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异步编程利器CompletableFuture

代码小郭...大约 12 分钟JAVA并发编程

异步编程利器CompletableFuture

一、简介

FutureTask的get()方法在Future计算完成之前会一直处于阻塞状态下，isDone()方法容易耗费CPU资源，对于真正的异步处理我们是希望能通过传入回调函数，在Future结束时自动调用该回调函数，这样，我们就不用等待结果。阻塞的方式和异步编程的设计理念相违背，而轮询的方式也会耗费CPU资源。因此JDK8中出现了一种新的工具类：CompletableFuture。

CompletableFuture是FutureTask的增强版，提供的是一种类似观察者模式的机制，可以让任务执行完成后通知监听的一方。在任务执行完成之前，监听方可以去干别的事情。

在Java8中，CompletableFuture提供了非常强大的Future的扩展功能，可以帮助我们简化异步编程的复杂性，并且提供了函数式编程的能力，可以通过回调的方式处理计算结果，也提供了转换和组合CompletableFuture的方法。它能代表一个明确完成的Future，也有可能代表一个完成阶段(CompletableStage)。它支持在计算完成后触发一些函数或者执行某些动作。它实现了Future和CompletionStage接口。

类结构
类结构

CompletionStage代表异步计算过程中的某一个阶段，一个阶段完成以后可能会触发另外一个阶段。
一个阶段的计算可以是一个Function，Consumer或者Runnable，比如：stage.thenApply(x -> square(x)).thenAccept(x -> System.out.print(x)).thenRun(() -> System.out.println());
一个阶段的执行可能是被单个阶段的完成触发，也可能是由多个阶段一起触发

二、四大静态方法入门

java不推荐使用构造方法构造CompletableFuture，推荐使用下面的四大静态方法。

1、runAsync无返回值(默认线程池)

public static CompletableFuture<Void> runAsync(Runnable runnable);

2、runAsync无返回值(自定义线程池)

public static CompletableFuture<Void> runAsync(Runnable runnable,Executor executor);

3、supplyAsync有返回值(默认线程池)

public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier);

4、supplyAsync有返回值(自定义线程池)

public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier,Executor executor);

5、源码测试

package com.gyd;

import java.util.concurrent.*;

public class CompletableFutureDemo {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        System.out.println("======CompletableFuture.runAsync的演示(无返回值，默认线程池)");
        runAsync1();
        System.out.println("======CompletableFuture.runAsync的演示(无返回值，自定义线程池)");
        runAsync2();
        System.out.println("======CompletableFuture.supplyAsync1的演示(有返回值，默认线程池)");
        supplyAsync1();
        System.out.println("======CompletableFuture.supplyAsync1的演示(有返回值，自定义线程池)");
        supplyAsync2();
    }

    public static void runAsync1() throws ExecutionException, InterruptedException {
        //不推荐
        //CompletableFuture completableFuture = new CompletableFuture();
        CompletableFuture<Void> completableFuture = CompletableFuture.runAsync(()->{
           System.out.println(Thread.currentThread().getName());
           try {TimeUnit.SECONDS.sleep(1);}catch (Exception e) {e.printStackTrace();}
        });
        System.out.println(completableFuture.get());
    }

    public static void runAsync2() throws ExecutionException, InterruptedException {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        CompletableFuture<Void> completableFuture = CompletableFuture.runAsync(()->{
            System.out.println(Thread.currentThread().getName());
            try {TimeUnit.SECONDS.sleep(1);}catch (Exception e) {e.printStackTrace();}
        },executorService);
        System.out.println(completableFuture.get());
        executorService.shutdown();
    }

    public static void supplyAsync1() throws ExecutionException, InterruptedException {
         CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(() ->{
             System.out.println(Thread.currentThread().getName());
             try {TimeUnit.SECONDS.sleep(1);}catch (Exception e) {e.printStackTrace();}
             return "Hello Supply";
         });
         System.out.println(completableFuture.get());
    }
    public static void supplyAsync2() throws ExecutionException, InterruptedException {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(() ->{
            System.out.println(Thread.currentThread().getName());
            try {TimeUnit.SECONDS.sleep(1);}catch (Exception e) {e.printStackTrace();}
            return "Hello Supply";
        },executorService);
        System.out.println(completableFuture.get());
        executorService.shutdown();
    }
}

输出结果:

======CompletableFuture.runAsync的演示(无返回值，默认线程池)
ForkJoinPool.commonPool-worker-25
null
======CompletableFuture.runAsync的演示(无返回值，自定义线程池)
pool-1-thread-1
null
======CompletableFuture.supplyAsync1的演示(有返回值，默认线程池)
ForkJoinPool.commonPool-worker-25
Hello Supply
======CompletableFuture.supplyAsync1的演示(有返回值，自定义线程池)
pool-2-thread-1
Hello Supply

Process finished with exit code 0

三、常用方法

1、获得结果和触发计算

package com.gyd;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class CompletableFutureDemo4 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(() ->{
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            return "abc";
        });

        //不见不散
//        System.out.println(completableFuture.get());
        //过时不候（抛出TimeoutException）
//        System.out.println(completableFuture.get(2,TimeUnit.SECONDS));
        //立即返回(立即获取结果不阻塞，没有计算完成的情况 给一个默认值)
//        System.out.println(completableFuture.getNow("xxx"));

        //complete方法用于判断是否执行完成，未执行完成则返回默认值，注意该方法只能被执行一次
        TimeUnit.SECONDS.sleep(4);
        System.out.println(completableFuture.complete("completeValue")+" "+completableFuture.join());

    }
}

2、对计算结果进行处理

thenApply：计算结果存在依赖，不同步骤的线程执行串行化，若某个步骤发生异常，则不进入下一步骤并直接进入异常处理流程。

package com.gyd;

import java.util.concurrent.*;

public class CompletableFutureDemo5 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        CompletableFuture<Integer> completableFuture = CompletableFuture.supplyAsync(() ->{
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println("111");
            return 1;
        },executorService).thenApply(f ->{
            System.out.println("222");
            //若当前步骤有异常，则不会继续执行后续步骤，直接进入异常处理流程exceptionally
            return f+2;
        }).thenApply(f->{
            System.out.println("333");
            return f+3;
        }).whenComplete((v,e) ->{
            if (e == null) System.out.println("v: "+v);
        }).exceptionally( e ->{
            //发生异常时的处理
            e.printStackTrace();
            System.out.println("发生异常");
            return null;
        });

        System.out.println(Thread.currentThread().getName()+"先去忙别的事情");

    }
}

handle：计算结果存在依赖，不同步骤的线程执行串行化，若某个步骤发生异常，携带异常信息继续执行下一步骤。

package com.gyd;

import java.util.concurrent.*;

public class CompletableFutureDemo6 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        CompletableFuture<Integer> completableFuture = CompletableFuture.supplyAsync(() ->{
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println("111");
            return 1;
        },executorService).handle((f,e) ->{
            System.out.println("222");
            //若当前步骤有异常，则携带异常信息继续执行后续步骤
            int i = 10/0;
            return f+2;
        }).handle((f,e)->{
            System.out.println("上一步骤异常信息："+e);
            System.out.println("333");
            return f+3;
        }).whenComplete((v,e) ->{
            if (e == null) System.out.println("v: "+v);
        }).exceptionally( e ->{
            //发生异常时的处理
            e.printStackTrace();
            System.out.println("发生异常");
            return null;
        });

        System.out.println(Thread.currentThread().getName()+"先去忙别的事情");

    }
}

3、对计算结果进行消费

thenAccept: 任务A执行完继续执行任务B，任务B需要依赖任务A的计算结果，但任务B无返回值

package com.gyd;

import java.util.concurrent.*;

public class CompletableFutureDemo7 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        CompletableFuture.supplyAsync(() ->{
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println("111");
            return 1;
        },executorService).thenApply(f ->{
            System.out.println("222");
            return f+2;
        }).thenApply(f->{
            return f+3;
        }).thenAccept(f -> {
            //接受任务的计算结果，进行消费处理，无返回结果
            System.out.println("完成前两个步骤的任务，消费结果="+f);
        });
        executorService.shutdown();
    }
}

thenRun: 任务A执行完继续执行任务B，任务B不需要任务A的计算结果

package com.gyd;

import java.util.concurrent.*;

public class CompletableFutureDemo8 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        CompletableFuture.supplyAsync(() ->{
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println("111");
            return 1;
        },executorService).thenApply(f ->{
            System.out.println("222");
            return f+2;
        }).thenApply(f->{
            return f+3;
        }).thenRun(() ->{
            System.out.println("执行完前两个步骤后，继续执行当前步骤");
        });

        executorService.shutdown();
     }

    
}

4、比较哪个步骤的任务执行快

applyToEither: 可以用来选出执行速度快的步骤。

package com.gyd;

import java.util.concurrent.*;

public class CompletableFutureDemo10 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {

       CompletableFuture<String> playA = CompletableFuture.supplyAsync(() ->{
           try {TimeUnit.SECONDS.sleep(3);} catch (InterruptedException e) {throw new RuntimeException(e);}
            return "playA";
       });

        CompletableFuture<String> playB = CompletableFuture.supplyAsync(() ->{
            try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {throw new RuntimeException(e);}
            return "playB";
        });
        CompletableFuture<String> winer = playA.applyToEither(playB,f -> f+" is winer");
        System.out.println(Thread.currentThread().getName()+"\t"+"-----:" + winer.join());
     }

}

5、对多个步骤的任务结果进行合并输出

thenCombine： 等待多个CompletionStage任务都完成后，最终把多个任务的结果合并处理输出。

示例将两个CompletionStage结果进行合并输出：

package com.gyd;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class CompletableFutureDemo11 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {

       CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() ->{
           try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {throw new RuntimeException(e);}
            return "task1";
       });
       CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() ->{
            try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {throw new RuntimeException(e);}
            return "task2";
        });
        CompletableFuture<String> future3 = future1.thenCombine(future2, (x , y) -> {
            return "合并："+x+y;
        });
        System.out.println(future3.join());
     }

}

6、等待多个并行任务执行完成后返回

比如需要获取完整订单数据，可能需要填充订单的付款信息、地址信息、商品信息,这三部分数据都来自不同系统,互相没有依赖，可以并行获取。

package com.gyd;

import java.util.Map;
import java.util.concurrent.*;

public class CompletableFutureDemo12 {

    public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {
        Map<String,Object> orderMap = new ConcurrentHashMap<>();
       CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() ->{
           try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("获取地址信息...");
           orderMap.put("address","湖南");
            return "task1";
       });
       CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() ->{
            try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("获取支付信息...");
           orderMap.put("payinfo","10元");
           return "task2";
        });
        CompletableFuture<String> future3 = CompletableFuture.supplyAsync(() ->{
            try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {throw new RuntimeException(e);}
            System.out.println("获取商品信息...");
            orderMap.put("goods","袜子");
            return "task3";
        });
        CompletableFuture<Void> result = CompletableFuture.allOf(future1,future2,future3);
        result.join();

        System.out.println("完整的订单数据:"+orderMap.toString());
     }

}

四、线程池的运行选择

废话少说，直接上代码:

package com.gyd;

import java.util.concurrent.*;

public class CompletableFutureDemo9 {

   public static void main(String[] args) throws ExecutionException, InterruptedException, TimeoutException {

       //a.模拟任务执行速度快的情况，系统底层会使用main线程处理任务
       //运行结果：
       //1号任务	ForkJoinPool.commonPool-worker-25
       //2号任务	ForkJoinPool.commonPool-worker-25
       //3号任务	main
       //4号任务	main
       System.out.println("===模拟任务执行速度快的情况，系统底层会使用main线程处理任务====");
       test0();

        //b.使用内置默认线程池+thenRun执行任务
       //运行结果：
       //1号任务	ForkJoinPool.commonPool-worker-25
       //2号任务	ForkJoinPool.commonPool-worker-25
       //3号任务	ForkJoinPool.commonPool-worker-25
       //4号任务	ForkJoinPool.commonPool-worker-25
       //null
       System.out.println("===使用内置默认线程池+thenRun执行任务====");
       test1();

        //c.使用自定义线程池+thenRun执行任务
       //运行结果：
       //1号任务	pool-1-thread-1
       //2号任务	pool-1-thread-1
       //3号任务	pool-1-thread-1
       //4号任务	pool-1-thread-1
       System.out.println("===使用自定义线程池+thenRun执行任务====");
       test2();

        //d.使用内置默认线程池+thenRunAsync执行任务
       //运行结果：
       //1号任务	ForkJoinPool.commonPool-worker-25
       //2号任务	ForkJoinPool.commonPool-worker-25
       //3号任务	ForkJoinPool.commonPool-worker-25
       //4号任务	ForkJoinPool.commonPool-worker-25
       System.out.println("===使用内置默认线程池+thenRunAsync执行任务====");
       test3();

        //e.使用自定义线程池+thenRunAsync执行任务
       //运行结果：
       //1号任务	pool-2-thread-1
       //2号任务	ForkJoinPool.commonPool-worker-25
       //3号任务	ForkJoinPool.commonPool-worker-25
       //4号任务	ForkJoinPool.commonPool-worker-25
       System.out.println("===使用自定义线程池+thenRunAsync执行任务====");
       test4();

    }

    //a.使用内置默认线程池+thenRun执行任务
   private static void test1() throws ExecutionException, InterruptedException, TimeoutException {
       CompletableFuture<Void> completableFuture = CompletableFuture.supplyAsync(() ->{
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("1号任务"+"\t"+Thread.currentThread().getName());
           return "abcd";
       }).thenRun(() ->{
           try {Thread.sleep(200);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("2号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("3号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("4号任务"+"\t"+Thread.currentThread().getName());
       });

       System.out.println(completableFuture.get(2L,TimeUnit.SECONDS));
       System.out.println("===============");
   }

   //b.使用自定义线程池+thenRun执行任务
   private static void test2() throws ExecutionException, InterruptedException, TimeoutException {
       ExecutorService executorService = Executors.newFixedThreadPool(5);
       CompletableFuture<Void> completableFuture = CompletableFuture.supplyAsync(() ->{
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("1号任务"+"\t"+Thread.currentThread().getName());
           return "abcd";
       },executorService).thenRun(() ->{
           try {Thread.sleep(200);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("2号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("3号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("4号任务"+"\t"+Thread.currentThread().getName());
       });

       System.out.println(completableFuture.get(2L,TimeUnit.SECONDS));
       executorService.shutdown();
       System.out.println("===============");

   }

   //c.使用内置默认线程池+thenRunAsync执行任务
   private static void test3() throws ExecutionException, InterruptedException, TimeoutException {
       CompletableFuture<Void> completableFuture = CompletableFuture.supplyAsync(() ->{
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("1号任务"+"\t"+Thread.currentThread().getName());
           return "abcd";
       }).thenRunAsync(() ->{
           try {Thread.sleep(200);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("2号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("3号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("4号任务"+"\t"+Thread.currentThread().getName());
       });

       System.out.println(completableFuture.get(2L,TimeUnit.SECONDS));
       System.out.println("===============");

   }
   //d.使用自定义线程池+thenRunAsync执行任务
   private static void test4() throws ExecutionException, InterruptedException, TimeoutException {
       ExecutorService executorService = Executors.newFixedThreadPool(4);
       CompletableFuture<Void> completableFuture = CompletableFuture.supplyAsync(() ->{
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("1号任务"+"\t"+Thread.currentThread().getName());
           return "abcd";
       },executorService).thenRunAsync(() ->{
           try {Thread.sleep(200);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("2号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("3号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}
           System.out.println("4号任务"+"\t"+Thread.currentThread().getName());
       });

       System.out.println(completableFuture.get(2L,TimeUnit.SECONDS));
       executorService.shutdown();
       System.out.println("===============");

   }

   //e.模拟任务执行速度快的情况，系统底层会使用main线程处理任务
   private static void test0() throws ExecutionException, InterruptedException, TimeoutException {
       CompletableFuture<Void> completableFuture = CompletableFuture.supplyAsync(() ->{
           System.out.println("1号任务"+"\t"+Thread.currentThread().getName());
           return "abcd";
       }).thenRun(() ->{
           System.out.println("2号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           System.out.println("3号任务"+"\t"+Thread.currentThread().getName());
       }).thenRun(() -> {
           System.out.println("4号任务"+"\t"+Thread.currentThread().getName());
       });

       System.out.println(completableFuture.get(2L,TimeUnit.SECONDS));
       System.out.println("===============");
   }
}

线程池运行选择总结

1）没有传入自定义线程池时，都使用默认线程池ForkJoinPool；

2）执行第一个任务时传入了一个自定义线程池 则当使用thenRun执行之后的任务时，都共用同一个自定义线程池；

3）执行第一个任务时传入了一个自定义线程池 则当使用thenRunAsync执行之后的任务时，只有第一个任务使用的自定义线程池，后续任务都使用的是默认ForkJoin线程池；

4）有可能处理太快的时候，由于系统底层优化原则，直接利用main线程处理任务。

5）其它如thenAccept、thenAcceptAsync、thenApply和thenApplyAsync等，它们之间的区别也同理。

五、应用场景

先A后B的场景应用

package com.gyd;

import java.util.Random;
import java.util.concurrent.*;

public class CompletableFutureDemo2 {
    //先A后B的场景应用
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        //推荐配置自定义的线程池！！
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        try {
            CompletableFuture<Integer> completableFuture = CompletableFuture.supplyAsync(() -> {
                //第一步
                int result = new Random().nextInt(10);
                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + " 出结果:" + result);
                return result;
            },executorService).whenComplete((v, e) -> {
                //第二步，获取第一步的结果
                if (null == e) {
                    System.out.println(Thread.currentThread().getName() + " 接收到结果:" + v);
                }
            }).exceptionally(e -> {
                e.printStackTrace();
                System.out.println("发生异常了：" + e);
                return null;
            });

            System.out.println(Thread.currentThread().getName()+"主线程去忙别的事情");
            //主线程不要立刻结束，否则CompletableFuture默认使用的线程池会立刻关闭，这里暂停3秒钟
            try{ Thread.sleep(3000);}catch (Exception e){e.printStackTrace();}
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            executorService.shutdown();
        }
    }
}

并发执行并获取汇总结果的场景应用

package com.gyd;

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.*;
import java.util.stream.Collectors;

public class CompletableFutureDemo3 {
    static List<NetMall> list = Arrays.asList(
            new NetMall("jd"),
            new NetMall("dangdang"),
            new NetMall("taobao"));

    //串行版本
    public static List<String> getPrice(List<NetMall> list,String productName) {
       return list.stream()
                  .map(netMall ->
                          String.format(productName+" in %s price is %.2f",
                                  netMall.getNetMallName(),
                                  netMall.calPrice(productName)))
                  .collect(Collectors.toList());
    }

    //并行版本
    public static List<String> getPriceByCompletableFuture(List<NetMall> list,String productName) {
        return list.stream().map(netMall -> CompletableFuture.supplyAsync(() ->
                        String.format(productName+"in %s price is %.2f",netMall.getNetMallName(),netMall.calPrice(productName))))
                .collect(Collectors.toList())
                .stream()
                .map(s -> s.join())
                .collect(Collectors.toList());

    }

    public static void main(String[] args) throws ExecutionException, InterruptedException {
       long startTime = System.currentTimeMillis();
       List<String> list1 = getPrice(list,"mysql");
       for (String element : list1) {
           System.out.println(element);
       }
       long endTime = System.currentTimeMillis()-startTime;
       System.out.println("costTime:"+endTime);

       System.out.println("=======================");
       startTime = System.currentTimeMillis();
       List<String> list2 = getPriceByCompletableFuture(list,"mysql");
       for (String element : list2) {
           System.out.println(element);
       }
       endTime = System.currentTimeMillis()-startTime;
       System.out.println("======costTime2 "+endTime);
    }
}

class NetMall {
    String netMallName;

    public String getNetMallName() {
        return netMallName;
    }

    public NetMall(String netMallName){this.netMallName = netMallName;}

    public Double calPrice(String productName){
        try {
            TimeUnit.SECONDS.sleep(1);
        }catch (Exception e) {
            e.printStackTrace();
        }
        return ThreadLocalRandom.current().nextDouble()*2+productName.charAt(0);
    }

}
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