Is CompletableFuture ordering guaranteed when chaining with 'then..' methods is not involved?

Question

Suppose

CompletableFuture<T> cf = CompletableFuture.completedFuture(...);
cf.thenApplyAsync(f)
  .thenApplyAsync(g);

cf.thenApplyAsync(h);

By chanining the calls to f and g we get a guaranteed ordering. What about h ? Do we get any guarentees that h will allways be executed after g ?

I tried putting g to sleep in order to test this and I always see h executing after g but this is not evidence. If it is the case that h is always executed after g where can I find some documentation about this ? I read the CompletionStage, ExecutorService and CompletableFuture docs but I haven't really found any info that would lead me to a conclusion on ordering when chaining like in the case of (g . f) is not involved.

Answer 1

If it's not in javadocs, it's not guaranteed. In your case, h can easily be executed before g . All it would take for this to happen is for f to take a while:

cf.thenApplyAsync(i -> {
    try {
        System.out.println("F");
        Thread.sleep(5000);
        return i;
    } catch(InterruptedException e) {
        throw new RuntimeException(e);
    }
}).thenApplyAsync(i -> {
        System.out.println("G");
        return i;
});

cf.thenApplyAsync(i -> {
        System.out.println("H");
        return i;
});