Completable Futures - thenApply vs thenCompose

January 6, 2024

Let’s wrap a blocking operation in a CompletablFuture

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public static Integer blockingCall(int x) {
    return x * 2;
}

public static CompletableFuture<Integer> asyncRunner(int x) {
    return CompletableFuture.supplyAsync(() -> blockingCall(x));
}

Here we chain the outputs so that it prints the aggregated results. But this will print the future not the value (16)

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public static void main(String[] args) {
    asyncRunner(4)
        .thenApply(data -> asyncRunner(data)) // returns CompletableFuture<Integer> yikes!  - analogous to map operation
        .thenAccept(System.out::println) // returns CompletableFuture<Void>
}

But I want to return 16 as output but not wrapped in a CompletableFuture.

Solution: simply replace thenApply with thenCompose

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asyncRunner(4)
        .thenACompose(data -> asyncRunner(data)) // returns 16 :)  - analogous to flatMap operation
        .thenAccept(System.out::println) // returns CompletableFuture<Void>

Notes

Non-blocking operations may run in main thread or in a background thread depending on the duration of the operation!

runAsync - For fire and forget tasks
supplyAsync - Get a response back from the task - by default uses the ForkJoin pool
thenApply - Just like the map method in Stream API
thenCompose - Aanalogous to flatMap method in Stream API