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Coroutine context and dispatchers

Coroutines always execute in some context which is represented by the value of CoroutineContext 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

Coroutine context includes a coroutine dispatcher (see CoroutineDispatcher) that determines what thread or threads the corresponding coroutine uses for its execution. Coroutine dispatcher can confine coroutine execution to a specific thread, dispatch it to a thread pool, or let it run unconfined.

All coroutines builders like launch and async accept an optional CoroutineContext parameter that can be used to explicitly specify the dispatcher for new coroutine and other context elements.

Try the following example:

fun main(args: Array<String>) = runBlocking<Unit> {
    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}")
    }
}

You can get full code here

It produces the following output (maybe in different order):

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 that 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 shared background pool of threads, so launch(Dispatchers.Default) { ... } uses the same dispatcher as GlobalScope.launch { ... }.

newSingleThreadContext creates a new 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 close function, or stored in a top-level variable and reused throughout the application.

Unconfined vs confined dispatcher

The Dispatchers.Unconfined coroutine dispatcher starts coroutine in the caller thread, but only until the first suspension point. After suspension it resumes in the thread that is fully determined by the suspending function that was invoked. Unconfined dispatcher is appropriate when coroutine does not consume CPU time nor updates any shared data (like UI) that is confined to a specific thread.

On the other side, by default, a dispatcher for the outer CoroutineScope is inherited. The default dispatcher for runBlocking coroutine, in particular, is confined to the invoker thread, so inheriting it has the effect of confining execution to this thread with a predictable FIFO scheduling.

fun main(args: Array<String>) = runBlocking<Unit> {
    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}")
    }
}

You can get full code here

Produces the output:

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 that had inherited context of runBlocking {...} continues to execute in the main thread, while the unconfined one had resumed in the default executor thread that delay function is using.

Unconfined dispatcher is an advanced mechanism that can be helpful in certain corner cases where dispatching of coroutine for its execution later is not needed or produces undesirable side-effects, because some operation in a coroutine must be performed right away. 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 coroutine was doing, where, and when. The common approach to debugging applications with threads 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:

fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")

fun main(args: Array<String>) = runBlocking<Unit> {
    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()}")
}

You can get full code here

There are three coroutines. The main coroutine (#1) – runBlocking one, and two coroutines computing 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:

[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, but the identifier of the currently executing coroutine is appended to it. This identifier is consecutively assigned to all created coroutines when debugging mode is turned on.

You can read more about debugging facilities in the documentation for newCoroutineContext function.

Jumping between threads

Run the following code with -Dkotlinx.coroutines.debug JVM option (see debug):

fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")

fun main(args: Array<String>) {
    newSingleThreadContext("Ctx1").use { ctx1 ->
        newSingleThreadContext("Ctx2").use { ctx2 ->
            runBlocking(ctx1) {
                log("Started in ctx1")
                withContext(ctx2) {
                    log("Working in ctx2")
                }
                log("Back to ctx1")
            }
        }
    }
}

You can get full code here

It demonstrates several new techniques. One is using runBlocking with an explicitly specified context, and the other one is using withContext function to change a context of a coroutine while still staying in the same coroutine as you can see in the output below:

[Ctx1 @coroutine#1] Started in ctx1
[Ctx2 @coroutine#1] Working in ctx2
[Ctx1 @coroutine#1] Back to ctx1

Note, that this example also uses use function from the Kotlin standard library to release threads that are created with newSingleThreadContext when they are no longer needed.

Job in the context

The coroutine's Job is part of its context. The coroutine can retrieve it from its own context using coroutineContext[Job] expression:

fun main(args: Array<String>) = runBlocking<Unit> {
    println("My job is ${coroutineContext[Job]}")
}

You can get full code here

It produces something like that when running in debug mode:

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, it is not tied to the scope it was launched from and operates independently.

fun main(args: Array<String>) = runBlocking<Unit> {
    // 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?")
}

You can get full code here

The output of this code is:

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. 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:

fun main(args: Array<String>) = runBlocking<Unit> {
    // 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")
}

You can get full code here

The result is going to be:

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 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. CoroutineName context element serves the same function as a thread name. It'll get displayed in the thread name that is executing this coroutine when debugging mode is turned on.

The following example demonstrates this concept:

fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")

fun main(args: Array<String>) = runBlocking(CoroutineName("main")) {
    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()}")
}

You can get full code here

The output it produces with -Dkotlinx.coroutines.debug JVM option is similar to:

[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 coroutine context. We can use + operator for that. For example, we can launch a coroutine with an explicitly specified dispatcher and an explicitly specified name at the same time:

fun main(args: Array<String>) = runBlocking<Unit> {
    launch(Dispatchers.Default + CoroutineName("test")) {
        println("I'm working in thread ${Thread.currentThread().name}")
    }
}

You can get full code here

The output of this code with -Dkotlinx.coroutines.debug JVM option is:

I'm working in thread DefaultDispatcher-worker-1 @test#2

Cancellation via explicit job

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 activity is destroyed to avoid memory leaks.

We manage a lifecycle of our coroutines by creating an instance of Job that is tied to the lifecycle of our activity. A job instance is created using Job() factory function when activity is created and it is cancelled when an activity is destroyed like this:

class Activity : CoroutineScope {
    lateinit var job: Job

    fun create() {
        job = Job()
    }

    fun destroy() {
        job.cancel()
    }
    // to be continued ...

We also implement CoroutineScope interface in this Actvity class. We only need to provide an override for its CoroutineScope.coroutineContext property to specify the context for coroutines launched in its scope. We combine the desired dispatcher (we used Dispatchers.Default in this example) and a job:

    // class Activity continues
    override val coroutineContext: CoroutineContext
        get() = Dispatchers.Default + job
    // to be continued ...

Now, we can launch coroutines in the scope of this Activity without having to explicitly specify their context. For the demo, we launch ten coroutines that delay for a different time:

    // class Activity continues
    fun doSomething() {
        // launch ten coroutines for a demo, each working for a different time
        repeat(10) { i ->
            launch {
                delay((i + 1) * 200L) // variable delay 200ms, 400ms, ... etc
                println("Coroutine $i is done")
            }
        }
    }
} // class Activity ends

In our main function we create activity, call our test doSomething function, and destroy activity after 500ms. This cancels all the coroutines that were launched which we can confirm by noting that it does not print onto the screen anymore if we wait:

fun main(args: Array<String>) = runBlocking<Unit> {
    val activity = Activity()
    activity.create() // create an 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
}

You can get full code here

The output of this example is:

Launched coroutines
Coroutine 0 is done
Coroutine 1 is done
Destroying activity!

As you can see, only the first two coroutines had printed a message and the others were cancelled by a single invocation of job.cancel() in Activity.destroy().

Thread-local data

Sometimes it is convenient to have an ability to pass some thread-local data, but, for coroutines, which are not bound to any particular thread, it is hard to achieve it manually without writing a lot of boilerplate.

For ThreadLocal, asContextElement extension function is here for the rescue. It creates an additional context element, which keep the value of the given ThreadLocal and restores it every time the coroutine switches its context.

It is easy to demonstrate it in action:

val threadLocal = ThreadLocal<String?>() // declare thread-local variable

fun main(args: Array<String>) = runBlocking<Unit> {
    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()}'")
}

You can get full code here

In this example we launch new coroutine in a background thread pool using Dispatchers.Default, so it works on a different threads from a thread pool, but it still has the value of thread local variable, that we've specified using threadLocal.asContextElement(value = "launch"), no matter on what thread the coroutine is executed. Thus, output (with debug) is:

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'

ThreadLocal has first-class support and can be used with any primitive kotlinx.coroutines provides. It has one key limitation: when thread-local is mutated, a new value is not propagated to the coroutine caller (as context element cannot track all ThreadLocal object accesses) and 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, is 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 documentation for ThreadContextElement interface that should be implemented.