如何使用 CompletableFuture 的 supplyAsync 每次运行具有多个输入的相同方法？

Question

I have the following code where I create a supplier and use the completableFuture's supplyAsync method to invoke another method after async execution.

public void runParallelFunctions(MyInput myInput) {
    Supplier<Map<String, String>> taskSupplier = () -> {
        try {
            return invokeLambda("input1");
        } catch (Exception e) {
            System.out.println(e);
        }
        return new HashMap<>();
    };
    
    for (int i = 0; i < 5; i++) {
        CompletableFuture.supplyAsync(taskSupplier::get, executorService)
                         .thenAccept(this::printResultsFromParallelInvocations);
    }
    System.out.println("Doing other work....");
}

Below is the method I call after the execution completes.

private void printResultsFromParallelInvocations(Map<String, String> result) {
        result.forEach((key, value) -> System.out.println(key + ": " + value));
}

In the above code, how can I call the method invokeLambda passing multiple arguments like "input1", "input2" etc.? I can generate the inputs through a loop, but how can I use some sort of a list with the supplier so that I can call the entire list for supplyAsync method? I cannot use runAsync method because I have a return value that I need to call printResultsFromParallelInvocations with. I'm new to futures and async callbacks and would appreciate any help. Thanks in advance.

Answer 1

You can not create a single Supplier<Map<String, String>> and expect it to behave differently for the five evaluations. It would require external mutable state to make it detectable that an evaluation is the n'th evaluation which at the same time contradicts the idea of performing five concurrent evaluations which have no order.

Simply create five different suppliers, eg

for(int i = 0; i < 5; i++) {
    String input = "input" + i;
    CompletableFuture.supplyAsync(() -> invokeLambda(input), executorService)
        .thenAccept(this::printResultsFromParallelInvocations);
}

In each loop iteration, the lambda expression () -> invokeLambda(input) captures the current value of input and creates an appropriate Supplier instance.

Side notes:

• Don't name methods after technical aspects like invokeLambda but rather try to express their purpose .

• the taskSupplier::get in your original code was an unnecessary method reference as it produced a Supplier invoking the method on an object that was already a Supplier . So taskSupplier could have been passed to supplyAsync directly if getting the same behavior for every evaluation was intended.

Answer 2

You can create new Supplier on the fly inside the loop.

public static Supplier<Map<String, String>> supplierFunc(Object... args) {
    return () -> {
        try {
            return invokeLambda(args);
        } catch (Exception e) {
            System.out.println(e);
        }
        return new HashMap<>();
    };
}

public void runParallelFunctions(Object myInput) {
    for (int i = 0; i < 5; i++) {
        CompletableFuture.supplyAsync(supplierFunc("input1", "input2"), executorService)
                .thenAccept(this::printResultsFromParallelInvocations);
    }
    System.out.println("Doing other work....");
}