Exception handling in coroutines in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of try-catch blocks in Kotlin coroutines?

They are used to catch and handle exceptions that occur inside a coroutine, preventing the coroutine from crashing the program.

intermediate

How does CoroutineExceptionHandler help in exception handling?

It is a special coroutine context element that catches uncaught exceptions in coroutines and allows centralized handling of those exceptions.

intermediate

What happens if an exception is thrown in a child coroutine launched with launch?

The exception is propagated to the parent coroutine and can cancel the parent and its other children unless handled properly.

advanced

Why should exceptions in async coroutines be handled differently?

Because async coroutines return a Deferred result, exceptions are thrown only when await() is called, so handling should be done around await().

advanced

What is the role of supervisorScope in exception handling?

It allows child coroutines to fail independently without cancelling their siblings or the parent, helping to isolate exceptions.

Which coroutine builder propagates exceptions immediately to its parent?

A async

B launch

C runBlocking

D supervisorScope

The launch builder propagates exceptions immediately to its parent coroutine.

Where should you handle exceptions when using async coroutines?

A Exceptions are ignored in <code>async</code>

B Inside the coroutine block

C In <code>CoroutineExceptionHandler</code>

D When calling <code>await()</code>

Exceptions in async coroutines are thrown when await() is called, so handling should be done there.

What does CoroutineExceptionHandler NOT catch?

A Exceptions in <code>async</code> coroutines before <code>await()</code>

B Exceptions in <code>runBlocking</code>

C Uncaught exceptions in coroutines

D Exceptions in <code>launch</code> coroutines

CoroutineExceptionHandler does not catch exceptions in async coroutines before await() is called.

What is the effect of using supervisorScope?

A Allows child coroutines to fail independently

B Cancels all child coroutines on any failure

C Ignores all exceptions silently

D Runs coroutines sequentially

supervisorScope lets child coroutines fail without cancelling siblings or the parent.

Which statement about exception handling in coroutines is TRUE?

A Exceptions in coroutines always crash the whole application

B Exceptions in <code>async</code> are ignored

C Exceptions in <code>launch</code> are propagated immediately

D <code>try-catch</code> cannot be used inside coroutines

Exceptions in launch coroutines are propagated immediately to the parent.

Explain how exception handling differs between launch and async coroutines in Kotlin.

Think about when exceptions become visible in each coroutine builder.

Describe the purpose and benefits of using supervisorScope for exception handling in coroutines.

Consider how failures affect other coroutines inside the scope.

Practice

1. What is the primary way to catch exceptions inside a Kotlin coroutine launched with launch?

easy

A. Use try-catch block inside the coroutine code

B. Use catch method on the coroutine builder

C. Use finally block outside the coroutine

D. Exceptions cannot be caught in coroutines

Solution

1. Step 1: Understand coroutine exception handling basics

   Exceptions inside launch coroutines are caught by try-catch blocks within the coroutine code itself.

2. Step 2: Identify correct method to catch exceptions

   The catch method does not exist on coroutine builders, and finally outside coroutine does not catch coroutine exceptions.

3. Final Answer:

   Use try-catch block inside the coroutine code -> Option A

4. Quick Check:

   try-catch inside coroutine = correct way [OK]

Hint: Catch exceptions inside coroutine with try-catch [OK]

Common Mistakes:

• Thinking catch method exists on coroutine builders
• Using finally block outside coroutine to catch exceptions
• Assuming exceptions cannot be caught in coroutines

2. Which of the following is the correct syntax to define a CoroutineExceptionHandler in Kotlin?

easy

A. val handler = CoroutineExceptionHandler { println(exception) }

B. val handler = CoroutineExceptionHandler { exception -> println(exception) }

C. val handler = CoroutineExceptionHandler(context, exception -> println(exception))

D. val handler = CoroutineExceptionHandler { context, exception -> println(exception) }

Solution

1. Step 1: Recall CoroutineExceptionHandler signature

   It takes a lambda with two parameters: context and exception.

2. Step 2: Match correct syntax

   val handler = CoroutineExceptionHandler { context, exception -> println(exception) } correctly uses the two parameters in the lambda. Other options miss parameters or have wrong syntax.

3. Final Answer:

   val handler = CoroutineExceptionHandler { context, exception -> println(exception) } -> Option D

4. Quick Check:

   CoroutineExceptionHandler requires (context, exception) lambda [OK]

Hint: CoroutineExceptionHandler lambda needs context and exception [OK]

Common Mistakes:

• Omitting context parameter in lambda
• Passing parameters incorrectly
• Using invalid constructor syntax

3. What will be the output of the following Kotlin code?

import kotlinx.coroutines.*

fun main() = runBlocking {
    val handler = CoroutineExceptionHandler { _, exception ->
        println("Caught: ${exception.message}")
    }
    val job = launch(handler) {
        throw IllegalArgumentException("Invalid argument")
    }
    job.join()
    println("Done")
}

medium

A. Caught: Invalid argument
Done

B. Exception in thread "main" java.lang.IllegalArgumentException: Invalid argument

C. Done

D. Caught: null
Done

Solution

1. Step 1: Understand CoroutineExceptionHandler usage

   The handler catches exceptions thrown inside the launch coroutine and prints the message.

2. Step 2: Trace program output

   The exception message "Invalid argument" is printed by the handler, then "Done" is printed after job.join().

3. Final Answer:

   Caught: Invalid argument Done -> Option A

4. Quick Check:

   Exception caught by handler, then Done printed [OK]

Hint: ExceptionHandler prints message, then main continues [OK]

Common Mistakes:

• Expecting uncaught exception crash
• Missing that handler catches exception
• Ignoring job.join() effect

4. Identify the error in this Kotlin coroutine exception handling code:

import kotlinx.coroutines.*

fun main() = runBlocking {
    val handler = CoroutineExceptionHandler { _, exception ->
        println("Error: ${exception.message}")
    }
    val job = async(handler) {
        throw RuntimeException("Failure")
    }
    job.await()
}

medium

A. Missing try-catch block inside async coroutine

B. The handler lambda parameters are incorrect

C. CoroutineExceptionHandler does not catch exceptions from async coroutines

D. Using runBlocking incorrectly

Solution

1. Step 1: Understand exception handling difference between launch and async

   Exceptions in async are propagated via the deferred result and must be caught by try-catch on await().

2. Step 2: Analyze why CoroutineExceptionHandler fails here

   CoroutineExceptionHandler does not catch exceptions from async coroutines because they are deferred until await().

3. Final Answer:

   CoroutineExceptionHandler does not catch exceptions from async coroutines -> Option C

4. Quick Check:

   async exceptions need try-catch on await() [OK]

Hint: Use try-catch on await() for async exceptions [OK]

Common Mistakes:

• Expecting CoroutineExceptionHandler to catch async exceptions
• Ignoring need for try-catch on await()
• Misunderstanding handler lambda parameters

5. You want to handle exceptions from multiple launch coroutines in a Kotlin app and log errors without stopping other coroutines. Which approach is best?

hard

A. Use async coroutines and catch exceptions on await()

B. Attach a shared CoroutineExceptionHandler to the parent scope and launch coroutines within it

C. Wrap each coroutine body in a try-catch block and ignore exceptions

D. Use SupervisorJob without any exception handling

Solution

1. Step 1: Understand exception handling for multiple launch coroutines

   Using a shared CoroutineExceptionHandler on the parent scope catches exceptions from all child launch coroutines without cancelling siblings.

2. Step 2: Evaluate other options

   Wrapping each coroutine in try-catch is verbose and error-prone; async with await is for deferred results; SupervisorJob alone does not handle exceptions or logging.

3. Final Answer:

   Attach a shared CoroutineExceptionHandler to the parent scope and launch coroutines within it -> Option B

4. Quick Check:

   Shared handler on parent scope manages launch exceptions [OK]

Hint: Use shared CoroutineExceptionHandler on parent for launch coroutines [OK]

Common Mistakes:

• Ignoring shared handler benefits
• Using async instead of launch for fire-and-forget
• Not handling exceptions leads to crashes