Exception handling in coroutines in Kotlin

Introduction

Sometimes things go wrong when your program runs. Exception handling in coroutines helps you catch and manage these problems smoothly without crashing your app.

When you want to catch errors inside a coroutine to prevent your app from stopping.

When you need to handle network failures or timeouts in asynchronous tasks.

When you want to clean up resources or show a message after an error happens in a coroutine.

When you want to log errors from background tasks without crashing the main program.

Syntax

Kotlin

try {
    // coroutine code that might throw an exception
} catch (e: Exception) {
    // handle the exception here
}

You can use try-catch blocks inside coroutines just like in normal code.

For coroutine builders like launch, exceptions are handled differently and may need a CoroutineExceptionHandler.

Examples

This example shows catching an exception inside a coroutine using try-catch.

Kotlin

runBlocking {
    try {
        val result = riskyOperation()
        println("Result: $result")
    } catch (e: Exception) {
        println("Caught error: ${e.message}")
    }
}

This example uses CoroutineExceptionHandler to catch exceptions from a launch coroutine.

Kotlin

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

val job = GlobalScope.launch(handler) {
    throw RuntimeException("Oops!")
}
job.join()

Sample Program

This program starts a coroutine that throws an exception. The CoroutineExceptionHandler catches it and prints a message. The program continues running after handling the error.

Kotlin

import kotlinx.coroutines.*

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

    val job = launch(handler) {
        println("Starting coroutine")
        delay(100)
        throw IllegalArgumentException("Something went wrong")
    }

    job.join()
    println("Coroutine finished")
}

Important Notes

Exceptions in launch coroutines are uncaught by default unless you provide a CoroutineExceptionHandler.

Exceptions in async coroutines are thrown when you call await(), so handle exceptions there.

Always handle exceptions to keep your app stable and user-friendly.

Summary

Use try-catch inside coroutines to catch errors directly.

Use CoroutineExceptionHandler to catch exceptions from launch coroutines.

Handling exceptions keeps your app running smoothly even when errors happen.

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