Future的onComplete后面的内容输出如何显示？ 原因主线程结束时，future线程还没执行完 增加Thread.sleep(5000)

PromiseFutures.scala

package neophyte.guide

import concurrent._
import ExecutionContext.Implicits.global
import neophyte.guide.PromiseFutures.TaxCut
import scala.util.{Failure, Success}

object PromiseFutures {

  case class TaxCut(reduction: Int)
  
  def main(args: Array[String]): Unit = {

    val taxCutF: Future[TaxCut] = Government.redeemCampaignPledge()

    println("Now that they're elected, let's see if they remember their promise...")
    taxCutF.onComplete {
      case Success(TaxCut(reduction)) => println(s"A miracle! They really cut our taxes by $reduction percentage points!")
      case Failure(ex) => println(s"They broke their promises! Again! Because of a ${ex.getMessage}")
    }
    Thread.sleep(5000)
  }
}

object Government {
  def redeemCampaignPledge(): Future[TaxCut] = {
    val p = Promise[TaxCut]()
    future {
      println("Starting the new legislative period")
      Thread.sleep(2000)
      p.success(TaxCut(20))
      println("We reduced the taxes! You must reelect us!!!!1111")
    }

    p.future
  }
}