When we want to return a result after finishing a task asynchronously, we can use CompletableFuture.supplyAsync(). It takes Supplier<T> as a parameter and returns CompletableFuture<T>.

Let's check the supplyAsync() method by creating another controller method in our BankController class, with the following example:

@RequestMapping(value = "/synccustbal")
  @ResponseBody
  public CompletableFuture<String> syncCustomerBalance() {
    LOGGER.info("Entering in controller");

    CompletableFuture<String> completableFuture = 
    CompletableFuture.supplyAsync(new Supplier<String>() {
      
      @Override
      public String get() {
        try {
          return syncService.syncCustomerBalance().get();
        } catch (InterruptedException | ExecutionException e) {
          LOGGER.error(e);
        }
        return "No balance found";
      }
    }); 
      LOGGER.info("Leaving from controller");
      return completableFuture;
  }

The CompletableFuture object uses the global thread pool, ForkJoinPool.commonPool(), to execute tasks in a separate thread. We can create a thread pool and pass it to runAsync() and supplyAsync() methods.

The following are two variants of the runAsync() and supplyAsync() methods:

CompletableFuture<Void> runAsync(Runnable runnable)
CompletableFuture<Void> runAsync(Runnable runnable, Executor executor)
CompletableFuture<U> supplyAsync(Supplier<U> supplier)
CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor)