Java 8, among other changes, had introduced CompletableFuture which has made writing asynchronous programs in Java easy. In this article I will be using CompletableFuture to explain how asynchronous programs are written and what value asynchronous programs bring in the context of backend services’ response times. I will also be covering different features of the CompletableFuture itself.

Though the concepts explained here use Java as an example, it can be applied in any other language. In fact CompletableFutures are similar to Promises in JavaScript.

## Why do we need asynchronous programs

With growing dependence of business on information technology, expectations from business serving applications have become high. They need to be fast and highly available. While serving requests, applications rarely work in isolation. For serving a request, applications have to talk to one or more external services like databases, third-party services, and internal applications. All of these happen over network and thus have a tendency to increase response times and increase chances of errors. We may not have much control over network errors, but we can use asynchronous programming techniques to reduce response time. Let’s see, using an example, how this reduction can be achieved.

Suppose that to serve a request an application has to talk to two services, Service A and Service B. Let’s assume that average response time of Service A is 2 seconds and that of Service B is 3 seconds.

• If we call them serially, then minimum response time of the application is 5 seconds (2 seconds + 3 seconds).
• If Service A and Service B is called in parallel, then minimum response time of the application becomes 3 seconds (maximum time among 2 seconds and 3 seconds). This is assuming that both services can be called in parallel i.e. to call one service there is no dependency on any data from the other service.

You will have to use asynchronous programming techniques to be able to make calls in parallel which will lead to a reduction in response times.

Let’s see what it means to use asynchronous programming techniques in the applications you develop.

## Designing asynchronous interfaces

Let’s say we have an interface as below. Any implementation of this interface can either extract the data from a database or call another service to get the details. In either of the cases, the processing will be done on the calling thread, the thread from which the interface method is called, and the execution will be stopped till the order value is received. Such kind of interface definitions are blocking.

interface OrderService {
Order findOrderByOrderId(String orderId); // synchronous
}


Now let’s transform this interface to make it non-blocking a.k.a asynchronous. In the code snippet below, notice that I have changed the return type from Order to CompletableFuture<Order>. Instead of returning the value of order, which blocks the calling thread, a container or holder of order is returned. Any implementation of the interface can now:

• Immediately return the container with no value populated in the beginning.
• Start the processing, of order retrieval, in a thread different from the calling thread.
• Populate the container with the order value when the processing completes.

The advantage to the calling thread is that it can do something else till the time the result, the order, is available in the container.

interface OrderService {
CompletableFuture<Order> findOrderByOrderId(String orderId);  // asynchronous
}


### What all interfaces should be asynchronous

The implementations of interfaces that perform IO operations or long running computations are good candidates for making asynchronous.

## CompletableFuture Recipes

I will be highlighting the features of CompletableFuture using some recipes.

### Recipes for an implementor of an asynchronous interface

We saw in the previous section that, while writing asynchronous code, we returned a container of a value rather than the value itself. This changes the way you have been writing implementations of service interfaces. Let’s see the changes using a few recipes.

#### Returning the container and populating the container with the computed value at a later time

Let’s say you have an interface and it’s implementation as below. This represents the synchronous way.

interface SomeSyncService {
SomeReponse someMethod(String someParam);
}

class SomeSyncServiceImpl implements SomeSyncService {
@Override SomeReponse someMethod(String someParam) {
return retrieveResponse(someParam);
}

private SomeResponse retrieveResponse(String someParam) { ... }
}


Let’s convert this to asynchronous using CompletableFuture. Note the use of supplyAsync method. CompletableFuture will execute the task passed to supplyAsync in a separate thread provided by ForkJoinPool. This method will also take care to populate the container with the computed value if processing completes successfully. It will populate with the error if any RuntimeException occurs during execution. We will see later how to handle success or failure responses.

interface SomeAsyncService {
CompletableFuture<SomeReponse> someMethod(String someParam);
}

class SomeAsyncServiceImpl implements SomeAsyncService {
@Override CompletableFuture<SomeReponse> someMethod(String someParam) {
return CompletableFuture.supplyAsync(() -> retrieveResponse(someParam));
}

private SomeResponse retrieveResponse(String someParam) { ... }
}


#### Handling checked exceptions in implementations

Let’s modify to the previous example by letting the retrieveResponse private method to now throw a checked exception. The following points are worth noting:

• In (1) an empty container is created
• In (2) the asynchronous task is started by submitting the task to an ExecutorService which runs the task in a separate thread
• In (5) the empty container is immediately returned hence does not block the calling thread
• At a later point in time submitted task can complete successfully as in (3) or complete with error as in (4)
class SomeAsyncServiceImpl implements SomeAsyncService {
@Override CompletableFuture<SomeReponse> someMethod(String someParam) {
CompletableFuture<SomeResponse> future = new CompletableFuture<>(); // 1
executorService.submit(() -> {  // 2
try {
future.complete(retrieveResponse(someParam)); // 3
} catch (IOException e) {
future.completeExceptionally(e); // 4
}
});
return future; // 5
}

private SomeResponse retrieveResponse(String someParam) throws IOException { ... }

// fields and constructors not shown for brevity
}


### Recipes for a client using the asynchronous interface

Let’s see some recipes of how to code a client using the interfaces that return CompletableFuture.

#### Running two computations in parallel

Suppose we have two services AsyncServiceA and AsyncServiceB and we want to execute them in parallel. When both complete, we want to join the responses ResponseA and ResponseB to return the result. In the example below:

• The thenCombine method collects the result of both the futures, futureA and futureB. Once both of them complete successfully, the combiner function is called.
• The join method which blocks till the combinedFuture completes.
// start task A asynchronously
CompletableFuture<ResponseA> futureA = asyncServiceA.someMethod(someParam);
CompletableFuture<ResponseB> futureB = asyncServiceB.someMethod(someParam);

CompletableFuture<String> combinedFuture = futureA
.thenCombine(futureB, (a, b) -> a.toString() + b.toString());

// wait till both A and B complete
String finalValue = combinedFuture.join();


#### Handling errors that occur during asynchronous task execution

We have seen that the CompletableFuture is a container, which when completes, has the successfully computed value or the exception when computation completes exceptionally. In the last example, calling the join directly is dangerous because in case of exceptional completion, it may throw CompletionException. CompletableFuture provides some handler methods like exceptionally, handle and whenComplete to provide error handling.

Let’s see below how we can use the exceptionally method to make our code more robust. In the example below:

• The join is safe because we have declared in (2) that return Optional.empty() whenever any exception occurs.
• The function passed to thenApply in (1) will get executed only when futureA completes without any error.
CompletableFuture<SomeResponseA> futureA = someAsyncServiceA.someMethod(someParam);

Optional<SomeResponseA> safeResponse = futureA
.thenApply(Optional::ofNullable)  // 1
.exceptionally(ex -> Optional.empty()) // 2
.join();


#### Adding timeout to asynchronous executions

In all the examples that we have seen till now, there is a chance that the asynchronous tasks takes too long to complete. You may want to apply some sensible timeout properties. This blog post does a very good job of explaining this. I will mention here a summary for this recipe.

Suppose you want to add a timeout of 30 seconds to the example shown in the previous section. In the example below:

• Notice the use of applyToEither method on the CompletableFuture to achieve the timeout. It waits asynchronously for either of the two futures to complete first.
• When either of the futureA or timeout completes, the value from it is transferred to subsequent computations using Function.identity().
// start task A asynchronously
CompletableFuture<SomeResponseA> futureA = someAsyncServiceA.someMethod(someParam);
// Start a timeout task asynchronously
CompletableFuture<SomeResponseA> timeout = failAfter(Duration.ofSeconds(30));

Optional<SomeResponseA> safeResponse = futureA
.applyToEither(timeout, Function.identity())
.thenApply(Optional::ofNullable)
.exceptionally(ex -> Optional.empty())
.join();

public static <T> CompletableFuture<T> failAfter(Duration duration) {
// schedule a task that throws exception after specified duration
}


## Summary

I hope that after reading this blog you will agree with me that asynchronous programming brings benefits when we want to achieve faster response times. I also hope that after seeing the recipes you will agree that CompletableFuture has made it easy to write asynchronous code in Java. It provides a rich set of methods for defining flows. I assure you that this will definitely be a good addition to your technology tool kit.

There are other libraries like RxJava that make writing asynchronous programs in Java easy. Check out this link to know more.