CompletableFuture thenApplyAsync与thenCompose及其用例[重复]

Question

CompletableFuture<String> completableFuture = CompletableFuture.supplyAsync(() -> {
    System.out.println(Thread.currentThread().getName() + " Printing hello");
    return "Hello";
}).thenCompose((String s) -> {
  return CompletableFuture.supplyAsync(() -> {
      System.out.println(Thread.currentThread().getName() + " Adding abc");
      return "abc "+s;});
}).thenApplyAsync((String s) -> {
    System.out.println(Thread.currentThread().getName() + " Adding world");
    return s + " World";
}).thenApplyAsync((String s) -> {
    System.out.println(Thread.currentThread().getName() + " Adding name");
    if (false) {
       throw new RuntimeException("Oh no exception");
    }
    return s + " player!";
}).handle((String s, Throwable t) -> {
    System.out.println(s != null ? s : "BLANK");
    System.out.println(t != null ? t.getMessage() : "BLANK Exception");
    return s != null ? s : t.getMessage();
});

我浏览了这个链接，但它讨论的是thenApply，这有点不同：CompletableFuture thenApply vs thenCompose

Answer 1

当操作返回CompletionStage时，可以使用thencompose，当操作不返回CompletionStage时，可以使用thenapply。->这是在enapply vs encompose中

但是，CompletionStage接口的Async变体有细微的区别和罕见的用例。

让我们考虑一下这个例子：

import java.util.concurrent.CompletableFuture;

public class Futures {

    public static void main(String[] args) throws InterruptedException {

        CompletableFuture<Void> c = CompletableFuture.runAsync(() -> {

            System.out.println("run1: " + Thread.currentThread().getId());
        });

        c.whenComplete((r, t) -> {
            System.out.println("I'm completed");
        });

        c.thenCompose(__ -> {
            System.out.println("thenCompose1: " + Thread.currentThread().getId());
            return CompletableFuture.runAsync(() -> {
                System.out.println("run2: " + Thread.currentThread().getId());
            });
        }).thenRunAsync(() -> {
            System.out.println("RunAsync1: " + Thread.currentThread().getId());
        });

        Thread.sleep(5000);

        System.out.println("Finished");
    }
}

它输出类似于：

run1: 11
thenCompose1: 11
run2: 12
I'm completed
RunAsync1: 11
Finished

static CompletionStage<String> insert(Object database) {
    throw new UnsupportedOperationException();
}

static CompletionStage<Object> get(String id) {
    throw new UnsupportedOperationException();
}

public static void main(String[] args) throws InterruptedException {

    Object db = new Object(); // pretend this connects to a database
    CompletionStage<Object> recordInserted = insert(db).thenCompose(id -> get(id));
}

然后，您将使用ThenApplyAsync启动发送电子邮件逻辑，因为它对您的系统并不重要。用户总是可以回去说“再给我发一封电子邮件”

static CompletionStage<String> register(String username) {
    throw new UnsupportedOperationException();
}

static void sendConfirmationEmail(String username) {
    throw new UnsupportedOperationException();
}

public static void main(String[] args) throws InterruptedException {

    register("user").thenAcceptAsync(username -> sendConfirmationEmail(username));

}

在这里，当注册完成时，您的系统会响应，但它不会等待电子邮件发送，从而提高了您系统的响应能力。

到目前为止，我发现的Async变体的用例很少，但它们确实存在。