Blog Series: Mastering Kotlin Coroutines

Coroutine Exception Handling

In this article, we’ll explore how Kotlin coroutines handle exceptions. Exception handling is a critical aspect of any asynchronous programming model, and coroutines provide powerful tools to manage errors gracefully. By understanding these mechanisms, you can build robust applications that recover seamlessly from unexpected failures.

How Exceptions Work in Coroutines

Kotlin coroutines propagate exceptions differently than traditional synchronous code. Exceptions in coroutines are:

1. Propagated to the Parent Coroutine:

   • When a child coroutine encounters an exception, it propagates up to its parent unless handled explicitly.

2. Bound to Coroutine Scope:

   • Coroutines running in the same scope share exception handling, ensuring structured concurrency.

3. Cancellable:

   • A coroutine can be canceled when an exception occurs, preventing further execution.

Example: Basic Exception Propagation

fun main() = runBlocking {
    val job = launch {
        throw RuntimeException("Test exception")
    }

    job.join()
    println("End of main")
}

Output:

Exception in thread "main" java.lang.RuntimeException: Test exception

In this example, the exception terminates the coroutine and propagates to the parent.

Handling Exceptions with try-catch

You can handle exceptions within a coroutine using try-catch blocks, just like in synchronous code.

Example: Using try-catch

fun main() = runBlocking {
    val job = launch {
        try {
            throw RuntimeException("Handled exception")
        } catch (e: Exception) {
            println("Caught: ${e.message}")
        }
    }

    job.join()
    println("End of main")
}

Output:

Caught: Handled exception
End of main

CoroutineExceptionHandler

The CoroutineExceptionHandler is a specialized tool for handling uncaught exceptions. It provides a centralized way to handle exceptions in a coroutine.

Example: Using CoroutineExceptionHandler

fun main() = runBlocking {
    val handler = CoroutineExceptionHandler { _, exception ->
        println("Caught: ${exception.message}")
    }

    val job = launch(handler) {
        throw RuntimeException("Unhandled exception")
    }

    job.join()
}

Output:

Caught: Unhandled exception

SupervisorScope and SupervisorJob

In a normal coroutine scope, if a child coroutine fails, the parent and all other children are canceled. However, SupervisorScope and SupervisorJob allow independent error handling for child coroutines.

Example: Using SupervisorScope

fun main() = runBlocking {
    supervisorScope {
        launch {
            println("Child 1 starts")
            throw RuntimeException("Child 1 failed")
        }

        launch {
            println("Child 2 starts")
            delay(1000)
            println("Child 2 completes")
        }
    }
}

Output:

Child 1 starts
Child 2 starts
Child 2 completes
Exception in thread "main" java.lang.RuntimeException: Child 1 failed

Here, the failure of Child 1 does not cancel Child 2 because they are in a SupervisorScope.

Best Practices for Exception Handling

1. Use Lifecycle-Aware Scopes:

   • Use viewModelScope or lifecycleScope in Android to tie coroutine lifetimes to the UI lifecycle.

2. Centralize Exception Handling:

   • Leverage CoroutineExceptionHandler for a consistent error-handling strategy.

3. Avoid Silent Failures:

   • Always log or report exceptions to ensure they don’t go unnoticed.

4. Combine try-catch with onCompletion:

   • Use onCompletion to clean up resources regardless of success or failure.

Debugging Coroutines

Kotlin provides tools to debug coroutines effectively:

1. Debug Probes:

   • Add DebugProbes.install() to enable detailed coroutine debugging.

2. Thread Dump Analysis:

   • Use thread dumps to identify active coroutines.

3. Structured Logging:

   • Use structured logging libraries to trace coroutine activity.

Conclusion

Exception handling in Kotlin coroutines is a robust and flexible system that supports structured concurrency. By leveraging tools like CoroutineExceptionHandler, try-catch, and SupervisorScope, you can build resilient applications that gracefully recover from errors.

In the next article, we’ll explore coroutine performance optimization techniques to write highly efficient asynchronous code.

Stay tuned!