很久以前多线程是这样创建：
Thread t1 = new Thread();
Thread t2 = new Thread();
t1.start(); // 启动新线程
t2.start(); // 启动新线程
由于创建和销毁线程是非常耗资源，因此改成线程建好后不销毁，可以重用，用户只需提供run方法的具体实现：
public static void main(String[] args) throws Exception {
ExecutorService executor = Executors.newSingleThreadExecutor();
Future<String> stringFuture = executor.submit(new Callable<String>() {
@Override
public String call() throws Exception {
Thread.sleep(2000);
return "async thread";
}
});
Thread.sleep(1000);
System.out.println("main thread");
System.out.println(stringFuture.get());
}
但如果很多线程被创建，并且线程间有依赖，即按流程和条件执行线程，实现起来就有点复杂，于是CompletableFuture横空出世。一共有50各方法可供使用。
CompletableFuture.supplyAsync()，相当于创建了ExecutorService，同时也创建了Callable，然后提交到线程池中执行。
CompletableFuture<String> futureA = CompletableFuture.supplyAsync(() -> "任务A");
CompletableFuture<String> futureB = CompletableFuture.supplyAsync(() -> "任务B");
CompletableFuture<String> futureC = futureB.thenApply(b -> {
System.out.println("执行任务C.");
System.out.println("参数:" + b);//参数:任务B
return "a";
});
!!How to use CompletableFuture and Callable in Java
https://ducmanhphan.github.io/2020-02-10-How-to-use-CompletableFuture-Callable-in-Java/使用CompletableFuture优化你的代码执行效率
https://www.cnblogs.com/fingerboy/p/9948736.htmlCompletableFuture 使用详解
https://www.jianshu.com/p/6bac52527ca4使用CompletableFuture
https://www.liaoxuefeng.com/wiki/1252599548343744/1306581182447650https://github.com/eugenp/tutorials/blob/master/core-java-modules/core-java-concurrency-basic/src/test/java/com/baeldung/completablefuture/CompletableFutureLongRunningUnitTest.java