JUC源码解析CompletableFuture

Posted 2020-11-19 aniao

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简介

先说Future, 它用来描述一个异步计算的结果。isDone方法可以用来检查计算是否完成，get方法可以用来获取结果，直到完成前一直阻塞当前线程，cancel方法可以取消任务。而对于结果的获取，只能通过阻塞(get())或者轮询的方式[while(!isDone)]. 阻塞的方式违背了异步编程的理念，轮询的方式耗费无谓的CPU资源（CPU空转）。于是，CompletableFuture应运而生。

样例

后面介绍的源码都会以下面的用例为切入点，循着调用轨迹理解源码。

 1     public void whenComplete() {
 2         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
 3         future.whenComplete((l, r) -> System.out.println(l));
 4     }
 5 
 6     public void thenApply() {
 7         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
 8         future.thenApply(i -> -i);
 9     }
10 
11     public void thenAccept() {
12         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
13         future.thenAccept(System.out::println);
14     }
15 
16     public void thenRun() {
17         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
18         future.thenRun(() -> System.out.println("Done"));
19     }
20 
21     public void thenAcceptBoth() {
22         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
23         CompletableFuture<Integer> other = CompletableFuture.supplyAsync(() -> 200);
24         future.thenAcceptBoth(other, (x, y) -> System.out.println(x + y));
25     }
26 
27     public void acceptEither() {
28         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
29         CompletableFuture<Integer> other = CompletableFuture.supplyAsync(() -> 200);
30         future.acceptEither(other, System.out::println);
31 
32     }
33 
34     public void allOf() {
35         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
36         CompletableFuture<Integer> second = CompletableFuture.supplyAsync(() -> 200);
37         CompletableFuture<Integer> third = CompletableFuture.supplyAsync(() -> 300);
38         CompletableFuture.allOf(future, second, third);
39 
40     }
41 
42     public void anyOf() throws InterruptedException, ExecutionException {
43         CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 100);
44         CompletableFuture<Integer> second = CompletableFuture.supplyAsync(() -> 200);
45         CompletableFuture<Integer> third = CompletableFuture.supplyAsync(() -> 300);
46         CompletableFuture.anyOf(future, second, third);
47     }