CompletableFuture用法介绍

Posted 2020-11-15 tenwood

一、CompletableFuture用法入门介绍

入门介绍的一个例子：

 1 package com.cy.java8;
 2 
 3 import java.util.Random;
 4 import java.util.concurrent.CompletableFuture;
 5 
 6 public class CompletableFutureInAction {
 7     private final static Random RANDOM = new Random(System.currentTimeMillis());
 8 
 9     public static void main(String[] args){
10         CompletableFuture<Double> completableFuture = new CompletableFuture<>();
11         new Thread(() -> {
12             double value = get();
13             completableFuture.complete(value);
14         }).start();
15 
16         System.out.println("do other things...");
17 
18         completableFuture.whenComplete((t, e) -> {
19             System.out.println("complete. value = "+ t);
20             if(e != null){
21                 e.printStackTrace();
22             }
23         });
24     }
25 
26     private static double get(){
27         try {
28             Thread.sleep(RANDOM.nextInt(3000));
29         } catch (InterruptedException e) {
30             e.printStackTrace();
31         }
32         return RANDOM.nextDouble();
33     }
34 }

console打印：

do other things...
complete. value = 0.8244376567363494

　

二、CompletableFuture.supplyAsync

CompletableFuture很少有直接new出来的方式去用的，一般都是通过提供的静态方法来使用。

1.使用CompletableFuture.supplyAsync来构造CompletableFuture：

 1 package com.cy.java8;
 2 
 3 import java.util.concurrent.*;
 4 
 5 import static com.cy.java8.CompletableFutureInAction.get;
 6 
 7 public class CompletableFutureInAction2 {
 8 
 9     public static void main(String[] args) {
10         /**
11          * 可以发现value=..没有被打印，为什么呢？
12          * 因为此方法构造出来的线程是demon的，守护进程，main执行结束之后就消失了，所以
13          * 根本没来得及执行whenComplete中的语句
14          */
15         CompletableFuture.supplyAsync(() -> get())
16                 .whenComplete((v, e) -> {
17                     System.out.println("value = " + v);
18                     if (e != null) {
19                         e.printStackTrace();
20                     }
21                 });
22 
23         System.out.println("do other things...");
24     }
25 
26 
27 }

2.要将上面whenComplete中的语句执行，进行改造：

 1 package com.cy.java8;
 2 
 3 import java.util.concurrent.*;
 4 import java.util.concurrent.atomic.AtomicBoolean;
 5 import static com.cy.java8.CompletableFutureInAction.get;
 6 
 7 public class CompletableFutureInAction2 {
 8 
 9     public static void main(String[] args) throws InterruptedException {
10         AtomicBoolean finished = new AtomicBoolean(false);
11 
12         CompletableFuture.supplyAsync(() -> get())
13                 .whenComplete((v, e) -> {
14                     System.out.println("value = " + v);
15                     if (e != null) {
16                         e.printStackTrace();
17                     }
18                     finished.set(true);
19                 });
20 
21         System.out.println("do other things...");
22 
23         while(!finished.get()){
24             Thread.sleep(1);
25         }
26     }
27 
28 
29 }

改写之后， main线程发现如果finished没有变为true就会一直等1ms，直到whenComplete执行将finished变为true。

 

3.上面的改写很low，其实只要将守护线程变为前台进程，main结束后不会消失就行了。

 1 package com.cy.java8;
 2 
 3 import java.util.concurrent.*;
 4 import static com.cy.java8.CompletableFutureInAction.get;
 5 
 6 public class CompletableFutureInAction2 {
 7 
 8     public static void main(String[] args){
 9         ExecutorService executorService = Executors.newFixedThreadPool(2, r -> {
10             Thread t = new Thread(r);
11             t.setDaemon(false);     //非守护线程
12             return t;
13         });
14 
15         CompletableFuture.supplyAsync(() -> get(), executorService)
16                 .whenComplete((v, e) -> {
17                     System.out.println("value = " + v);
18                     if (e != null) {
19                         e.printStackTrace();
20                     }
21                 });
22 
23         System.out.println("do other things...");
24 
25         //main执行结束之后，executorService线程不会结束，需要手动shutdown
26         executorService.shutdown();
27     }
28 
29 
30 }

 

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