实现有状态的函数

背景：实现传感器的警报。监控每一个传感器的温度差异，如果温度差大于一个阈值，则添加到报警列表。代码如下：
package com.zy.chapter07

import com.zy.chapter05.SensorReading
import org.apache.flink.api.common.functions.RichFlatMapFunction
import org.apache.flink.api.common.state.{ValueState, ValueStateDescriptor}
import org.apache.flink.configuration.Configuration
import org.apache.flink.util.Collector

/**

• create 2020-01-29
• author zhouyu
• desc 温度警报函数，测试Flink的valuestate状态控制
  */
  class TemperatureAlertFunction(val thresold:Double)
  extends RichFlatMapFunction[SensorReading,(String,Double,Double)]{
  //状态对象引用
  private var lastTempState : ValueState[Double] = _

override def open(parameters: Configuration): Unit = {
//create state descriptor
val descriptor = new ValueStateDescriptorDouble
//get state reference
lastTempState = getRuntimeContext().getStateDouble
}

override def flatMap(value: SensorReading, out: Collector[(String, Double, Double)]): Unit = {
//从状态中获取上一次的温度
val lastTemp = lastTempState.value()
val tempDiff = (value.temperature - lastTemp).abs
if(tempDiff > thresold){
out.collect((value.id,value.timestamp,value.temperature))
}
this.lastTempState.update(value.temperature)
}
}