问：e3不应该先于l4吗？
使用 ProcessFunction . 像这样：

public static class SortFunction extends ProcessFunction<Event, Event> {
  private ValueState<PriorityQueue<Event>> queueState = null;

  @Override
  public void open(Configuration config) {
    ValueStateDescriptor<PriorityQueue<Event>> descriptor = new ValueStateDescriptor<>(
        // state name
        "sorted-events",
        // type information of state
        TypeInformation.of(new TypeHint<PriorityQueue<Event>>() {
        }));
    queueState = getRuntimeContext().getState(descriptor);
  }

  @Override
  public void processElement(Event event, Context context, Collector<Event> out) throws Exception {
    TimerService timerService = context.timerService();

    if (context.timestamp() > timerService.currentWatermark()) {
      PriorityQueue<Event> queue = queueState.value();
      if (queue == null) {
        queue = new PriorityQueue<>(10);
      }
      queue.add(event);
      queueState.update(queue);
      timerService.registerEventTimeTimer(event.timestamp);
    }
  }

  @Override
  public void onTimer(long timestamp, OnTimerContext context, Collector<Event> out) throws Exception {
    PriorityQueue<Event> queue = queueState.value();
    Long watermark = context.timerService().currentWatermark();
    Event head = queue.peek();
    while (head != null && head.timestamp <= watermark) {
      out.collect(head);
      queue.remove(head);
      head = queue.peek();
    }
  }
}

更新：请参阅how to sort an out-order event time stream using flink，以了解通常更好的方法的描述。