Driver stuck when thread pool is not shut down

object SparkTest {
  def main(args: Array[String]): Unit = {
    val spark = SparkSession.builder()
      .appName("zt-test")
      .config("spark.logConf",true)
      .getOrCreate()
    spark.sparkContext.setLogLevel("INFO")
    val threadPool = Executors.newFixedThreadPool(5)

    for (i <- 0 until 10) {
      threadPool.execute(new Task("Task " + i))
      // do not shut down threadPool
    }
    val rdd = spark.sparkContext.makeRDD(Seq(1,2,4))
    val res = rdd.collect()
    // throw 
    throw new Throwable()
  }
}

class Task(private var name: String) extends Runnable {
  override def run(): Unit = {
    System.out.println("Executing task: " + name + " by " + Thread.currentThread.getName)
  }
}

ugi.doAs(new PrivilegedExceptionAction[Unit]() {
  override def run(): Unit = System.exit(master.run()) 
})