在Java 8中对具有聚合的多个字段进行分组

问题描述

我有一个与Web访问记录相关的域对象列表。这些域对象可以扩展到数千个。

I have a list of domain objects that relate to web access records. These domain objects can stretch into the thousands in number.

我没有资源或要求以原始格式将它们存储在数据库中，所以相反我想要预计算聚合并将聚合数据放入数据库。

I don't have the resources or requirement to store them in a database in raw format, so instead I want to precompute aggregations and put the aggregated data in a database.

我需要聚合在5分钟窗口中传输的总字节数，喜欢以下SQL查询

I need to aggregate the total bytes transferred in 5 minute windows, like the following SQL query

select
  round(request_timestamp, '5') as window, --round timestamp to the nearest 5 minute
  cdn,
  isp,
  http_result_code,
  transaction_time,
  sum(bytes_transferred)
from web_records
group by
    round(request_timestamp, '5'),
    cdn,
    isp,
    http_result_code,
    transaction_time

在Java 8中，我的第一个当前刺戳看起来像这样，我知道这个解决方案类似于

In Java 8 my first current stab looks like this, I am aware this solution is similar to this response in Group by multiple field names in java 8

Map<Date, Map<String, Map<String, Map<String, Map<String, Integer>>>>>>> aggregatedData =
webRecords
    .stream()
    .collect(Collectors.groupingBy(WebRecord::getFiveMinuteWindow,
               Collectors.groupingBy(WebRecord::getCdn,
                 Collectors.groupingBy(WebRecord::getIsp,
                   Collectors.groupingBy(WebRecord::getResultCode,
                       Collectors.groupingBy(WebRecord::getTxnTime,
                         Collectors.reducing(0,
                                             WebRecord::getReqBytes(),
                                             Integer::sum)))))));

这很有效，但它很难看，所有这些嵌套地图都是噩梦！要将地图展平或展开成行，我必须这样做

This works, but it's ugly, all those nested maps are a nightmare! To "flatten" or "unroll" the map out into rows I have to do this

for (Date window : aggregatedData.keySet()) {
  for (String cdn : aggregatedData.get(window).keySet()) {
    for (String isp : aggregatedData.get(window).get(cdn).keySet()) {
      for (String resultCode : aggregatedData.get(window).get(cdn).get(isp).keySet()) {
        for (String txnTime : aggregatedData.get(window).get(cdn).get(isp).get(resultCode).keySet()) {

           Integer bytesTransferred = aggregatedData.get(window).get(cdn).get(distId).get(isp).get(resultCode).get(txnTime);
           AggregatedRow row = new AggregatedRow(window, cdn, distId...

尽可能看到这是非常混乱和难以维护。

As you can see this is pretty messy and difficult to maintain.

任何人都有更好的方法来做到这一点吗？任何帮助将不胜感激。

Anyone have any ideas of a better way to do this? Any help would be greatly appreciated.

我想知道是否有更好的方法来展开嵌套地图，或者是否有一个允许你对集合进行GROUP BY的库。

I'm wondering if there is a nicer way to unroll the nested maps, or if there is a library that allows you to do a GROUP BY on a collection.

推荐答案

您应该为地图创建自定义键。最简单的方法是使用 Arrays.asList ：

You should create the custom key for your map. The simplest way is to use Arrays.asList:

Function<WebRecord, List<Object>> keyExtractor = wr ->
    Arrays.<Object>asList(wr.getFiveMinuteWindow(), wr.getCdn(), wr.getIsp(),
             wr.getResultCode(), wr.getTxnTime());
Map<List<Object>, Integer> aggregatedData = webRecords.stream().collect(
      Collectors.groupingBy(keyExtractor, Collectors.summingInt(WebRecord::getReqBytes)));

在这种情况下，键是固定顺序的5个元素的列表。不是面向对象，而是简单。或者，您可以定义自己的类型来表示自定义键，并创建正确的 hashCode / 等于实现。

In this case the keys are lists of 5 elements in fixed order. Not quite object-oriented, but simple. Alternatively you can define your own type which represents the custom key and create proper hashCode/equals implementations.

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