I'd like to understand how coroutines work in Kotlin. What problems do they solve, how is asynchrony organized without blocking threads, and what pitfalls might arise when using them? Are there any recommendations on code structure and best practices? Please share your experience and thoughts on how to best apply coroutines in real projects.
How do coroutines work in Kotlin and where are they effectively used?
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Coroutines in Kotlin are a lightweight way to write asynchronous and concurrent code that runs on top of regular JVM threads without forcing you to manage them manually. Compared to the classic `Thread`/`ExecutorService` approach or RxJava, where each operator often spawns its own thread or scheduler, a coroutine is a single `Continuation` object that can "suspend" and later resume on the same thread pool without blocking it. This works because when you call `delay` or any other `suspend` function, the current thread is freed, and the scheduler (e.g., `Dispatchers.IO`) picks it up again when the result is ready.
The main pitfalls are: forgetting about contexts (`Dispatchers`) and accidentally running heavy operations on `Dispatchers.Main`, which leads to ANRs; not controlling the lifecycle of coroutines (e.g., launching them in `GlobalScope` without cancellation), which can cause memory leaks; and mixing "classic" blocking code with coroutines without `withContext(Dispatchers.IO)`, which defeats the purpose. Best practices include keeping business logic in `suspend` functions, using `viewModelScope`/`lifecycleScope` for automatic cancellation, explicitly specifying `Dispatcher` for network or file operations, and using `async/await` inside `coroutineScope` or `supervisorScope` for parallel tasks. In real projects, this approach eliminates nested callbacks and complex Rx chains, making the code linear and easier to debug.
Coroutines are lightweight "green" threads managed by a scheduler (Dispatcher). When you use `launch` or `async`, you create a task that immediately goes into a waiting state, and the actual execution happens in a thread pool (e.g., `Dispatchers.IO` for I/O operations or `Dispatchers.Default` for CPU-intensive tasks). Thanks to structured concurrency (`coroutineScope`, `supervisorScope`), all child coroutines are automatically canceled when exiting the block, which prevents most resource leaks. In my projects, I usually keep business logic in `viewModelScope` (for Android) and perform long-running requests in `withContext(Dispatchers.IO)`, while UI operations remain on `Dispatchers.Main`.
Pitfalls: Don’t forget about cancellation (`isActive`, `ensureActive`)—a coroutine can only be interrupted if it periodically checks its status; exceptions in child coroutines can "bubble up" if you don’t use `supervisorScope`. Also, avoid `GlobalScope` in production code—it disables the cancellation structure and leads to hanging tasks. Tip: Move all network/database interactions into separate repositories, return `Flow` or `suspend` functions, and test them with `runBlockingTest`. This approach makes the code predictable, easy to scale, and simplifies debugging in real projects.