未来/承诺无需等待结果？返回单位

要在里面转换Future,只需使用map和flatMap.当回调完成时,实际操作是异步执行的,但它们是类型安全的.

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

for(file <- directory) {
 if(timestamp > filetimestamp) {
   val future = Future {
   // do a convert job that returns UNIT
   } map {
     file => // whatever you want.
   }
}

警告! 如果任何Future抛出"NonFatal"错误,它将被吞下.使用Future [Unit]时这是一个严重的问题:如果没有代码评估未来,错误就会消失在黑洞中.(它会影响任何Future [_],但是如果要返回一个值,通常会对它执行某些操作,因此会发现错误.)

scala> import scala.concurrent.ExecutionContext.Implicits.global
scala.concurrent.Future { throw new IllegalArgumentException("foo") }

scala> res16: scala.concurrent.Future[Nothing] = scala.concurrent.impl.Promise$DefaultPromise@64dd3f78

scala> import scala.concurrent.ExecutionContext.Implicits.global
scala.concurrent.Future { throw new IllegalArgumentException("foo"); 42 }

scala> res11: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@65c8295b

另一种方法可以完成相同的操作,但不会隐藏错误:

scala> val context = scala.concurrent.ExecutionContext.Implicits.global
context.execute(new Runnable {
  def run() = throw new IllegalArgumentException("foo")
})
context: scala.concurrent.ExecutionContextExecutor = scala.concurrent.impl.ExecutionContextImpl@1fff4cac

scala>      |      | java.lang.IllegalArgumentException: foo
at $line48.$read$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$anon$1.run(<console>:34)
at $line48.$read$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$iw$$anon$1.run(<console>:33)
at scala.concurrent.impl.ExecutionContextImpl$$anon$3.exec(ExecutionContextImpl.scala:107)
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)