flink对watermark的处理

DataStream waterPowerStream = inputStream.map(new MapFunction() {
            @Override
            public WaterPowerData map(String s) throws Exception {
                WaterPowerData waterPowerData = JSONObject.parseObject(s, WaterPowerData.class);
                return waterPowerData;
            }
        }).assignTimestampsAndWatermarks(new AssignerWithPeriodicWatermarks() {
            private long currentTimestamp = Long.MIN_VALUE;
            private final long maxTimeLag = 5000;

            @Override
            public long extractTimestamp(WaterPowerData waterPowerData, long l) {
                long timestamp = waterPowerData.getOutputTime();
                currentTimestamp = Math.max(timestamp, currentTimestamp);
                return timestamp;
            }

            @Nullable
            @Override
            public Watermark getCurrentWatermark() {
                return new Watermark(currentTimestamp == Long.MIN_VALUE ? Long.MIN_VALUE : currentTimestamp - maxTimeLag);
            }
        });