# What is CoroutineContext and how does it work?

This is a chapter from the book Kotlin Coroutines. You can find it on LeanPub.

If you take a look at the coroutine builders’ definitions, you will see that their first parameter is of type CoroutineContext.

public fun CoroutineScope.launch( context: CoroutineContext = EmptyCoroutineContext, start: CoroutineStart = CoroutineStart.DEFAULT, block: suspend CoroutineScope.() -> Unit ): Job { ... }

The receiver and the last argument’s receiver are of type CoroutineScope1. This CoroutineScope seems to be an important concept, so let's check out its definition:

public interface CoroutineScope { public val coroutineContext: CoroutineContext }

It seems to be just a wrapper around CoroutineContext. So, you might want to recall how Continuation is defined.

public interface Continuation<in T> { public val context: CoroutineContext public fun resumeWith(result: Result<T>) }

Continuation contains CoroutineContext as well. This type is used by the most important Kotlin coroutine elements. This must be a really important concept, so what is it?

### CoroutineContext interface

CoroutineContext is an interface that represents an element or a collection of elements. It is conceptually similar to a map or a set collection: it is an indexed set of Element instances like Job, CoroutineName, CouroutineDispatcher, etc. The unusual thing is that each Element is also a CoroutineContext. So, every element in a collection is a collection in itself.

This concept is quite intuitive. Imagine a mug. It is a single element, but it is also a collection that contains a single element. When you add another mug, you have a collection with two elements.

In order to allow convenient context specification and modification, each CoroutineContext element is a CoroutineContext itself, as in the example below (adding contexts and setting a coroutine builder context will be explained later). Just specifying or adding contexts is much easier than creating an explicit set.

launch(CoroutineName("Name1")) { ... } launch(CoroutineName("Name2") + Job()) { ... }

Every element in this set has a unique Key that is used to identify it. These keys are compared by reference.

For example CoroutineName or Job implement CoroutineContext.Element, which implements the CoroutineContext interface.

import kotlinx.coroutines.CoroutineName import kotlinx.coroutines.Job import kotlin.coroutines.CoroutineContext //sampleStart fun main() { val name: CoroutineName = CoroutineName("A name") val element: CoroutineContext.Element = name val context: CoroutineContext = element val job: Job = Job() val jobElement: CoroutineContext.Element = job val jobContext: CoroutineContext = jobElement } //sampleEnd

It’s the same with SupervisorJob, CoroutineExceptionHandler and dispatchers from the Dispatchers object. These are the most important coroutine contexts. They will be explained in the next chapters.

### Finding elements in CoroutineContext

Since CoroutineContext is like a collection, we can find an element with a concrete key using get. Another option is to use square brackets, because in Kotlin the get method is an operator and can be invoked using square brackets instead of an explicit function call. Just like in Map: when an element is in the context, it will be returned. If it is not, null will be returned instead.

import kotlinx.coroutines.CoroutineName import kotlinx.coroutines.Job import kotlin.coroutines.CoroutineContext //sampleStart fun main() { val ctx: CoroutineContext = CoroutineName("A name") val coroutineName: CoroutineName? = ctx[CoroutineName] // or ctx.get(CoroutineName) println(coroutineName?.name) // A name val job: Job? = ctx[Job] // or ctx.get(Job) println(job) // null } //sampleEnd

CoroutineContext is part of the built-in support for Kotlin coroutines, so it is imported from kotlin.coroutines, while contexts like Job or CoroutineName are part of the kotlinx.coroutines library, so they need to be imported from kotlinx.coroutines.

To find a CoroutineName, we use just CoroutineName. This is not a type or a class: it is a companion object. It is a feature of Kotlin that a name of a class used by itself acts as a reference to its companion object, so ctx[CoroutineName] is just a shortcut to ctx[CoroutineName.Key].

data class CoroutineName( val name: String ) : AbstractCoroutineContextElement(CoroutineName) { override fun toString(): String = "CoroutineName($name)" companion object Key : CoroutineContext.Key<CoroutineName> } It is common practice in the kotlinx.coroutines library to use companion objects as keys to elements with the same name. This makes it easier to remember2. A key might point to a class (like CoroutineName) or to an interface (like Job) that is implemented by many classes with the same key (like Job and SupervisorJob). interface Job : CoroutineContext.Element { companion object Key : CoroutineContext.Key<Job> // ... } ### Adding contexts What makes CoroutineContext truly useful is the ability to merge two of them together. When two elements with different keys are added, the resulting context responds to both keys. import kotlinx.coroutines.CoroutineName import kotlinx.coroutines.Job import kotlin.coroutines.CoroutineContext //sampleStart fun main() { val ctx1: CoroutineContext = CoroutineName("Name1") println(ctx1[CoroutineName]?.name) // Name1 println(ctx1[Job]?.isActive) // null val ctx2: CoroutineContext = Job() println(ctx2[CoroutineName]?.name) // null println(ctx2[Job]?.isActive) // true, because "Active" // is the default state of a job created this way val ctx3 = ctx1 + ctx2 println(ctx3[CoroutineName]?.name) // Name1 println(ctx3[Job]?.isActive) // true } //sampleEnd When another element with the same key is added, just like in a map, the new element replaces the previous one. import kotlinx.coroutines.CoroutineName import kotlin.coroutines.CoroutineContext //sampleStart fun main() { val ctx1: CoroutineContext = CoroutineName("Name1") println(ctx1[CoroutineName]?.name) // Name1 val ctx2: CoroutineContext = CoroutineName("Name2") println(ctx2[CoroutineName]?.name) // Name2 val ctx3 = ctx1 + ctx2 println(ctx3[CoroutineName]?.name) // Name2 } //sampleEnd ### Empty coroutine context Since CoroutineContext is like a collection, we also have an empty context. Such a context by itself returns no elements; if we add it to another context, it behaves exactly like this other context. import kotlinx.coroutines.CoroutineName import kotlinx.coroutines.Job import kotlin.coroutines.CoroutineContext import kotlin.coroutines.EmptyCoroutineContext //sampleStart fun main() { val empty: CoroutineContext = EmptyCoroutineContext println(empty[CoroutineName]) // null println(empty[Job]) // null val ctxName = empty + CoroutineName("Name1") + empty println(ctxName[CoroutineName]) // CoroutineName(Name1) } //sampleEnd ### Subtracting elements Elements can also be removed from a context by their key using the minusKey function. The minus operator is not overloaded for CoroutineContext. I believe this is because its meaning would not be clear enough, as explained in Effective Kotlin Item 12: An operator’s meaning should be consistent with its function name. import kotlinx.coroutines.CoroutineName import kotlinx.coroutines.Job //sampleStart fun main() { val ctx = CoroutineName("Name1") + Job() println(ctx[CoroutineName]?.name) // Name1 println(ctx[Job]?.isActive) // true val ctx2 = ctx.minusKey(CoroutineName) println(ctx2[CoroutineName]?.name) // null println(ctx2[Job]?.isActive) // true val ctx3 = (ctx + CoroutineName("Name2")) .minusKey(CoroutineName) println(ctx3[CoroutineName]?.name) // null println(ctx3[Job]?.isActive) // true } //sampleEnd ### Folding context If we need to do something for each element in a context, we can use the fold method, which is similar to fold for other collections. It takes: • an initial accumulator value; • an operation to produce the next state of the accumulator, based on the current state, and the element it is currently invoked in. import kotlinx.coroutines.CoroutineName import kotlinx.coroutines.Job import kotlin.coroutines.CoroutineContext //sampleStart fun main() { val ctx = CoroutineName("Name1") + Job() ctx.fold("") { acc, element -> "$acc$element " } .also(::println) // CoroutineName(Name1) JobImpl{Active}@dbab622e val empty = emptyList<CoroutineContext>() ctx.fold(empty) { acc, element -> acc + element } .joinToString() .also(::println) // CoroutineName(Name1), JobImpl{Active}@dbab622e } //sampleEnd ### Coroutine context and builders So CoroutineContext is just a way to hold and pass data. By default, the parent passes its context to the child, which is one of the parent-child relationship effects. We say that the child inherits context from its parent. import kotlinx.coroutines.* //sampleStart fun CoroutineScope.log(msg: String) { val name = coroutineContext[CoroutineName]?.name println("[$name] $msg") } fun main() = runBlocking(CoroutineName("main")) { log("Started") // [main] Started val v1 = async { delay(500) log("Running async") // [main] Running async 42 } launch { delay(1000) log("Running launch") // [main] Running launch } log("The answer is${v1.await()}") // [main] The answer is 42 } //sampleEnd

Each child might have a specific context defined in the argument. This context overrides the one from the parent.

import kotlinx.coroutines.* fun CoroutineScope.log(msg: String) { val name = coroutineContext[CoroutineName]?.name println("[$name]$msg") } //sampleStart fun main() = runBlocking(CoroutineName("main")) { log("Started") // [main] Started val v1 = async(CoroutineName("c1")) { delay(500) log("Running async") // [c1] Running async 42 } launch(CoroutineName("c2")) { delay(1000) log("Running launch") // [c2] Running launch } log("The answer is ${v1.await()}") // [main] The answer is 42 } //sampleEnd A simplified formula to calculate a coroutine context is: defaultContext + parentContext + childContext Since new elements always replace old ones with the same key, the child context always overrides elements with the same key from the parent context. The defaults are used only for keys that are not specified anywhere else. Currently, the defaults only set Dispatchers.Default when no ContinuationInterceptor is set, and they only set CoroutineId when the application is in debug mode. There is a special context called Job, which is mutable and is used to communicate between a coroutine’s child and its parent. The next chapters will be dedicated to the effects of this communication. ### Accessing context in a suspending function CoroutineScope has a coroutineContext property that can be used to access the context. But what if we are in a regular suspending function? As you might remember from the Coroutines under the hood chapter, context is referenced by continuations, which are passed to each suspending function. So, it is possible to access a parent’s context in a suspending function. To do this, we use the coroutineContext property, which is available on every suspending scope. import kotlinx.coroutines.* import kotlin.coroutines.coroutineContext suspend fun printName() { println(coroutineContext[CoroutineName]?.name) } suspend fun main() = withContext(CoroutineName("Outer")) { printName() // Outer launch(CoroutineName("Inner")) { printName() // Inner } delay(10) printName() // Outer } ### Creating our own context It is not a common need, but we can create our own coroutine context pretty easily. To do this, the easiest way is to create a class that implements the CoroutineContext.Element interface. Such a class needs a property key of type CoroutineContext.Key<*>. This key will be used as the key that identifies this context. The common practice is to use this class’s companion object as a key. This is how a very simple coroutine context can be implemented: class MyCustomContext : CoroutineContext.Element { override val key: CoroutineContext.Key<*> = Key companion object Key : CoroutineContext.Key<MyCustomContext> } Such a context will behave a lot like CoroutineName: it will propagate from parent to child, but any children will be able to override it with a different context with the same key. To see this in practice, below you can see an example context that is designed to print consecutive numbers. import kotlinx.coroutines.launch import kotlinx.coroutines.withContext import kotlin.coroutines.CoroutineContext import kotlin.coroutines.coroutineContext class CounterContext( private val name: String ) : CoroutineContext.Element { override val key: CoroutineContext.Key<*> = Key private var nextNumber = 0 fun printNext() { println("$name: \$nextNumber") nextNumber++ } companion object Key:CoroutineContext.Key<CounterContext> } suspend fun printNext() { coroutineContext[CounterContext]?.printNext() } suspend fun main(): Unit = withContext(CounterContext("Outer")) { printNext() // Outer: 0 launch { printNext() // Outer: 1 launch { printNext() // Outer: 2 } launch(CounterContext("Inner")) { printNext() // Inner: 0 printNext() // Inner: 1 launch { printNext() // Inner: 2 } } } printNext() // Outer: 3 }

I have seen custom contexts in use as a kind of dependency injection - to easily inject different values in production than in tests. However, I don't think this will become standard practice.

import kotlinx.coroutines.withContext import java.util.* import kotlin.coroutines.CoroutineContext import kotlin.coroutines.coroutineContext import kotlin.test.assertEquals data class User(val id: String, val name: String) abstract class UuidProviderContext : CoroutineContext.Element { abstract fun nextUuid(): String override val key: CoroutineContext.Key<*> = Key companion object Key : CoroutineContext.Key<UuidProviderContext> } class RealUuidProviderContext : UuidProviderContext() { override fun nextUuid(): String = UUID.randomUUID().toString() } class FakeUuidProviderContext( private val fakeUuid: String ) : UuidProviderContext() { override fun nextUuid(): String = fakeUuid } suspend fun nextUuid(): String = checkNotNull(coroutineContext[UuidProviderContext]) { "UuidProviderContext not present" } .nextUuid() // function under test suspend fun makeUser(name: String) = User( id = nextUuid(), name = name ) suspend fun main(): Unit { // production case withContext(RealUuidProviderContext()) { println(makeUser("Michał")) // e.g. User(id=d260482a-..., name=Michał) } // test case withContext(FakeUuidProviderContext("FAKE_UUID")) { val user = makeUser("Michał") println(user) // User(id=FAKE_UUID, name=Michał) assertEquals(User("FAKE_UUID", "Michał"), user) } }

### Summary

CoroutineContext is conceptually similar to a map or a set collection. It is an indexed set of Element instances, where each Element is also a CoroutineContext. Every element in it has a unique Key that is used to identify it. This way, CoroutineContext is just a universal way to group and pass objects to coroutines. These objects are kept by the coroutines and can determine how these coroutines should be running (what their state is, in which thread, etc). In the next chapters, we will discuss the most essential coroutine contexts in the Kotlin coroutines library.

1:

Let’s clear up the nomenclature. launch is an extension function on CoroutineScope, so CoroutineScope is its receiver type. The extension function’s receiver is the object we reference with this.

2:

The companion object below is named Key. We can name companion objects, but this changes little in terms of how they are used. The default companion object name is Companion, so this name is used when we need to reference this object using reflection or when we define an extension function on it. Here we use Key instead.