Scala的Future介绍

Posted 2021-04-25 ScalaCoder

流程解析

将任务提交到线程池

先看一个 Future 用法的 例子

Future { println("start future") }

在 scala 的语法里，以下三种写法作用一样

Future {}Future ()Future.apply()

进入到 Future.apply 方法

def apply[T](body: =>T)(implicit @deprecatedName("execctx") executor: ExecutionContext): Future[T] = unit.map(_ => body)

unit 为

val unit: Future[Unit] = successful(())

再看 successful

def successful[T](result: T): Future[T] = Promise.successful(result).future

进入到 Promise.successful()

 
   
   
 

  
    
    
  
 
   
 
   
 
  
  
    
    
  
def successful[T](result: T): Promise[T] = fromTry(Success(result))def fromTry[T](result: Try[T]): Promise[T] = impl.Promise.KeptPromise[T](result)
 
   
   
 

进入到 Promise.KeptPromise apply方法

def apply[T](result: Try[T]): scala.concurrent.Promise[T] = resolveTry(result) match { case s @ Success(_) => new Successful(s) case f @ Failure(_) => new Failed(f) }

最终构造了一个 Kept 对象 Kept 是 Promise 的子类，Promise 是 Future 的子类

Kept[T] extends Promise[T] 
Promise[T] extends scala.concurrent.Promise[T] with scala.concurrent.Future[T]

执行任务 body : => T 我们重回 Future.apply，看看 unit.map(_ => body) 的逻辑

def map[S](f: T => S)(implicit executor: ExecutionContext): Future[S] = transform(_ map f)

transform 是一个抽象方法，所以我们去看子类 Promise.transform 的实现

import scala.concurrent.Futureimport scala.concurrent.impl.Promise.DefaultPromise
override def transform[S](f: Try[T] => Try[S])(implicit executor: ExecutionContext): Future[S] = { val p = new DefaultPromise[S]()  onComplete { result => p.complete(try f(result) catch { case NonFatal(t) => Failure(t) }) } p.future}

根据 DefaultPromise 类的定义

class DefaultPromise[T] extends AtomicReference[AnyRef](Nil) with Promise[T]
val p = new DefaultPromise[S]() //实际上是初始化了一个 AtomicReference[AnyRef](空的list)Nil = List.empty

接着我们看 DefaultPromise.onComplete 的实现，DefaultPromise 是 AtomicReference 无锁的对象引用的子类

final def onComplete[U](func: Try[T] => U)(implicit executor: ExecutionContext): Unit = dispatchOrAddCallback(new CallbackRunnable[T](executor.prepare(), func))//尾递归优化@tailrecprivate def dispatchOrAddCallback(runnable: CallbackRunnable[T]): Unit = { get() match { case r: Try[_] => runnable.executeWithValue(r.asInstanceOf[Try[T]]) case dp: DefaultPromise[_] => compressedRoot(dp).dispatchOrAddCallback(runnable) case listeners: List[_] => if (compareAndSet(listeners, runnable :: listeners)) () else dispatchOrAddCallback(runnable) }}

最终 runable.executeWithValue 执行，也就是 CallbackRunnable.executeWithValue 提交任务到线程池去执行

private final class CallbackRunnable[T](val executor: ExecutionContext, val onComplete: Try[T] => Any) extends Runnable with OnCompleteRunnable { // must be filled in before running it var value: Try[T] = null
 override def run() = { require(value ne null) // must set value to non-null before running! try onComplete(value) catch { case NonFatal(e) => executor reportFailure e } }
 def executeWithValue(v: Try[T]): Unit = { require(value eq null) // can't complete it twice value = v // Note that we cannot prepare the ExecutionContext at this point, since we might // already be running on a different thread! try executor.execute(this) catch { case NonFatal(t) => executor reportFailure t } }}

看 CallbackRunnalbe 的定义, 函数onComplete的实现为 Promise.transform 的实现中的代码

result => p.complete(try f(result) catch { case NonFatal(t) => Failure(t) })

最后 p.complete 会返回一个 Promise 对象也就是 Future对象本身

def complete(result: Try[T]): this.type = if (tryComplete(result)) this else throw new IllegalStateException("Promise already completed.")

总结

创建Future 交给 Promise 对象管理，并将线程池引用传入到 Promise 对象中， Promise 对 Future 里的任务进行调度执行