java - 停止供应商内部的 CompletableFuture 链

Question

我有这样的操作，

public void createFuture() {
    CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 5);
    future.thenApply(i -> {
        if (i == 5) {
            System.out.println("hello, i equals to 5 so expensive operations are unnecessary");
        }
        return i;
    }).thenApply(i -> {
        if (i != 5) {
            System.out.println("Expensive operation");
        }
        return i;
    }).thenApply(i -> {
        if (i != 5) {
            System.out.println("Expensive operation");
        }
        return i;
    }).thenApply(i -> {
        if (i != 5) {
            System.out.println("Expensive operation");
        }
        return i;
    }).thenApply(i -> {
        if (i != 5) {
            System.out.println("Expensive operation");
        }
        return i;
    });

}

在第一个 block 中，我检查一些条件(i == 5)，如果为真，则不需要其余操作。我不想抛出异常来取消其余的，因为这不是特殊情况。除了向每个操作传递一些 boolean 值之外，还有其他好方法吗？

Answer 1

CompletableFuture.thenCompose() 应该可以解决问题。

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.ExecutionException;

class Scratch {

  public static void main(String[] args) throws ExecutionException, InterruptedException {
    CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 5);
    CompletableFuture<Integer> composed = future.thenCompose(number -> {
      if (number == 5)
        return simpleFuture(number);
      else
        return complexFuture(number);
    });
    System.out.println(composed.get());
  }

  private static CompletionStage<Integer> complexFuture(Integer number) {
    return CompletableFuture.completedFuture(number)
        .thenApply(i -> {
          System.out.println("I am expensive");
          return i;
        });
  }

  private static CompletableFuture<Integer> simpleFuture(Integer number) {
    return CompletableFuture.completedFuture(number)
        .thenApply(i -> {
          System.out.println("I am cheap");
          return i;
        });
  }
}