在 Java 中并行执行依赖任务

在Scala中，这很容易做到，我认为使用Scala更好。以下是我从这里抽取的一个例子 http://danielwestheide.com/（The Neophyte's Guide to Scala Part 16： Where to Go From Here）这个家伙有一个很棒的博客（我不是那个家伙）

让我们带一个律师煮咖啡。要执行的任务是：

1. 研磨所需的咖啡豆（无前置任务）
2. 加热一些水（没有前面的任务）
3. 使用研磨咖啡和热水冲泡浓缩咖啡（取决于1和2）
4. 泡一些牛奶（没有前面的任务）
5. 将泡沫牛奶和浓缩咖啡混合（取决于3，4）

或作为一棵树：

Grind   _
Coffe    \
          \   
Heat    ___\_Brew____ 
Water                \_____Combine
                     /
Foam    ____________/
Milk

在使用并发 API 的 java 中，这将是：

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class Barrista {

    static class HeatWater implements Callable<String> {
        @Override
        public String call() throws Exception {
            System.out.println("Heating Water");
            Thread.sleep(1000);
            return "hot water";
        }
    }

    static class GrindBeans implements Callable<String> {
        @Override
        public String call() throws Exception {
            System.out.println("Grinding Beans");
            Thread.sleep(2000);
            return "grinded beans";
        }
    }

    static class Brew implements Callable<String> {

        final Future<String> grindedBeans;
        final Future<String> hotWater;

        public Brew(Future<String> grindedBeans, Future<String> hotWater) {
            this.grindedBeans = grindedBeans;
            this.hotWater = hotWater;
        }

        @Override
        public String call() throws Exception
        {
            System.out.println("brewing coffee with " + grindedBeans.get()
                    + " and " + hotWater.get());
            Thread.sleep(1000);
            return "brewed coffee";
        }
    }

    static class FrothMilk implements Callable<String> {

        @Override
        public String call() throws Exception {
            Thread.sleep(1000);
            return "some milk";
        }
    }

    static class Combine implements Callable<String> {

        public Combine(Future<String> frothedMilk, Future<String> brewedCoffee) {
            super();
            this.frothedMilk = frothedMilk;
            this.brewedCoffee = brewedCoffee;
        }

        final Future<String> frothedMilk;
        final Future<String> brewedCoffee;

        @Override
        public String call() throws Exception {
            Thread.sleep(1000);
            System.out.println("Combining " + frothedMilk.get() + " "
                    + brewedCoffee.get());
            return "Final Coffee";
        }

    }

    public static void main(String[] args) {

        ExecutorService executor = Executors.newFixedThreadPool(2);

        FutureTask<String> heatWaterFuture = new FutureTask<String>(new HeatWater());
        FutureTask<String> grindBeans = new FutureTask<String>(new GrindBeans());
        FutureTask<String> brewCoffee = new FutureTask<String>(new Brew(grindBeans, heatWaterFuture));
        FutureTask<String> frothMilk = new FutureTask<String>(new FrothMilk());
        FutureTask<String> combineCoffee = new FutureTask<String>(new Combine(frothMilk, brewCoffee));

        executor.execute(heatWaterFuture);
        executor.execute(grindBeans);
        executor.execute(brewCoffee);
        executor.execute(frothMilk);
        executor.execute(combineCoffee);


        try {

            /**
             *  Warning this code is blocking !!!!!!!
             */         
            System.out.println(combineCoffee.get(20, TimeUnit.SECONDS));
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        } catch (TimeoutException e) {
            System.out.println("20 SECONDS FOR A COFFEE !!!! I am !@#! leaving!!");
            e.printStackTrace();
        } finally{
                executor.shutdown();
            }
        }
    }

确保你添加了超时，以确保你的代码不会永远等待某些事情完成，这是通过使用Future.get（long，TimeUnit）完成的，然后相应地处理失败。

然而，在scala中它要好得多，在这里它就像在博客上一样：准备一些咖啡的代码看起来像这样：

def prepareCappuccino(): Try[Cappuccino] = for {
  ground <- Try(grind("arabica beans"))
  water <- Try(heatWater(Water(25)))
  espresso <- Try(brew(ground, water))
  foam <- Try(frothMilk("milk"))
} yield combine(espresso, foam)

其中，所有方法都返回一个 future（类型化 future），例如 grind 将如下所示：

def grind(beans: CoffeeBeans): Future[GroundCoffee] = Future {
   // grinding function contents
}

对于所有实现，请查看博客，但仅此而已。你也可以很容易地集成Scala和Java。我真的建议在Scala而不是Java中做这种事情。Scala需要更少的代码，更干净和事件驱动。

具有依赖关系的任务的常规编程模型是数据流。简化的模型，其中每个任务只有一个，尽管重复，依赖关系是Actor模型。Java有很多执行组件库，但数据流很少。另请参阅：哪个-actor-model-library-framework-for-java，java-pattern-for-nested-callbacks