Prompt Title: Every time we create and launch a new coroutine, does it create a new thread?

When you launch a coroutine in Kotlin, it does not necessarily create a new thread. Instead, coroutines operate on existing threads by suspending and resuming execution without blocking the thread they are running on. This is a fundamental aspect of how coroutines manage efficiency and concurrency.

Understanding Coroutine Dispatchers and Threads

Coroutines are launched within a context determined by a CoroutineDispatcher which specifies which thread or thread pool the coroutine will run on. Here’s how this affects threading:

1. Default Dispatcher (Dispatchers.Default):

   • This dispatcher is backed by a shared pool of threads. The number of threads used by default is limited to the number of CPU cores (but at least two).
   • When you launch coroutines on this dispatcher, they use these shared threads without creating new ones.

2. IO Dispatcher (Dispatchers.IO):

   • Similar to the Default dispatcher but tuned for offloading blocking IO tasks to a shared pool of threads.
   • It also reuses threads from a pool and is capable of expanding the number of threads temporarily if needed for IO-intensive operations.

3. Main Dispatcher (Dispatchers.Main):

   • This dispatcher is designed for interacting with the main thread of an application, such as the UI thread in Android.
   • Coroutines launched on this dispatcher do not create new threads but post tasks to the main thread's event queue.

4. Unconfined Dispatcher (Dispatchers.Unconfined):

   • This dispatcher starts a coroutine in the current call-frame until the first suspension point. After suspension, it resumes in the thread that is fully determined by the suspending function, which might be a different thread.
   • This dispatcher is not confined to any specific thread and does not create new threads.

5. New Single-Thread Context:

   • If specific needs dictate that a coroutine must run in its dedicated thread, you can create a new single-threaded context using newSingleThreadContext. This does indeed create a new thread:

     kotlin
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     val newThreadContext = newSingleThreadContext("NewThread")
     launch(newThreadContext) {
         // This coroutine runs in a new dedicated thread.
     }
     

   • This approach is more resource-intensive and generally not recommended unless necessary due to the overhead of managing additional threads.

Example of Coroutine Not Creating a New Thread

Here’s an example using the IO dispatcher, which reuses threads from a pool:

kotlin
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import kotlinx.coroutines.*

fun main() = runBlocking {
    launch(Dispatchers.IO) {
        println("This is running on a pooled thread: ${Thread.currentThread().name}")
    }
}

Why Coroutines Don’t Usually Need New Threads

• Efficiency: Creating and destroying threads is resource-intensive. Reusing existing threads from a pool minimizes overhead.
• Suspend and Resume: Coroutines are built around the concept of suspending and resuming without blocking threads. This allows multiple coroutines to share fewer threads, as they only occupy threads when they are actively working and release them when they are waiting (suspended).

Conclusion

Coroutines in Kotlin are a powerful feature designed to write asynchronous code more efficiently. They do not inherently create new threads unless explicitly directed to do so (such as using newSingleThreadContext). Instead, they leverage existing thread pools or the main thread to execute tasks, making asynchronous programming more efficient and less resource-heavy compared to traditional thread management. This design helps achieve high levels of concurrency without the overhead associated with thread management in traditional multithreading environments.