**Table of contents**
* [Coroutine Basics](#coroutine-basics)
* [Your first coroutine](#your-first-coroutine)
* [Bridging blocking and non-blocking worlds](#bridging-blocking-and-non-blocking-worlds)
* [Waiting for a job](#waiting-for-a-job)
* [Structured concurrency](#structured-concurrency)
* [Scope builder](#scope-builder)
* [Extract function refactoring](#extract-function-refactoring)
* [Coroutines ARE light-weight](#coroutines-are-light-weight)
* [Global coroutines are like daemon threads](#global-coroutines-are-like-daemon-threads)
## Coroutine Basics
This section covers basic coroutine concepts.
### Your first coroutine
Run the following code:
```kotlin
import kotlinx.coroutines.*
fun main() {
GlobalScope.launch { // launch a new coroutine in background and continue
delay(1000L) // non-blocking delay for 1 second (default time unit is ms)
println("World!") // print after delay
}
println("Hello,") // main thread continues while coroutine is delayed
Thread.sleep(2000L) // block main thread for 2 seconds to keep JVM alive
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-01.kt).
You will see the following result:
```text
Hello,
World!
```
Essentially, coroutines are light-weight threads.
They are launched with [launch] _coroutine builder_ in a context of some [CoroutineScope].
Here we are launching a new coroutine in the [GlobalScope], meaning that the lifetime of the new
coroutine is limited only by the lifetime of the whole application.
You can achieve the same result by replacing
`GlobalScope.launch { ... }` with `thread { ... }`, and `delay(...)` with `Thread.sleep(...)`.
Try it (don't forget to import `kotlin.concurrent.thread`).
If you start by replacing `GlobalScope.launch` with `thread`, the compiler produces the following error:
```
Error: Kotlin: Suspend functions are only allowed to be called from a coroutine or another suspend function
```
That is because [delay] is a special _suspending function_ that does not block a thread, but _suspends_ the
coroutine, and it can be only used from a coroutine.
### Bridging blocking and non-blocking worlds
The first example mixes _non-blocking_ `delay(...)` and _blocking_ `Thread.sleep(...)` in the same code.
It is easy to lose track of which one is blocking and which one is not.
Let's be explicit about blocking using the [runBlocking] coroutine builder:
```kotlin
import kotlinx.coroutines.*
fun main() {
GlobalScope.launch { // launch a new coroutine in background and continue
delay(1000L)
println("World!")
}
println("Hello,") // main thread continues here immediately
runBlocking { // but this expression blocks the main thread
delay(2000L) // ... while we delay for 2 seconds to keep JVM alive
}
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-02.kt).
The result is the same, but this code uses only non-blocking [delay].
The main thread invoking `runBlocking` _blocks_ until the coroutine inside `runBlocking` completes.
This example can be also rewritten in a more idiomatic way, using `runBlocking` to wrap
the execution of the main function:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking { // start main coroutine
GlobalScope.launch { // launch a new coroutine in background and continue
delay(1000L)
println("World!")
}
println("Hello,") // main coroutine continues here immediately
delay(2000L) // delaying for 2 seconds to keep JVM alive
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-03.kt).
Here `runBlocking { ... }` works as an adaptor that is used to start the top-level main coroutine.
We explicitly specify its `Unit` return type, because a well-formed `main` function in Kotlin has to return `Unit`.
This is also a way to write unit tests for suspending functions:
```kotlin
class MyTest {
@Test
fun testMySuspendingFunction() = runBlocking {
// here we can use suspending functions using any assertion style that we like
}
}
```
### Waiting for a job
Delaying for a time while another coroutine is working is not a good approach. Let's explicitly
wait (in a non-blocking way) until the background [Job] that we have launched is complete:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
val job = GlobalScope.launch { // launch a new coroutine and keep a reference to its Job
delay(1000L)
println("World!")
}
println("Hello,")
job.join() // wait until child coroutine completes
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-04.kt).
Now the result is still the same, but the code of the main coroutine is not tied to the duration of
the background job in any way. Much better.
### Structured concurrency
There is still something to be desired for practical usage of coroutines.
When we use `GlobalScope.launch`, we create a top-level coroutine. Even though it is light-weight, it still
consumes some memory resources while it runs. If we forget to keep a reference to the newly launched
coroutine, it still runs. What if the code in the coroutine hangs (for example, we erroneously
delay for too long), what if we launched too many coroutines and ran out of memory?
Having to manually keep references to all the launched coroutines and [join][Job.join] them is error-prone.
There is a better solution. We can use structured concurrency in our code.
Instead of launching coroutines in the [GlobalScope], just like we usually do with threads (threads are always global),
we can launch coroutines in the specific scope of the operation we are performing.
In our example, we have a `main` function that is turned into a coroutine using the [runBlocking] coroutine builder.
Every coroutine builder, including `runBlocking`, adds an instance of [CoroutineScope] to the scope of its code block.
We can launch coroutines in this scope without having to `join` them explicitly, because
an outer coroutine (`runBlocking` in our example) does not complete until all the coroutines launched
in its scope complete. Thus, we can make our example simpler:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking { // this: CoroutineScope
launch { // launch a new coroutine in the scope of runBlocking
delay(1000L)
println("World!")
}
println("Hello,")
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-05.kt).
### Scope builder
In addition to the coroutine scope provided by different builders, it is possible to declare your own scope using the
[coroutineScope][_coroutineScope] builder. It creates a coroutine scope and does not complete until all launched children complete.
[runBlocking] and [coroutineScope][_coroutineScope] may look similar because they both wait for their body and all its children to complete.
The main difference is that the [runBlocking] method _blocks_ the current thread for waiting,
while [coroutineScope][_coroutineScope] just suspends, releasing the underlying thread for other usages.
Because of that difference, [runBlocking] is a regular function and [coroutineScope][_coroutineScope] is a suspending function.
It can be demonstrated by the following example:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking { // this: CoroutineScope
launch {
delay(200L)
println("Task from runBlocking")
}
coroutineScope { // Creates a coroutine scope
launch {
delay(500L)
println("Task from nested launch")
}
delay(100L)
println("Task from coroutine scope") // This line will be printed before the nested launch
}
println("Coroutine scope is over") // This line is not printed until the nested launch completes
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-06.kt).
Note that right after the "Task from coroutine scope" message (while waiting for nested launch)
"Task from runBlocking" is executed and printed — even though the [coroutineScope][_coroutineScope] is not completed yet.
### Extract function refactoring
Let's extract the block of code inside `launch { ... }` into a separate function. When you
perform "Extract function" refactoring on this code, you get a new function with the `suspend` modifier.
This is your first _suspending function_. Suspending functions can be used inside coroutines
just like regular functions, but their additional feature is that they can, in turn,
use other suspending functions (like `delay` in this example) to _suspend_ execution of a coroutine.
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
launch { doWorld() }
println("Hello,")
}
// this is your first suspending function
suspend fun doWorld() {
delay(1000L)
println("World!")
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-07.kt).
But what if the extracted function contains a coroutine builder which is invoked on the current scope?
In this case, the `suspend` modifier on the extracted function is not enough. Making `doWorld` an extension
method on `CoroutineScope` is one of the solutions, but it may not always be applicable as it does not make the API clearer.
The idiomatic solution is to have either an explicit `CoroutineScope` as a field in a class containing the target function
or an implicit one when the outer class implements `CoroutineScope`.
As a last resort, [CoroutineScope(coroutineContext)][CoroutineScope()] can be used, but such an approach is structurally unsafe
because you no longer have control on the scope of execution of this method. Only private APIs can use this builder.
### Coroutines ARE light-weight
Run the following code:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
repeat(100_000) { // launch a lot of coroutines
launch {
delay(5000L)
print(".")
}
}
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-08.kt).
It launches 100K coroutines and, after 5 seconds, each coroutine prints a dot.
Now, try that with threads. What would happen? (Most likely your code will produce some sort of out-of-memory error)
### Global coroutines are like daemon threads
The following code launches a long-running coroutine in [GlobalScope] that prints "I'm sleeping" twice a second and then
returns from the main function after some delay:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
GlobalScope.launch {
repeat(1000) { i ->
println("I'm sleeping $i ...")
delay(500L)
}
}
delay(1300L) // just quit after delay
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-basic-09.kt).
You can run and see that it prints three lines and terminates:
```text
I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
```
Active coroutines that were launched in [GlobalScope] do not keep the process alive. They are like daemon threads.
[launch]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/launch.html
[CoroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/index.html
[GlobalScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-global-scope/index.html
[delay]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/delay.html
[runBlocking]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/run-blocking.html
[Job]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html
[Job.join]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/join.html
[_coroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html
[CoroutineScope()]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope.html