How to trigger only once for each window when using TumblingProcessingTimeWindows？

Read Kafka message and keyBy by tableName, then write the message list to DataBase with batchUpdate

keyedStream.window(TumblingProcessingTimeWindows.of(Time.seconds(1))).aggregate(new ListAggregate()).addSink(new TemplateMySQLSink());

It seems that for every record comming, the aggregate function will be trigged. 

But I want to trigger only once for every window.

How can i implement this?

Thanks,

Lei

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