在 Java 8 中对具有聚合的多个字段进行分组
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Group by in Java 8 on multiple fields with aggregations
提问by djhworld
I have a list of domain objects that relate to web access records. These domain objects can stretch into the thousands in number.
我有一个与 Web 访问记录相关的域对象列表。这些域对象的数量可以扩展到数千个。
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.
我没有资源或要求以原始格式将它们存储在数据库中，因此我想预先计算聚合并将聚合数据放入数据库中。
I need to aggregate the total bytes transferred in 5 minute windows, likethe following SQL query
我需要汇总 5 分钟窗口内传输的总字节数，如下面的 SQL 查询
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
In Java 8 my first current stab looks like this, I am aware this solution is similar to this responsein Group by multiple field names in java 8
在Java 8成我的第一电流刺这个样子的，我知道这个解决方案与此类似反应在集团的Java中的多个字段名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)))))));
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
这有效，但它很丑陋，所有那些嵌套的地图都是一场噩梦！要将地图“展平”或“展开”成行，我必须这样做
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...
As you can see this is pretty messy and difficult to maintain.
如您所见，这非常混乱且难以维护。
Anyone have any ideas of a better way to do this? Any help would be greatly appreciated.
任何人都有更好的方法来做到这一点的任何想法？任何帮助将不胜感激。
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.
我想知道是否有更好的方法来展开嵌套映射，或者是否有一个库允许您对集合执行 GROUP BY。
回答by Tagir Valeev
You should create the custom key for your map. The simplest way is to use Arrays.asList:
您应该为地图创建自定义键。最简单的方法是使用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)));
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/equalsimplementations.
在这种情况下，键是按固定顺序包含 5 个元素的列表。不太面向对象，但很简单。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/equals实现。
