RichAllWindowFunction的open方法在什么时候被触发

全局window必须通过触发器设置触发条件来进行触发。
例如：

val env = StreamExecutionEnvironment.getExecutionEnvironment
env.setStreamTimeCharacteristic(TimeCharacteristic.EventTime)
val source = env.addSource(new SimpleSource)

var triggerTest  = EventTimeTrigger.create()

val window = source
  .keyBy(_._1)
  .window(GlobalWindows.create())
  .trigger(CountTrigger.of(1))
  .apply(new WindowFunctionTest)
  .print().name("GlobalWindows")

env.execute("GlobalWindows")