CoroutineExceptionHandler

Learn error handling with CoroutineExceptionHandler.

What is CoroutineExceptionHandler?

CoroutineExceptionHandler catches uncaught exceptions inside a coroutine. It is a piece of CoroutineContext and is passed to launch as context.

Signature: val handler = CoroutineExceptionHandler { _, throwable -> println(throwable.message) }. The first parameter is the context, the second is the thrown Throwable. The handler is only invoked on root coroutines; it does not work for async. You put recovery logic inside, such as logging, alerts or fallback data.

Using the handler

You pass the handler to launch as context: launch(handler) { throw RuntimeException("Error") }. The thrown exception goes to the handler and appears as the throwable parameter.

Inside the handler you can log the exception, show a user alert or start a fallback flow. After the handler runs, the coroutine is considered crashed; the cancelled state is preserved. So instead of starting a new coroutine inside the handler, log the error and retry from outside for safety.

Exercise Define a handler and print

Define val handler = CoroutineExceptionHandler { _, throwable -> println("Error caught") }. Inside main, print println("Ready"). Showing the handler is enough; you do not need to throw.

Show example solution

fun main() = runBlocking {
    val handler = CoroutineExceptionHandler { _, throwable ->
        println("Error caught")
    }
    println("Ready")
}

Test yourself

Complete the handler parameter

val handler = CoroutineExceptionHandler { _, ____ -> println("Error") }

Show answer

val handler = CoroutineExceptionHandler { _, throwable -> println("Error") }

What is CoroutineExceptionHandler for?
- Catches uncaught exceptions
- Starts a coroutine
- Creates threads
- Picks a dispatcher
Show answer

Catches uncaught exceptions — The handler catches uncaught exceptions.
Match the handler parts

CoroutineExceptionHandler

Catches exception
throwable

Thrown exception
context

First parameter
launch

Takes context

Next lesson: Summary - Advanced coroutines

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