A Guide to Error Handling with Scala Future

In Scala, Future is a powerful abstraction for working with asynchronous computations. When working with Futures, it’s important to handle errors appropriately to ensure the stability and reliability of your code. Here are some techniques for error handling with Scala Future :

Error Handling in Futures

failed

In the scenario where we know that the given future will fail with an exception, we’ll treat failure as success. For that Future.failed method allows you to create a failed Future with a specific exception or error. This is useful when you want to explicitly indicate that a computation has failed.

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

// Creating a failed Future with a specific exception
val failedFuture: Future[String] = Future.failed(new RuntimeException("Something went wrong!"))

In this example, we use the Future.failed method to create a failed Future with a RuntimeException indicating that something went wrong. The Future.failed method takes an exception or error as its parameter.

fallbackTo

In Scala, the fallbackTo method is used with a Future to specify an alternative Future that will execute if the original Future fails or completes with an exception. The fallbackTo method allows you to provide a backup computation in case of failure.

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

def computeSomething(): Future[Int] = {
  // Simulate some asynchronous computation
  Future {
    // Perform computation here
    // let's assume it throws an exception
    throw new Exception("Computation failed")
  }
}

// Define a backup computation as a Future
def backupComputation(): Future[Int] = {
  // Perform an alternative computation here
  Future.successful(42) // Returning a successful Future with a value of 42
}

// Use fallbackTo to specify the backup computation
val result: Future[Int] = computeSomething().fallbackTo(backupComputation())

In this example, the computeSomething function represents the original computation that may fail or complete with an exception. The backupComputation function represents an alternative computation that should be executed if the original computation fails.

The fallbackTo method is invoked on the original Future, specifying the backup computation to be executed. It returns a new Future one that represents either the successful result of the original computation or the backup computation if the original computation fails. If both future fails then the resulting future will also fail.

recover

Using recover method you can handle exceptions and recover from failures within a Future. recover takes a partial function that matches the Throwable type and returns a fallback value. Here’s an example:

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

// Example future that may throw an exception
val future: Future[Int] = Future {
  // Some computation that may throw an exception
  if (scala.util.Random.nextBoolean()) {
    throw new RuntimeException("Oops, something went wrong!")
  } else {
    42
  }
}

// Handling exceptions using recover
val recoveredFuture: Future[Int] = future.recover {
  case e: RuntimeException =>
    println(s"Exception occurred: ${e.getMessage}")
    -1 // Fallback value
}

recoveredFuture.foreach(result => println(s"Result: $result"))

In the above example, the future is a Future that represents some computation that may throw a RuntimeException. The recover method will call on the future to provide a partial function that handles the specific type of exception (RuntimeException in this case). If the future completes with a RuntimeException, the partial function will invoke, and a fallback value of -1 will be returning as a new Future (recoveredFuture). If the future completes successfully without any exception, the original result (42) will return in recoveredFuture.

recoverWith

In Scala, the recoverWith method is use to handle and recover from exceptions that occur during the execution of a Future. It allows you to specify a partial function that will apply to the failed Future to transform it into a new Future.

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

val future: Future[String] = Future {
  // Some computation that may throw an exception
  "Hello, World!"
}

val recoveredFuture: Future[String] = future.recoverWith {
  case ex: Exception =>
    // Handle the exception and return a new Future
    Future.successful("Recovery value")
}

recoveredFuture.foreach(println)

In the above example, the recoverWith method will apply to the future object. It takes a partial function that specifies the handling of exceptions. If an exception occurs during the execution of the future, the partial function will be applied, and the returned Future will be used instead.

In the recoverWith partial function, you can handle different types of exceptions separately using pattern matching. In this example, we’re handling all exceptions by returning a new Future one with a recovery value of "Recovery value" using Future.successful.

onComplete

The onComplete method allows you to specify a callback that will be invoked when the Future completes, regardless of whether it succeeded or failed. The callback receives a Try that contains either the successful result or the exception. Here’s an example:

val futureResult: Future[String] = // some asynchronous computation

futureResult.onComplete {
  case Success(result) => println("Success: " + result)
  case Failure(error) => println("Error: " + error.getMessage)
}

Conclusion

This blog contains the useful method of scala Future to handle an exception in Future. Choosing the appropriate technique depends on the specific requirements of your application and the desired error-handling behavior. If you want to know more about the concept of error handling in Scala you can go through it here.