**Table of contents**
* [Coroutine Context and Dispatchers](#coroutine-context-and-dispatchers)
* [Dispatchers and threads](#dispatchers-and-threads)
* [Unconfined vs confined dispatcher](#unconfined-vs-confined-dispatcher)
* [Debugging coroutines and threads](#debugging-coroutines-and-threads)
* [Debugging with IDEA](#debugging-with-idea)
* [Debugging using logging](#debugging-using-logging)
* [Jumping between threads](#jumping-between-threads)
* [Job in the context](#job-in-the-context)
* [Children of a coroutine](#children-of-a-coroutine)
* [Parental responsibilities](#parental-responsibilities)
* [Naming coroutines for debugging](#naming-coroutines-for-debugging)
* [Combining context elements](#combining-context-elements)
* [Coroutine scope](#coroutine-scope)
* [Thread-local data](#thread-local-data)
## Coroutine Context and Dispatchers
Coroutines always execute in some context represented by a value of the
[CoroutineContext](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-coroutine-context/)
type, defined in the Kotlin standard library.
The coroutine context is a set of various elements. The main elements are the [Job] of the coroutine,
which we've seen before, and its dispatcher, which is covered in this section.
### Dispatchers and threads
The coroutine context includes a _coroutine dispatcher_ (see [CoroutineDispatcher]) that determines what thread or threads
the corresponding coroutine uses for its execution. The coroutine dispatcher can confine coroutine execution
to a specific thread, dispatch it to a thread pool, or let it run unconfined.
All coroutine builders like [launch] and [async] accept an optional
[CoroutineContext](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-coroutine-context/)
parameter that can be used to explicitly specify the dispatcher for the new coroutine and other context elements.
Try the following example:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
launch { // context of the parent, main runBlocking coroutine
println("main runBlocking : I'm working in thread ${Thread.currentThread().name}")
}
launch(Dispatchers.Unconfined) { // not confined -- will work with main thread
println("Unconfined : I'm working in thread ${Thread.currentThread().name}")
}
launch(Dispatchers.Default) { // will get dispatched to DefaultDispatcher
println("Default : I'm working in thread ${Thread.currentThread().name}")
}
launch(newSingleThreadContext("MyOwnThread")) { // will get its own new thread
println("newSingleThreadContext: I'm working in thread ${Thread.currentThread().name}")
}
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-01.kt).
It produces the following output (maybe in different order):
```text
Unconfined : I'm working in thread main
Default : I'm working in thread DefaultDispatcher-worker-1
newSingleThreadContext: I'm working in thread MyOwnThread
main runBlocking : I'm working in thread main
```
When `launch { ... }` is used without parameters, it inherits the context (and thus dispatcher)
from the [CoroutineScope] it is being launched from. In this case, it inherits the
context of the main `runBlocking` coroutine which runs in the `main` thread.
[Dispatchers.Unconfined] is a special dispatcher that also appears to run in the `main` thread, but it is,
in fact, a different mechanism that is explained later.
The default dispatcher that is used when coroutines are launched in [GlobalScope]
is represented by [Dispatchers.Default] and uses a shared background pool of threads,
so `launch(Dispatchers.Default) { ... }` uses the same dispatcher as `GlobalScope.launch { ... }`.
[newSingleThreadContext] creates a thread for the coroutine to run.
A dedicated thread is a very expensive resource.
In a real application it must be either released, when no longer needed, using the [close][ExecutorCoroutineDispatcher.close]
function, or stored in a top-level variable and reused throughout the application.
### Unconfined vs confined dispatcher
The [Dispatchers.Unconfined] coroutine dispatcher starts a coroutine in the caller thread, but only until the
first suspension point. After suspension it resumes the coroutine in the thread that is fully determined by the
suspending function that was invoked. The unconfined dispatcher is appropriate for coroutines which neither
consume CPU time nor update any shared data (like UI) confined to a specific thread.
On the other side, the dispatcher is inherited from the outer [CoroutineScope] by default.
The default dispatcher for the [runBlocking] coroutine, in particular,
is confined to the invoker thread, so inheriting it has the effect of confining execution to
this thread with predictable FIFO scheduling.
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
launch(Dispatchers.Unconfined) { // not confined -- will work with main thread
println("Unconfined : I'm working in thread ${Thread.currentThread().name}")
delay(500)
println("Unconfined : After delay in thread ${Thread.currentThread().name}")
}
launch { // context of the parent, main runBlocking coroutine
println("main runBlocking: I'm working in thread ${Thread.currentThread().name}")
delay(1000)
println("main runBlocking: After delay in thread ${Thread.currentThread().name}")
}
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-02.kt).
Produces the output:
```text
Unconfined : I'm working in thread main
main runBlocking: I'm working in thread main
Unconfined : After delay in thread kotlinx.coroutines.DefaultExecutor
main runBlocking: After delay in thread main
```
So, the coroutine with the context inherited from `runBlocking {...}` continues to execute
in the `main` thread, while the unconfined one resumes in the default executor thread that the [delay]
function is using.
> The unconfined dispatcher is an advanced mechanism that can be helpful in certain corner cases where
dispatching of a coroutine for its execution later is not needed or produces undesirable side-effects,
because some operation in a coroutine must be performed right away.
The unconfined dispatcher should not be used in general code.
### Debugging coroutines and threads
Coroutines can suspend on one thread and resume on another thread.
Even with a single-threaded dispatcher it might be hard to
figure out what the coroutine was doing, where, and when if you don't have special tooling.
#### Debugging with IDEA
The Coroutine Debugger of the Kotlin plugin simplifies debugging coroutines in IntelliJ IDEA.
> Debugging works for versions 1.3.8 or later of `kotlinx-coroutines-core`.
The **Debug** tool window contains the **Coroutines** tab. In this tab, you can find information about both currently running and suspended coroutines.
The coroutines are grouped by the dispatcher they are running on.
With the coroutine debugger, you can:
* Check the state of each coroutine.
* See the values of local and captured variables for both running and suspended coroutines.
* See a full coroutine creation stack, as well as a call stack inside the coroutine. The stack includes all frames with
variable values, even those that would be lost during standard debugging.
* Get a full report that contains the state of each coroutine and its stack. To obtain it, right-click inside the **Coroutines** tab, and then click **Get Coroutines Dump**.
To start coroutine debugging, you just need to set breakpoints and run the application in debug mode.
Learn more about coroutines debugging in the [tutorial](https://kotlinlang.org/docs/tutorials/coroutines/debug-coroutines-with-idea.html).
#### Debugging using logging
Another approach to debugging applications with
threads without Coroutine Debugger is to print the thread name in the log file on each log statement. This feature is universally supported
by logging frameworks. When using coroutines, the thread name alone does not give much of a context, so
`kotlinx.coroutines` includes debugging facilities to make it easier.
Run the following code with `-Dkotlinx.coroutines.debug` JVM option:
```kotlin
import kotlinx.coroutines.*
fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")
fun main() = runBlocking {
//sampleStart
val a = async {
log("I'm computing a piece of the answer")
6
}
val b = async {
log("I'm computing another piece of the answer")
7
}
log("The answer is ${a.await() * b.await()}")
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-03.kt).
There are three coroutines. The main coroutine (#1) inside `runBlocking`
and two coroutines computing the deferred values `a` (#2) and `b` (#3).
They are all executing in the context of `runBlocking` and are confined to the main thread.
The output of this code is:
```text
[main @coroutine#2] I'm computing a piece of the answer
[main @coroutine#3] I'm computing another piece of the answer
[main @coroutine#1] The answer is 42
```
The `log` function prints the name of the thread in square brackets, and you can see that it is the `main`
thread with the identifier of the currently executing coroutine appended to it. This identifier
is consecutively assigned to all created coroutines when the debugging mode is on.
> Debugging mode is also turned on when JVM is run with `-ea` option.
You can read more about debugging facilities in the documentation of the [DEBUG_PROPERTY_NAME] property.
### Jumping between threads
Run the following code with the `-Dkotlinx.coroutines.debug` JVM option (see [debug](#debugging-coroutines-and-threads)):
```kotlin
import kotlinx.coroutines.*
fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")
fun main() {
//sampleStart
newSingleThreadContext("Ctx1").use { ctx1 ->
newSingleThreadContext("Ctx2").use { ctx2 ->
runBlocking(ctx1) {
log("Started in ctx1")
withContext(ctx2) {
log("Working in ctx2")
}
log("Back to ctx1")
}
}
}
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-04.kt).
It demonstrates several new techniques. One is using [runBlocking] with an explicitly specified context, and
the other one is using the [withContext] function to change the context of a coroutine while still staying in the
same coroutine, as you can see in the output below:
```text
[Ctx1 @coroutine#1] Started in ctx1
[Ctx2 @coroutine#1] Working in ctx2
[Ctx1 @coroutine#1] Back to ctx1
```
Note that this example also uses the `use` function from the Kotlin standard library to release threads
created with [newSingleThreadContext] when they are no longer needed.
### Job in the context
The coroutine's [Job] is part of its context, and can be retrieved from it
using the `coroutineContext[Job]` expression:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
println("My job is ${coroutineContext[Job]}")
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-05.kt).
In the [debug mode](#debugging-coroutines-and-threads), it outputs something like this:
```
My job is "coroutine#1":BlockingCoroutine{Active}@6d311334
```
Note that [isActive] in [CoroutineScope] is just a convenient shortcut for
`coroutineContext[Job]?.isActive == true`.
### Children of a coroutine
When a coroutine is launched in the [CoroutineScope] of another coroutine,
it inherits its context via [CoroutineScope.coroutineContext] and
the [Job] of the new coroutine becomes
a _child_ of the parent coroutine's job. When the parent coroutine is cancelled, all its children
are recursively cancelled, too.
However, when [GlobalScope] is used to launch a coroutine, there is no parent for the job of the new coroutine.
It is therefore not tied to the scope it was launched from and operates independently.
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
// launch a coroutine to process some kind of incoming request
val request = launch {
// it spawns two other jobs, one with GlobalScope
GlobalScope.launch {
println("job1: I run in GlobalScope and execute independently!")
delay(1000)
println("job1: I am not affected by cancellation of the request")
}
// and the other inherits the parent context
launch {
delay(100)
println("job2: I am a child of the request coroutine")
delay(1000)
println("job2: I will not execute this line if my parent request is cancelled")
}
}
delay(500)
request.cancel() // cancel processing of the request
delay(1000) // delay a second to see what happens
println("main: Who has survived request cancellation?")
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-06.kt).
The output of this code is:
```text
job1: I run in GlobalScope and execute independently!
job2: I am a child of the request coroutine
job1: I am not affected by cancellation of the request
main: Who has survived request cancellation?
```
### Parental responsibilities
A parent coroutine always waits for completion of all its children. A parent does not have to explicitly track
all the children it launches, and it does not have to use [Job.join] to wait for them at the end:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
// launch a coroutine to process some kind of incoming request
val request = launch {
repeat(3) { i -> // launch a few children jobs
launch {
delay((i + 1) * 200L) // variable delay 200ms, 400ms, 600ms
println("Coroutine $i is done")
}
}
println("request: I'm done and I don't explicitly join my children that are still active")
}
request.join() // wait for completion of the request, including all its children
println("Now processing of the request is complete")
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-07.kt).
The result is going to be:
```text
request: I'm done and I don't explicitly join my children that are still active
Coroutine 0 is done
Coroutine 1 is done
Coroutine 2 is done
Now processing of the request is complete
```
### Naming coroutines for debugging
Automatically assigned ids are good when coroutines log often and you just need to correlate log records
coming from the same coroutine. However, when a coroutine is tied to the processing of a specific request
or doing some specific background task, it is better to name it explicitly for debugging purposes.
The [CoroutineName] context element serves the same purpose as the thread name. It is included in the thread name that
is executing this coroutine when the [debugging mode](#debugging-coroutines-and-threads) is turned on.
The following example demonstrates this concept:
```kotlin
import kotlinx.coroutines.*
fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")
fun main() = runBlocking(CoroutineName("main")) {
//sampleStart
log("Started main coroutine")
// run two background value computations
val v1 = async(CoroutineName("v1coroutine")) {
delay(500)
log("Computing v1")
252
}
val v2 = async(CoroutineName("v2coroutine")) {
delay(1000)
log("Computing v2")
6
}
log("The answer for v1 / v2 = ${v1.await() / v2.await()}")
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-08.kt).
The output it produces with `-Dkotlinx.coroutines.debug` JVM option is similar to:
```text
[main @main#1] Started main coroutine
[main @v1coroutine#2] Computing v1
[main @v2coroutine#3] Computing v2
[main @main#1] The answer for v1 / v2 = 42
```
### Combining context elements
Sometimes we need to define multiple elements for a coroutine context. We can use the `+` operator for that.
For example, we can launch a coroutine with an explicitly specified dispatcher and an explicitly specified
name at the same time:
```kotlin
import kotlinx.coroutines.*
fun main() = runBlocking {
//sampleStart
launch(Dispatchers.Default + CoroutineName("test")) {
println("I'm working in thread ${Thread.currentThread().name}")
}
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-09.kt).
The output of this code with the `-Dkotlinx.coroutines.debug` JVM option is:
```text
I'm working in thread DefaultDispatcher-worker-1 @test#2
```
### Coroutine scope
Let us put our knowledge about contexts, children and jobs together. Assume that our application has
an object with a lifecycle, but that object is not a coroutine. For example, we are writing an Android application
and launch various coroutines in the context of an Android activity to perform asynchronous operations to fetch
and update data, do animations, etc. All of these coroutines must be cancelled when the activity is destroyed
to avoid memory leaks. We, of course, can manipulate contexts and jobs manually to tie the lifecycles of the activity
and its coroutines, but `kotlinx.coroutines` provides an abstraction encapsulating that: [CoroutineScope].
You should be already familiar with the coroutine scope as all coroutine builders are declared as extensions on it.
We manage the lifecycles of our coroutines by creating an instance of [CoroutineScope] tied to
the lifecycle of our activity. A `CoroutineScope` instance can be created by the [CoroutineScope()] or [MainScope()]
factory functions. The former creates a general-purpose scope, while the latter creates a scope for UI applications and uses
[Dispatchers.Main] as the default dispatcher:
```kotlin
class Activity {
private val mainScope = MainScope()
fun destroy() {
mainScope.cancel()
}
// to be continued ...
```
Now, we can launch coroutines in the scope of this `Activity` using the defined `scope`.
For the demo, we launch ten coroutines that delay for a different time:
```kotlin
// class Activity continues
fun doSomething() {
// launch ten coroutines for a demo, each working for a different time
repeat(10) { i ->
mainScope.launch {
delay((i + 1) * 200L) // variable delay 200ms, 400ms, ... etc
println("Coroutine $i is done")
}
}
}
} // class Activity ends
```
In our main function we create the activity, call our test `doSomething` function, and destroy the activity after 500ms.
This cancels all the coroutines that were launched from `doSomething`. We can see that because after the destruction
of the activity no more messages are printed, even if we wait a little longer.
```kotlin
import kotlinx.coroutines.*
class Activity {
private val mainScope = CoroutineScope(Dispatchers.Default) // use Default for test purposes
fun destroy() {
mainScope.cancel()
}
fun doSomething() {
// launch ten coroutines for a demo, each working for a different time
repeat(10) { i ->
mainScope.launch {
delay((i + 1) * 200L) // variable delay 200ms, 400ms, ... etc
println("Coroutine $i is done")
}
}
}
} // class Activity ends
fun main() = runBlocking {
//sampleStart
val activity = Activity()
activity.doSomething() // run test function
println("Launched coroutines")
delay(500L) // delay for half a second
println("Destroying activity!")
activity.destroy() // cancels all coroutines
delay(1000) // visually confirm that they don't work
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-10.kt).
The output of this example is:
```text
Launched coroutines
Coroutine 0 is done
Coroutine 1 is done
Destroying activity!
```
As you can see, only the first two coroutines print a message and the others are cancelled
by a single invocation of `job.cancel()` in `Activity.destroy()`.
> Note, that Android has first-party support for coroutine scope in all entities with the lifecycle.
See [the corresponding documentation](https://developer.android.com/topic/libraries/architecture/coroutines#lifecyclescope).
### Thread-local data
Sometimes it is convenient to have an ability to pass some thread-local data to or between coroutines.
However, since they are not bound to any particular thread, this will likely lead to boilerplate if done manually.
For [`ThreadLocal`](https://docs.oracle.com/javase/8/docs/api/java/lang/ThreadLocal.html),
the [asContextElement] extension function is here for the rescue. It creates an additional context element
which keeps the value of the given `ThreadLocal` and restores it every time the coroutine switches its context.
It is easy to demonstrate it in action:
```kotlin
import kotlinx.coroutines.*
val threadLocal = ThreadLocal() // declare thread-local variable
fun main() = runBlocking {
//sampleStart
threadLocal.set("main")
println("Pre-main, current thread: ${Thread.currentThread()}, thread local value: '${threadLocal.get()}'")
val job = launch(Dispatchers.Default + threadLocal.asContextElement(value = "launch")) {
println("Launch start, current thread: ${Thread.currentThread()}, thread local value: '${threadLocal.get()}'")
yield()
println("After yield, current thread: ${Thread.currentThread()}, thread local value: '${threadLocal.get()}'")
}
job.join()
println("Post-main, current thread: ${Thread.currentThread()}, thread local value: '${threadLocal.get()}'")
//sampleEnd
}
```
> You can get the full code [here](../kotlinx-coroutines-core/jvm/test/guide/example-context-11.kt).
In this example we launch a new coroutine in a background thread pool using [Dispatchers.Default], so
it works on a different thread from the thread pool, but it still has the value of the thread local variable
that we specified using `threadLocal.asContextElement(value = "launch")`,
no matter which thread the coroutine is executed on.
Thus, the output (with [debug](#debugging-coroutines-and-threads)) is:
```text
Pre-main, current thread: Thread[main @coroutine#1,5,main], thread local value: 'main'
Launch start, current thread: Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main], thread local value: 'launch'
After yield, current thread: Thread[DefaultDispatcher-worker-2 @coroutine#2,5,main], thread local value: 'launch'
Post-main, current thread: Thread[main @coroutine#1,5,main], thread local value: 'main'
```
It's easy to forget to set the corresponding context element. The thread-local variable accessed from the coroutine may
then have an unexpected value, if the thread running the coroutine is different.
To avoid such situations, it is recommended to use the [ensurePresent] method
and fail-fast on improper usages.
`ThreadLocal` has first-class support and can be used with any primitive `kotlinx.coroutines` provides.
It has one key limitation, though: when a thread-local is mutated, a new value is not propagated to the coroutine caller
(because a context element cannot track all `ThreadLocal` object accesses), and the updated value is lost on the next suspension.
Use [withContext] to update the value of the thread-local in a coroutine, see [asContextElement] for more details.
Alternatively, a value can be stored in a mutable box like `class Counter(var i: Int)`, which is, in turn,
stored in a thread-local variable. However, in this case you are fully responsible to synchronize
potentially concurrent modifications to the variable in this mutable box.
For advanced usage, for example for integration with logging MDC, transactional contexts or any other libraries
which internally use thread-locals for passing data, see the documentation of the [ThreadContextElement] interface
that should be implemented.
[Job]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html
[CoroutineDispatcher]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-dispatcher/index.html
[launch]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/launch.html
[async]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/async.html
[CoroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/index.html
[Dispatchers.Unconfined]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-unconfined.html
[GlobalScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-global-scope/index.html
[Dispatchers.Default]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-default.html
[newSingleThreadContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/new-single-thread-context.html
[ExecutorCoroutineDispatcher.close]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-executor-coroutine-dispatcher/close.html
[runBlocking]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/run-blocking.html
[delay]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/delay.html
[DEBUG_PROPERTY_NAME]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-d-e-b-u-g_-p-r-o-p-e-r-t-y_-n-a-m-e.html
[withContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html
[isActive]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/is-active.html
[CoroutineScope.coroutineContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/coroutine-context.html
[Job.join]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/join.html
[CoroutineName]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-name/index.html
[CoroutineScope()]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope.html
[MainScope()]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-main-scope.html
[Dispatchers.Main]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-main.html
[asContextElement]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/java.lang.-thread-local/as-context-element.html
[ensurePresent]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/java.lang.-thread-local/ensure-present.html
[ThreadContextElement]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-thread-context-element/index.html