In simple terms, a CoroutineScope in Kotlin is like a “container” that manages your coroutines. It defines the boundaries and the “lifetime” for the coroutines you start within it, ensuring that they are automatically canceled when they are no longer needed.

Job

Job Highlight

kdoc: Job

Job is the primary mechanism for controlling cancellation behavior of co-routines.

Job - provides another avenue of stopping work other than traditional exception flow. Separate cancellation mechanism that is not as apparent as typical exceptions, where failed co-routine cancel’s all co-routines that are related to it through Job hierarchy. (Note: Supervisor-Job is exception)

Exception/Cancellation Behavior with Regular Job

In ALL scopes that use Regular:

Uncaught, non-cancellation exception → Cancels Siblings + Propagates recursively through parent Job hierarchy, with each parent shutting down its children, Effectively shutting down the entire hierarchy up to the top most Job (or up to where you caught exception at error boundary).

Cancels siblings: functions-suspension-point in sibling functions will throw CancellationException

Propagates to parent: Parent scope will rethrow the original exception that was thrown in co-routine.

Highlight

Job - provide another avenue of stopping work other than traditional exceptions. Separate cancellation mechanism that is not as apparent as typical exception flow, where failed co-routine cancel’s all co-routines that are related to it through Job hierarchy.

Each coroutine has its own Job instance.

Each Job instance is tied to it’s parent job (if it has a parent Job). This tie in enables cancellation behavior.

Example

If regular Job() were used. If Coroutine#6 throws non-CancellationException exception the entire hierarchy is gonig to be cancelled and Coroutine#1 will rethrow the exception that Coroutine#6 threw.

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Creating Error Boundary

Look at scope-function-error-boundary to create error/ boundaries which allows us to try/catch exceptions stopping them from going higher up.

Gotchas

Cancellation Gotchas (With Regular Job)

⚠️ Firstly, make sure to understand intended ExceptionCancellationBehavior ⚠️.

evil-example-swallowing-cancel-exception

exceptions-from-async-under-coroutineScope-go-to-parent-and-not-await

cancel-call-does-not-stop-right-away

join

Link to original
Link to original
Link to original

Job states
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In the “Active” state, a job is running and doing its job. If the job is created with a coroutine builder, this is the state where the body of this coroutine will be executed. In this state, we can start child coroutines. Most coroutines will start in the “Active” state. Only those that are started lazily will start with the “New” state. These need to be started in order for them to move to the “Active” state. When a coroutine is executing its body, it is surely in the “Active” state. When it is done, its state changes to “Completing”, where it waits for its children. Once all its children are done, the job changes its state to “Completed”, which is a terminal one. Alternatively, if a job cancels or fails when running (in the “Active” or “Completing” state), its state will change to “Cancelling”. In this state, we have the last chance to do some clean-up, like closing connections or freeing resources (we will see how to do this in the next chapter). Once this is done, the job will move to the “Cancelled” state.

State isActive isCompleted isCancelled
New (optional initial state) false false false
Active (default initial state) true false false
Completing (transient state) true false false
Cancelling (transient state) false false true
Cancelled (final state) false true true
Completed (final state) false true false
Link to original

State	isActive	isCompleted	isCancelled
New (optional initial state)	false	false	false
Active (default initial state)	true	false	false
Completing (transient state)	true	false	false
Cancelling (transient state)	false	false	true
Cancelled (final state)	false	true	true
Completed (final state)	false	true	false

SupervisorJob

kdoc: SupervisorJob

kdoc: Exception Handling - SupervisorJob()

SupervisorJob is similar to a regular Job with the difference that cancellation is propagated only downwards, which means:

Children can fail independently of each other.
Does not auto-fail parent.

A failure or cancellation of a child does not cause the supervisor job to fail and does not affect its other children.

SupervisorJob() examples

Straightforward Examples

Straightforward SupervisorJob examples

SupervisorJob() added at scope level allows one child to finish, when the other child threw exception

Code

package com.glassthought.sandbox
 
import gt.sandbox.util.output.Emojis
import gt.sandbox.util.output.Out
import kotlinx.coroutines.CancellationException
import kotlinx.coroutines.CoroutineName
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.SupervisorJob
import kotlinx.coroutines.async
import kotlinx.coroutines.delay
import kotlinx.coroutines.runBlocking
import kotlin.system.exitProcess
 
fun main(): kotlin.Unit = runBlocking {
  val out = Out.standard()
 
  out.info("START")
 
  try {
    val coroutineScope = CoroutineScope(
      Dispatchers.Default + SupervisorJob()
    )
 
    val coRoutine1Deferred = coroutineScope.async(CoroutineName("WillFail")) {
      val timeMillis = 2000L
      out.info("This one will throw in $timeMillis ms")
      delay(timeMillis)
      val exc = MyExceptionWillThrowFromCoroutine("I-Failed-In-CoRoutine")
      out.warn("${Emojis.EXCEPTOIN} I am throwing [${exc::class.simpleName}/${exc.message}]! ${Emojis.EXCEPTOIN}")
      throw exc
 
      "ResultFromWillFail"
    }
 
    val coRoutine2Deferred = coroutineScope.async(CoroutineName("JustPrints")) {
      (0..10)
        .map { "a-${it}" }
        .forEach {
          out.info(it)
 
          try {
            delay(500)
          } catch (e: CancellationException) {
            val excMsg = e.message ?: e.toString()
            out.warn("${Emojis.OBIDIENT} I have caught [${e::class.simpleName}/$excMsg], and rethrowing it ${Emojis.OBIDIENT} ")
 
            throw e
          }
        }
    }
 
    // Notice we wait on co-routine 2 first. we do that since 2nd one is not the one that throws
    // MyExceptionWillThrowFromCoroutine
    out.info("co-routine-2 result: " + coRoutine2Deferred.await())
 
    out.info("Successfully awaited co-routine-2, now awaiting co-routine-1")
 
    out.info("co-routine-1 result: " + coRoutine1Deferred.await())
  } catch (e: Exception) {
    out.error("in main got an exception! of type=[${e::class.simpleName}] with msg=[${e.message}] cause=[${e.cause}]. Exiting with error code 1")
 
    exitProcess(1)
  }
 
  out.info("DONE - without errors on main")
}
 
class MyExceptionWillThrowFromCoroutine(msg: String) : RuntimeException(msg)

Command to reproduce:

gt.sandbox.checkout.commit fc38283452b69eaf78fc \
&& cd "${GT_SANDBOX_REPO}" \
&& cmd.run.announce "./gradlew run --quiet"

Recorded output of command:

Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
[INFO][elapsed:   14ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] START
[INFO][elapsed:   33ms][2️⃣][⓶][coroutname:@WillFail#2][tname:DefaultDispatcher-worker-1/tid:31] This one will throw in 2000 ms
[INFO][elapsed:   37ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-0
[INFO][elapsed:  538ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-1
[INFO][elapsed: 1039ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-2
[INFO][elapsed: 1539ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-3
[INFO][elapsed: 2040ms][2️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-1/tid:31] a-4
[WARN][elapsed: 2068ms][3️⃣][⓶][coroutname:@WillFail#2][tname:DefaultDispatcher-worker-2/tid:32] 💥  I am throwing [MyExceptionWillThrowFromCoroutine/I-Failed-In-CoRoutine]! 💥 
[INFO][elapsed: 2541ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-5
[INFO][elapsed: 3041ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-6
[INFO][elapsed: 3542ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-7
[INFO][elapsed: 4043ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-8
[INFO][elapsed: 4543ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-9
[INFO][elapsed: 5044ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-10
[INFO][elapsed: 5546ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] co-routine-2 result: kotlin.Unit
[INFO][elapsed: 5546ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] Successfully awaited co-routine-2, now awaiting co-routine-1
[ERROR][elapsed: 5560ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] in main got an exception! of type=[MyExceptionWillThrowFromCoroutine] with msg=[I-Failed-In-CoRoutine] cause=[com.glassthought.sandbox.MyExceptionWillThrowFromCoroutine: I-Failed-In-CoRoutine]. Exiting with error code 1
 
FAILURE: Build failed with an exception.
 
* What went wrong:
Execution failed for task ':app:run'.
> Process 'command '/home/nickolaykondratyev/.jdks/corretto-21.0.7/bin/java'' finished with non-zero exit value 1
 
* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.
 
BUILD FAILED in 5s

SupervisorJob() added at 'async' level of throwing sibling allows the other sibling to finish

Code

package com.glassthought.sandbox
 
import gt.sandbox.util.output.Emojis
import gt.sandbox.util.output.Out
import kotlinx.coroutines.CancellationException
import kotlinx.coroutines.CoroutineName
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.SupervisorJob
import kotlinx.coroutines.async
import kotlinx.coroutines.delay
import kotlinx.coroutines.runBlocking
import kotlin.system.exitProcess
 
fun main(): kotlin.Unit = runBlocking {
  val out = Out.standard()
 
  out.info("START")
 
  try {
    val coroutineScope = CoroutineScope(
      Dispatchers.Default
    )
 
    val coRoutine1Deferred = coroutineScope.async(
      CoroutineName("WillFail") + SupervisorJob()
    ) {
      val timeMillis = 2000L
      out.info("This one will throw in $timeMillis ms")
      delay(timeMillis)
      val exc = MyExceptionWillThrowFromCoroutine("I-Failed-In-CoRoutine")
      out.warn("${Emojis.EXCEPTOIN} I am throwing [${exc::class.simpleName}/${exc.message}]! ${Emojis.EXCEPTOIN}")
      throw exc
 
      "ResultFromWillFail"
    }
 
    val coRoutine2Deferred = coroutineScope.async(CoroutineName("JustPrints")) {
      (0..10)
        .map { "a-${it}" }
        .forEach {
          out.info(it)
 
          try {
            delay(500)
          } catch (e: CancellationException) {
            val excMsg = e.message ?: e.toString()
            out.warn("${Emojis.OBIDIENT} I have caught [${e::class.simpleName}/$excMsg], and rethrowing it ${Emojis.OBIDIENT} ")
 
            throw e
          }
        }
    }
 
    // Notice we wait on co-routine 2 first. we do that since 2nd one is not the one that throws
    // MyExceptionWillThrowFromCoroutine
    out.info("co-routine-2 result: " + coRoutine2Deferred.await())
 
    out.info("Successfully awaited co-routine-2, now awaiting co-routine-1")
 
    out.info("co-routine-1 result: " + coRoutine1Deferred.await())
  } catch (e: Exception) {
    out.error("in main got an exception! of type=[${e::class.simpleName}] with msg=[${e.message}] cause=[${e.cause}]. Exiting with error code 1")
 
    exitProcess(1)
  }
 
  out.info("DONE - without errors on main")
}
 
class MyExceptionWillThrowFromCoroutine(msg: String) : RuntimeException(msg)

Command to reproduce:

gt.sandbox.checkout.commit 2156e50526a56711a2e8 \
&& cd "${GT_SANDBOX_REPO}" \
&& cmd.run.announce "./gradlew run --quiet"

Recorded output of command:

Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
[INFO][elapsed:   15ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] START
[INFO][elapsed:   36ms][2️⃣][⓶][coroutname:@WillFail#2][tname:DefaultDispatcher-worker-1/tid:31] This one will throw in 2000 ms
[INFO][elapsed:   38ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-0
[INFO][elapsed:  540ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-1
[INFO][elapsed: 1041ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-2
[INFO][elapsed: 1542ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-3
[INFO][elapsed: 2042ms][2️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-1/tid:31] a-4
[WARN][elapsed: 2071ms][3️⃣][⓶][coroutname:@WillFail#2][tname:DefaultDispatcher-worker-2/tid:32] 💥  I am throwing [MyExceptionWillThrowFromCoroutine/I-Failed-In-CoRoutine]! 💥 
[INFO][elapsed: 2543ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-5
[INFO][elapsed: 3044ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-6
[INFO][elapsed: 3544ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-7
[INFO][elapsed: 4045ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-8
[INFO][elapsed: 4546ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-9
[INFO][elapsed: 5047ms][3️⃣][⓷][coroutname:@JustPrints#3][tname:DefaultDispatcher-worker-2/tid:32] a-10
[INFO][elapsed: 5548ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] co-routine-2 result: kotlin.Unit
[INFO][elapsed: 5549ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] Successfully awaited co-routine-2, now awaiting co-routine-1
[ERROR][elapsed: 5563ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] in main got an exception! of type=[MyExceptionWillThrowFromCoroutine] with msg=[I-Failed-In-CoRoutine] cause=[com.glassthought.sandbox.MyExceptionWillThrowFromCoroutine: I-Failed-In-CoRoutine]. Exiting with error code 1
 
FAILURE: Build failed with an exception.
 
* What went wrong:
Execution failed for task ':app:run'.
> Process 'command '/home/nickolaykondratyev/.jdks/corretto-21.0.7/bin/java'' finished with non-zero exit value 1
 
* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.
 
BUILD FAILED in 5s

We are able to cancel Deferred(Job) that used SupervisorJob() to be created by calling cancel() directly on deferred object

Code

package com.glassthought.sandbox
 
import gt.sandbox.util.output.Emojis
import gt.sandbox.util.output.Out
import kotlinx.coroutines.CancellationException
import kotlinx.coroutines.CoroutineName
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.SupervisorJob
import kotlinx.coroutines.async
import kotlinx.coroutines.delay
import kotlinx.coroutines.runBlocking
import kotlin.system.exitProcess
 
fun main(): kotlin.Unit = runBlocking {
  val out = Out.standard()
  out.info("START")
 
  try {
    val scope = CoroutineScope(
      Dispatchers.Default
    )
 
    val deferred = scope.async(CoroutineName("JustPrints") + SupervisorJob()) {
      (0..10)
        .map { "a-${it}" }
        .forEach {
          out.info(it)
 
          try {
            delay(500)
          } catch (e: CancellationException) {
            val excMsg = e.message ?: e.toString()
            out.warn("${Emojis.OBIDIENT} I have caught [${e::class.simpleName}/$excMsg], and rethrowing it ${Emojis.OBIDIENT} ")
 
            throw e
          }
        }
    }
 
    out.info("delay 1500 before cancelling co-routine 1")
    delay(1500)
    out.info("Cancelling co-routine 1 - ${Emojis.CANCELLATION}")
    deferred.cancel()
    out.info("delay before deferred.await()")
    delay(1500)
    out.info("calling deferred.await()")
 
    // Notice we wait on co-routine 2 first. we do that since 2nd one is not the one that throws
    // MyExceptionWillThrowFromCoroutine
    out.info("co-routine-1 result: " + deferred.await())
  } catch (e: Exception) {
    out.error("in main got an exception! of type=[${e::class.simpleName}] with msg=[${e.message}] cause=[${e.cause}]. Exiting with error code 1")
 
    exitProcess(1)
  }
 
  out.info("DONE - without errors on main")
}
 
class MyExceptionWillThrowFromCoroutine(msg: String) : RuntimeException(msg)

Command to reproduce:

gt.sandbox.checkout.commit c1640d7e2e9cfd437d54 \
&& cd "${GT_SANDBOX_REPO}" \
&& cmd.run.announce "./gradlew run --quiet"

Recorded output of command:

Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
[INFO][elapsed:   15ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] START
[INFO][elapsed:   34ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] delay 1500 before cancelling co-routine 1
[INFO][elapsed:   36ms][2️⃣][⓶][coroutname:@JustPrints#2][tname:DefaultDispatcher-worker-1/tid:31] a-0
[INFO][elapsed:  538ms][2️⃣][⓶][coroutname:@JustPrints#2][tname:DefaultDispatcher-worker-1/tid:31] a-1
[INFO][elapsed: 1038ms][2️⃣][⓶][coroutname:@JustPrints#2][tname:DefaultDispatcher-worker-1/tid:31] a-2
[INFO][elapsed: 1536ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] Cancelling co-routine 1 - ❌
[INFO][elapsed: 1538ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] delay before deferred.await()
[WARN][elapsed: 1583ms][2️⃣][⓶][coroutname:@JustPrints#2][tname:DefaultDispatcher-worker-1/tid:31] 🫡 I have caught [JobCancellationException/DeferredCoroutine was cancelled], and rethrowing it 🫡 
[INFO][elapsed: 3039ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] calling deferred.await()
[ERROR][elapsed: 3040ms][🥇][①][coroutname:@coroutine#1][tname:main/tid:1] in main got an exception! of type=[JobCancellationException] with msg=[DeferredCoroutine was cancelled] cause=[kotlinx.coroutines.JobCancellationException: DeferredCoroutine was cancelled; job="JustPrints#2":DeferredCoroutine{Cancelled}@3a03464]. Exiting with error code 1
 
FAILURE: Build failed with an exception.
 
* What went wrong:
Execution failed for task ':app:run'.
> Process 'command '/home/nickolaykondratyev/.jdks/corretto-21.0.7/bin/java'' finished with non-zero exit value 1
 
* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.
 
BUILD FAILED in 3s

Glassthought

Explorer

CoroutineScope

Job

Job Highlight

Exception/Cancellation Behavior with Regular Job

Highlight

Example

Creating Error Boundary

Gotchas

Cancellation Gotchas (With Regular Job)

Job states

SupervisorJob

SupervisorJob() examples

Straightforward Examples

Straightforward SupervisorJob examples

Code

Command to reproduce:

Recorded output of command:

Code

Command to reproduce:

Recorded output of command:

Code

Command to reproduce:

Recorded output of command:

Trickier examples

Trickier SupervisorJob() examples

+SupervisorJob()/+Job()

+Job()/+SupervisorJob() must-inherit-from-parent or it will ⚠️not respect parent cancellation⚠️

issue

Bug prone: +Job() Ignores parent cancellation

+SupervisorJob() completely unlinks from parent scope cancellation / Breaks the parent-child chain

+SupervisorJob() is likely NOT what you want.

Code

Command to reproduce:

Recorded output of command:

+Job() has the same issue of unlinking from parent cancellations

Code

Command to reproduce:

Recorded output of command:

fix

How to Fix the ignore of parent cancellation when creating new Jobs

Use +SupervisorJob(coroutineContext[Job]) OR +Job(coroutineContext[Job])

Code

Command to reproduce:

Recorded output of command:

Side note

Scope with Regular Job

What is a “Scope with Regular Job”?

Default Exception Behavior

Exception/Cancellation Behavior with Regular Job

Highlight

Example

Creating Error Boundary

Gotchas

Cancellation Gotchas (With Regular Job)

Code example

Scope examples with Regular Job

Core examples with regular Job

structured-parallelization

manually-with-scope with child parallelization launched co-routines

Code

Command to reproduce:

Recorded output of command:

coroutineScope-happy-case

Code

Command to reproduce:

Recorded output of command:

throw-exception-in-one-which-cancels-others

Code

Command to reproduce:

Recorded output of command:

throw-cancellation-exception

Related

Code

Command to reproduce:

Recorded output of command:

With await:

happy-case

Code

Uncaught exception behavior with `async`:

Uncaught exception behavior with `async`: