Java   java.util.concurrent
scala  scala.concurrent
Future ExecutionContext
Future 提供回调函数 避免阻塞


Future 两种状态
未完成
已完成 有返回值 或者 异常 ，Future完成后不可变 

Future object apply 创建 Future 
def apply[T](body: =>T)(implicit @deprecatedName('execctx) executor: ExecutionContext): Future[T] = impl.Future(body)
需要 ExecutionContext 或者 隐式提供
import scala.concurrent.ExecutionContext.Implicits.global


val future = Future[Int] {
  Thread.sleep(1000)
  1
}



回调函数
onComplete onFailure onSuccess 

future.onComplete {
  case Success(result) => println(result)
  case Failure(e) => println("error: " + e.getMessage)
}

future.onSuccess {
  case num => println(num)
}

future.onFailure {
  case e => println(e.getMessage)
}

ExecutionContext与Java中的线程池类似，用来管理线程

Java中 利用 Executors来定义线程池ExecutorService
newFixedThreadPool newSingleThreadExecutor newCachedThreadPool newScheduledThreadPool

ExecutionContext.fromExecutorService 
def fromExecutorService(e : java.util.concurrent.ExecutorService) : scala.concurrent.ExecutionContextExecutorService = {  ... }

val executor = Executors.newFixedThreadPool(3)
val executionContext = ExecutionContext.fromExecutor(executor)



 scala future 实例  
 scala future for 实例  
 scala Future sequence   

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