flink 1.11定义watermark 水位线 水印 新方法 替代过时assignTimestampsAndWatermarks方法
mapStream.assignTimestampsAndWatermarks(WatermarkStrategy.forBoundedOutOfOrderness(Duration.ofSeconds(3)).withTimestampAssigner(newSerializableTimestampAssigner[(Long,student)]{overridedefextractTimes