java CompletableFuture未按预期工作

Question

我试图从Java8理解CompletableFuture的工作原理。下面的代码按预期工作

CompletableFuture.supplyAsync(() -> {
    System.out.println("supplyAsync Thread name " + Thread.currentThread().getName());
    return "str";
}).thenApply(str -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    return str;
}).thenApply(str1 -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    return str1;
}).thenAccept(str3 -> {
    System.out.println("thenAccept Thread name " + Thread.currentThread().getName());
});
System.out.println("Thread name " + Thread.currentThread().getName());

输出：

supplyAsync Thread name ForkJoinPool.commonPool-worker-1
thenApply Thread name main
thenApply Thread name main
thenAccept Thread name main
Thread name main

但是当我进行一些计算时，它并没有像预期的那样工作。如果我遗漏了什么，请纠正我

CompletableFuture.supplyAsync(() -> {
    System.out.println("supplyAsync Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
    return "str";
}).thenApply(str -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
    return str;
}).thenApply(str1 -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
    return str1;
}).thenAccept(str3 -> {
    System.out.println("thenAccept Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
});

System.out.println("Thread name " + Thread.currentThread().getName());

输出为：

supplyAsync Thread name ForkJoinPool.commonPool-worker-1
Thread name main

我同意我没有将子线程连接到主线程。我的理解是子线程应该独立于主线程打印语句。问题是为什么它根本不打印

Answer 1

解释

您没有将子线程ForkJoinPool.commonPool-worker-1连接到主线程。所以一旦线程main完成，它就会被杀死

解决方案

在代码中的某个时刻，尝试调用.join()来描述你的完全未来。请注意，此方法正在阻止main线程。因此，连接点之后的执行将暂停，直到子线程完成其执行

CompletableFuture.supplyAsync(() -> {
    System.out.println("=> supplyAsync Thread name " + Thread.currentThread().getName());
    // ...
}).thenApply(str -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    // ...
}).thenApply(str1 -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    // ...
}).thenAccept(str3 -> {
    System.out.println("thenAccept Thread name " + Thread.currentThread().getName());
    // ...
}).join()

将打印：

supplyAsync Thread name ForkJoinPool.commonPool-worker-1
thenApply Thread name ForkJoinPool.commonPool-worker-1
thenApply Thread name ForkJoinPool.commonPool-worker-1
thenAccept Thread name ForkJoinPool.commonPool-worker-1
Thread name main

如果希望最后一个System.out.println(...)不依赖于子线程的执行，则将CompletableFuture分配给一个变量，并将其连接到main的最末端：

CompletableFuture<Void> future = CompletableFuture.supplyAsync(...) ... // 

System.out.println("Thread name " + Thread.currentThread().getName());

future.join();    

Answer 2

My understanding is child thread should print the statements independently of main thread.

你的理解是错误的

每the documentation

Actions supplied for dependent completions of non-async methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.

这并不要求依赖完成由完成异步操作的线程执行，它只允许它

不过，我同意另一个答案，即你应该安排在计算链完成之前不要退出主例程