1. Introduction

Exception handling is a crucial aspect of writing robust and reliable software. In asynchronous programming, managing exceptions becomes even more critical due to the potential challenges in handling errors that might occur in different threads or coroutines.

In this tutorial, we’ll explore how to handle exceptions in Kotlin coroutines, specifically focusing on various methods of catching exceptions in asynchronous code.

2. Understanding Exceptions

Before delving into exception handling in Kotlin coroutines, let’s briefly review exceptions. An exception is an abnormal event that occurs during the execution of a program, disrupting the normal flow of the application. These events might include errors, such as division by zero, accessing an array out of bounds, or network-related issues.

3. Handling Exceptions in Coroutines

Let’s also briefly revisit coroutines. Coroutines are a powerful tool for asynchronous programming that allows developers to write asynchronous code in a sequential and more readable manner. They provide a way to perform non-blocking operations without the complexities of callback-based code.

Tasks run concurrently in asynchronous programming, often in separate threads or coroutines. If an exception is not properly handled, it can propagate up through the call stack, potentially causing the entire program to crash. Effectively managing exceptions is crucial for maintaining the stability and reliability of our asynchronous code.

Now, let’s explore several different methods for handling exceptions in Kotlin coroutines.

Exceptions are propagated through the coroutine hierarchy. If an exception occurs inside a coroutine, it propagates to the parent coroutine or coroutine scope. To catch and handle exceptions in a coroutine, we use a try-catch block within the coroutine’s code:

fun main() = runBlocking {
    launch {
        try {
            val result = 10 / 0 
            println("Result: $result") 
        } catch (e: ArithmeticException) {
            println("Caught an ArithmeticException: $e")
        }
    }
    delay(1000)
}

In this example, the coroutine attempts to perform a division by zero, which triggers an ArithmeticException. The try-catch block catches this specific exception, and the catch block prints a message detailing the caught exception.

Thus, the output would be:

Caught an ArithmeticException: java.lang.ArithmeticException: / by zero

4. Using CoroutineExceptionHandler

Alternatively, the CoroutineExceptionHandler interface allows us to define a global exception handler for all coroutines within a specific scope. This is useful when we want a centralized location to handle exceptions.

Our next few examples will use a custom exception:

CustomException(message: String) : Exception(message)

Now, let’s create a reusable CoroutineExceptionHandler and attach it to our coroutine:

fun main() = runBlocking {
    val exceptionHandler = CoroutineExceptionHandler { _, exception ->
        println("Caught global exception: ${exception.message}")
    }
    val job = GlobalScope.launch(exceptionHandler) {
        delay(100)
        throw CustomException("An exception occurred!")
    }
    job.join()
}

We defined a global exception handler using CoroutineExceptionHandler and attached it to a coroutine launched in the GlobalScope. The handler catches the exception thrown within the coroutine while providing reusable exception-handling logic for other coroutines.

The output would be:

Caught global exception: An exception occurred!

5. Wrapping Async Calls With coroutineScope()

Now that we’ve explored ways to handle exceptions in coroutines, let’s look at a few strategies to deal with exceptions while running many coroutines at once.

The coroutineScope() function is a coroutine builder that ensures all child coroutines finish unless one fails, which cancels the entire scope:

fun main() = runBlocking {
    try {
        coroutineScope {
            launch {
                delay(100)
                throw CustomException("An exception occurred!")
            }
            launch {
                delay(200)
                println("This coroutine completes successfully.")
            }
        }
    } catch (e: CustomException) {
        println("Caught exception: ${e.message}")
    }
}

In this example, the coroutineScope() function creates a scope for two child coroutines. If one of the coroutines fails, the entire scope is canceled, and the exception is caught in the surrounding try-catch block.

Since the successful coroutine has a shorter delay, the output would be:

Caught exception: An exception occurred!

6. Wrapping Async Calls With supervisorScope()

Finally, supervisorScope() is another coroutine builder similar to coroutineScope(), but it differs in how it handles failures. While coroutineScope() cancels all child coroutines on the first failure, supervisorScope() allows the remaining coroutines to continue running even if one of them fails:

fun main(args: Array) = runBlocking {
val exceptionHandler = CoroutineExceptionHandler { _, exception ->
        println("Caught an exception: ${exception.message}")
    }
    supervisorScope {
    	val job1 = launch(exceptionHandler) {
            delay(100)
            println("This coroutine completes successfully.")
        }
        val job2 = launch(exceptionHandler) {
            throw Exception("An exception occurred!")
        }
        listOf(job1, job2).joinAll()
    }
}

In the example above, we define a CoroutineExceptionHandler to handle exceptions arising within the coroutines. We then establish a scope for child coroutines with supervisorScope(). This mechanism safeguards against the failure of one child coroutine from impacting the others or the supervisor coroutine. One of our coroutines intentionally throws an exception while the other completes its execution without encountering any issues.

The output would be:

Caught an exception: An exception occurred!
This coroutine completes successfully.

To synchronize the supervisor coroutine’s execution with its children, we call join() on the job. This ensures that the supervisor coroutine waits for all child coroutines to finish execution. Should an exception occur within any child coroutine, the CoroutineExceptionHandler() captures it, allowing us to print an appropriate message indicating the nature of the exception.

7. Conclusion

In this tutorial, we highlighted the importance of understanding exceptions and how they can disrupt the normal flow of an application. With asynchronous programming, the challenge lies in effectively handling exceptions that may occur in different threads or coroutines. We discussed how exceptions propagate through the coroutine hierarchy and demonstrated the use of try-catch blocks within coroutines to catch and handle specific exceptions.

Additionally, we explored the CoroutineExceptionHandler interface, which provides a centralized mechanism for handling exceptions across multiple coroutines within a scope. This approach allows for reusable exception handling logic and promotes cleaner code organization. Furthermore, we examined coroutine builders like coroutineScope() and supervisorScope() for managing groups of coroutines. These builders offer different strategies for handling exceptions, with coroutineScope() canceling all child coroutines on the first failure, while supervisorScope() allows remaining coroutines to continue executing independently.

As always, the examples from this article can be found over on GitHub.

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