使用compose创建CompletableFutures的链接列表

Question

I want to create a chain of CompletableFutures.

I'm trying to build things up as follows.

Task 1 performs something and returns a String as result when Task 1 is done I want to start Task 2 with as input the result of Task 1. Task 2 when this is ready it returns an Integer and so on...

So it should be very dynamical so I already have this:

      try {
            CompletableFuture<String> task1Future = CompletableFuture.supplyAsync(new Task1());
            CompletableFuture<String> result = task1Future.thenCompose(task1Result -> CompletableFuture.supplyAsync(new Task2(task1Result)));
            System.out.println(result.get());
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }


        public class Task1 implements Supplier<String> {

    public Task1() {
        System.out.println("Task 1 started");
    }

    @Override
    public String get() {
        try {
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
            throw new IllegalStateException(e);
        }
        return "Result Task 1";
    }
}

What I know try to achieve is build a Wrapper (a sort of linked list):

List of Task where Task should be:

public class Task {
    private Supplier startTask;
    private Task followUpTask;

    public Task(Supplier startTask, Task followUpTask) {
        this.startTask = startTask;
        this.followUpTask = followUpTask;
    }
}

But I'm now stuck because I don't know how to do the chaining and to make Task more generic that it can be initiated with whatever is the result of the previous task.

So I need to have a method to contruct the CompletableFuture and to just say start() and that everything happens.

Can someone help me on the way?

Answer 1

You should define Task2 as a Function, since it accepts previous String result and produces new Integer result:

public static class Task2 implements Function<String, Integer> {

    public Task2() {
        System.out.println("Task 2 started");
    }

    @Override
    public Integer apply(String s) {
        return s.length();
    }
}

then you can chain them as follows:

   CompletableFuture<String> task1Future = CompletableFuture.supplyAsync(new Task1());
   CompletableFuture<Integer> result = task1Future.thenApply(new Task2());
   System.out.println(result.get());

Answer 2

Just in case you want all your tasks to implement Function, you can start the chain as follows:

CompletableFuture<String> task1Future = CompletableFuture.completedFuture("S")
    .thenApply(new Task1());

where completedFuture("S") holds the argument for the first task:

public static class Task1 implements Function<String, String>  {

    public Task1() {
        System.out.println("Task 1 started");
    }

    @Override
    public String apply(String s) {
        try {
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
            throw new IllegalStateException(e);
        }
        return "Result Task 1";
    }
}