很久以前多線程是這樣創建：
Thread t1 = new Thread();
Thread t2 = new Thread();
t1.start(); // 啟動新線程
t2.start(); // 啟動新線程
由于創建和銷毀線程是非常耗資源，因此改成線程建好后不銷毀，可以重用，用戶只需提供run方法的具體實現：
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());
}
但如果很多線程被創建，并且線程間有依賴，即按流程和條件執行線程，實現起來就有點復雜，于是CompletableFuture橫空出世。一共有50各方法可供使用。
CompletableFuture.supplyAsync()，相當于創建了ExecutorService，同時也創建了Callable，然后提交到線程池中執行。
CompletableFuture<String> futureA = CompletableFuture.supplyAsync(() -> "任務A");
CompletableFuture<String> futureB = CompletableFuture.supplyAsync(() -> "任務B");
CompletableFuture<String> futureC = futureB.thenApply(b -> {
System.out.println("執行任務C.");
System.out.println("參數:" + b);//參數:任務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優化你的代碼執行效率
https://www.cnblogs.com/fingerboy/p/9948736.htmlCompletableFuture 使用詳解
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