Debugging Kotlin Code: Advanced Techniques and Tools for Pros

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Why Debugging Kotlin is an Art, Not a Routine?

Imagine: you write elegant Kotlin code, use coroutines, null-safety, and extension functions. Everything looks perfect until the app crashes with a NullPointerException in the most unexpected place. Or even worse — a coroutine just "falls asleep" forever, leaving no traces. Sound familiar?

Debugging Kotlin code is not just about finding bugs. It's the ability to "peek" at your application's work in real time, understand how coroutines work, and quickly find bottlenecks. In this article, we'll cover advanced Kotlin debugging techniques that will save you hours of frustration and make your code more stable.

Basic Kotlin Debugging Tools in IntelliJ IDEA

IntelliJ IDEA is not just an IDE; it's a true Swiss Army knife for a Kotlin developer. Let's start with what's on the surface but is often used incorrectly.

Breakpoints: More Than Just a Pause

A regular line breakpoint is a classic. But Kotlin offers much more:

Conditional breakpoints — trigger only when a condition is met. For example, stop when userId == 42.
Exception breakpoints — catch exceptions before they hit the log.
Method breakpoints — trigger on function entry and exit. Ideal for debugging complex call chains.

Example of setting up a conditional breakpoint:

fun processOrder(order: Order) {    // Set breakpoint and add condition: order.amount > 1000    val discount = calculateDiscount(order.amount)    // ...}

Simply right-click on the breakpoint and enter the condition. Saves a ton of time!

Debugging Coroutines: Peeking Inside the Async World

Coroutines are powerful, but debugging can become a nightmare. IntelliJ IDEA (starting from version 2020.3) offers the Coroutines Debugger. It shows:

The state of each coroutine (Active, Suspended, Completed).
The call stack for each coroutine.
The values of local variables inside the coroutine.

import kotlinx.coroutines.*

fun main() = runBlocking {    val job1 = launch {        delay(1000)        println("Job 1 done")    }    val job2 = launch {        delay(500)        println("Job 2 done")    }    // Set a breakpoint here and check the "Coroutines" tab    job1.join()    job2.join()}

Tip: enable the Show coroutines option in the debug panel — it will save you from headaches with asynchronous code.

Advanced Techniques: Debugging Without Pain

Now let's move on to techniques that separate a beginner from a pro.

Logging as the First Line of Defense

You can't always set a breakpoint (e.g., in production). That's where logging comes to the rescue. But not just println(), but meaningful logging.

import org.slf4j.LoggerFactory

private val log = LoggerFactory.getLogger(MyService::class.java)

fun fetchUserData(userId: String): User? {    log.debug("Fetching user data for userId: {}", userId)    return try {        val user = repository.findById(userId)        log.info("User found: {}", user?.name)        user    } catch (e: Exception) {        log.error("Failed to fetch user: {}", userId, e)        null    }}

Use logging levels: TRACE for details, DEBUG for debugging, INFO for important events. And never leave println() in production — that's bad practice.