Kotlin Coroutines

Kotlin Coroutines builders & scope

🧩 Builders

Builder	Returns	Blocks OS thread	Waits completion	New Job	Root Job	New coroutine	Root coroutine	Suspend point
runBlocking { } (plug, rarely used)	block result	✓ Yes	✓ Yes	✓ Yes	✓ Yes	✓ Yes	✓ Yes CEH doesn't work	✗ No
withContext(Dispatcher...) { } \| runInterruptible { }	block result	✗ No	✓ Yes	✗ No	—	✗ No	—	✓ Yes
coroutineScope { }	block result	✗ No	✓ Yes	✓ Yes	✗ No	✓ Yes	✗ No	✓ Yes
supervisorScope { }	block result	✗ No	✓ Yes	✓ Yes SupervisorJob	✗ No	✓ Yes	✗ No CEH works for children	✓ Yes
GlobalScope. ... { } (e.g., GlobalScope.launch { })	Job/Deffered	✗ No	✗ No	✓ Yes	✓ Yes	✓ Yes	✓ Yes	✗ No
launch { } / async { }	Job/Deffered	✗ No	✗ No	✓ Yes	✗ No	✓ Yes	✗ No	✗ No
launch(Job()) { } / async(Job()) { } ⚠️ anti-pattern breaks structured concurrency	Job/Deffered	✗ No	✗ No	✓ Yes	✓ Yes	✓ Yes	✓ Yes passed Job() becomes root CEH works for launch, NOT for async	✗ No

🧩 Scope constructor

Expression	Returns	Blocks thread	Waits completion	New Job	Root Job	New coroutine	Root coroutine	Suspend point
val my = CoroutineScope() \| CoroutineScope(Job()) \| CoroutineScope(SupervisorJob())	itself	✗ No	✗ No	✓ Yes	✓ Yes gets/creates root	✗ No	—	✗ No

Coroutine is a code block that:

has exactly one associated Job object
belongs to a single coroutineScope
executes, suspends, cancels and handles exceptions according to structured concurrency rules.

CEH (CoroutineExceptionHandler) — is a last hope mechanism. CEH works only for root coroutine, because unhandled exception of not-root coroutines spreads up the hierarcy in structured concurrency and CEH is ignored. Exceptions to this rule are marked in the table above.

SupervisorJob defines a rule for child coroutines: failure of any one of them is isolated – it does not cancel the others in the same scope (including the parent). So, its exception is only its problem, that's why CEH works for them.