Coroutine Exception Handling

Coroutine Exception Handling (Kotlin)

Coroutine Exception Handling in Kotlin is different from normal try–catch, because coroutines are asynchronous and follow structured concurrency.

This guide explains all important rules, patterns, and best practices.

1. Basic Rule (Most Important)

Exceptions in coroutines are propagated to the parent scope

• Child coroutine fails → parent fails

• Parent cancels all children

2. try–catch with launch (Won’t Work)

 This does NOT catch exceptions:

Reason:

• launch is fire-and-forget

• Exception happens later

3. Correct try–catch Inside Coroutine

 Catch inside the coroutine:

4. async Exception Handling (Important Difference)

async stores exception until await() is called.

5. CoroutineExceptionHandler

Used for uncaught exceptions in launch.

Rules:

• Works with launch

•  Does NOT work with async

• Handles last-resort exceptions

6. supervisorScope (Prevent Cascade Failure)

By default:

• One child fails → all siblings cancel

supervisorScope changes this behavior.

 Child 2 continues running

7. SupervisorJob

Used at scope level.

8. Exception Handling in coroutineScope

•  Exception propagates to parent

9. CancellationException (Special Case)

•  Not treated as error

•  Not caught by handler

• Used internally for cancellation

10. Flow Exception Handling

 try–catch won’t work outside

 Use catch operator

11. Best Practices (Very Important)

•  Use try–catch inside coroutine
•  Use async + await() for result-based error handling
•  Use CoroutineExceptionHandler for logging
•  Use SupervisorJob in ViewModels
•  Use Flow catch {} for streams

12. Android MVVM Example

Summary Table