What is the difference between the dispatchers (Main, IO, Default, Unconfined) in Android?

Short answer

Main is for the UI and interaction, IO is for input/output and network operations, Default is for CPU-intensive computation, and Unconfined starts on the current thread without restriction.

Explanation

Dispatchers.Main runs tasks on the UI thread. Dispatchers.IO has a flexible thread pool (up to 64 threads by default) and is designed for long-waiting operations such as reading files, network, and databases. Dispatchers.Default relies on the number of CPU cores and suits image processing, large JSON, and complex algorithms.

Code example

// Use withContext to switch smoothly between threads:
suspend fun loadAndParseData(): Result = withContext(Dispatchers.IO) {
    val rawJson = networkClient.download() // IO work
    withContext(Dispatchers.Default) {
        parseHeavyJson(rawJson) // CPU work
    }
}

Common mistakes

• Decoding huge images or sorting giant lists on Dispatchers.IO instead of Dispatchers.Default.
• Running database queries or network calls on Dispatchers.Main, causing UI freezes and ANRs.

Follow-up questions

• What is the difference between the launch coroutine builder and the async builder and when do we use each?

Sources

• Kotlin Docs — Coroutine context and dispatchers