper.getCount().longValue();
    };

    @Override
    public MyObject next() {
        iterCount++;
        return super.next();
    };
};

VelocityContext context = new VelocityContext();
// place an interator here instead of collection object
context.put("rows", rowsIter);

Template t = ve.getTemplate(template);
File file = new File(parthToFile);
FileWriter fw = new FileWriter(file);
// generate on the fly in my case 1 000 000 rows in Excel, watch out such an Excel may have 1GB size
t.merge(context, fw);

// The CustomResultSetIterator is a:

public class CustomResultSetIterator<E> implements Iterator<E> {
  //...implement all interface's methods
}