Exception handling in coroutines in Kotlin - Mini Project: Build & Apply

Exception handling in coroutines

📖 Scenario: You are building a simple Kotlin program that uses coroutines to perform tasks asynchronously. Sometimes, these tasks might fail with errors. You want to learn how to catch and handle these errors properly inside coroutines.

🎯 Goal: Learn how to write Kotlin coroutines that handle exceptions using try-catch blocks and coroutine builders.

📋 What You'll Learn

Create a coroutine that throws an exception

Add a variable to hold the error message

Use try-catch inside a coroutine to catch the exception

Print the caught error message

💡 Why This Matters

🌍 Real World

Handling errors in asynchronous tasks is important to keep apps stable and responsive.

💼 Career

Many Kotlin developers use coroutines for background work and must handle exceptions properly to avoid crashes.

1

Create a coroutine that throws an exception

Write a suspend function called riskyTask that throws an IllegalArgumentException with the message "Something went wrong".

Kotlin

# Write the suspend function riskyTask that throws IllegalArgumentException
# Your code here

Hint

Use throw IllegalArgumentException("Something went wrong") inside the suspend function.

2

Add a variable to hold the error message

Create a mutable variable called errorMessage of type String? and set it to null.

Kotlin

suspend fun riskyTask() {
    throw IllegalArgumentException("Something went wrong")
}

// Create a variable errorMessage of type String? and set it to null
# Your code here

Hint

Use var errorMessage: String? = null to create the variable.

3

Use try-catch inside a coroutine to catch the exception

Write a runBlocking block. Inside it, call riskyTask() inside a try block. Catch IllegalArgumentException and set errorMessage to the exception's message.

Kotlin

suspend fun riskyTask() {
    throw IllegalArgumentException("Something went wrong")
}

var errorMessage: String? = null

// Use runBlocking and try-catch to call riskyTask and catch the exception
# Your code here

Hint

Use runBlocking { try { riskyTask() } catch (e: IllegalArgumentException) { errorMessage = e.message } }.

4

Print the caught error message

Add a println statement after the runBlocking block to print the value of errorMessage.

Kotlin

import kotlinx.coroutines.runBlocking

suspend fun riskyTask() {
    throw IllegalArgumentException("Something went wrong")
}

var errorMessage: String? = null

fun main() = runBlocking {
    try {
        riskyTask()
    } catch (e: IllegalArgumentException) {
        errorMessage = e.message
    }
}

// Print the errorMessage here
# Your code here

Hint

Use println(errorMessage) after the coroutine block to show the error message.

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