Given the following snippet, i do not understand why my android app crashes. I tested in a standalone kotlin app but this does not happen.
class LoginActivity : AppCompatActivity(), CoroutineScope
{
lateinit var job: Job
override val coroutineContext: CoroutineContext
get() = Dispatchers.Main + job
override fun onCreate(savedInstanceState: Bundle?)
{
super.onCreate(savedInstanceState)
job = Job()
try
{
launch()
{
try
{
var res = async { test() }
res.await()
}
catch (e2: java.lang.Exception)
{
}
}
}
catch (e: java.lang.Exception)
{
}
}
fun test(): String
{
throw java.lang.Exception("test ex")
return "";
}
}
--------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
Process: ro.ingr.ingeeasafety, PID: 11298
java.lang.Exception: test ex
at ro.ingr.ingeeasafety.activities.LoginActivity.test(LoginActivity.kt:72)
at ro.ingr.ingeeasafety.activities.LoginActivity$onCreate$1$res$1.invokeSuspend(LoginActivity.kt:48)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:32)
at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:236)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Standalone kotlin app code, execution reaches "main end" println
class app
{
companion object :CoroutineScope
{
lateinit var job: Job
override val coroutineContext: CoroutineContext
get() = Dispatchers.Default+ job
init
{
job=Job()
}
@JvmStatic
fun main(args: Array<String>)
{
launch()
{
try
{
async()
{
println("async start")
throw Exception("aaa")
}.await()
}
catch (e: Exception)
{
println("async exception")
}
}
println("main end")
}
}
}
I am trying to create a flow where i load something from somewhere and if the load operation fails my app does not crash. I was expecting that the exception got caught in the handlers defined.
LE: I added the crash stack trace.
You can find your answer here: https://proandroiddev.com/kotlin-coroutines-patterns-anti-patterns-f9d12984c68e
To summarise, there are a few ways to catch exceptions with async
.
async
call with supervisorScope
launch {
supervisorScope {
val task = async {
methodThatThrowsException()
}
try {
updateUI("Ok ${task.await()}")
} catch (e: Throwable) {
showError("Erro! ${e.message}")
}
}
}
SupervisorJob
as paramlaunch {
// parentJob (optional) is the parent Job of the CoroutineContext
val task = async(SupervisorJob(parentJob)) {
methodThatThrowsException()
}
try {
updateUI("Ok ${task.await()}")
} catch (e: Throwable) {
showError("Erro! ${e.message}")
}
}
async
with coroutineScope
launch {
try {
coroutineScope {
val task = async {
methodThatThrowsException()
}
updateUI("Ok ${task.await()}")
}
} catch (e: Throwable) {
showError("Erro! ${e.message}")
}
}
In your second example, if you add Thread.sleep(1000)
after your println("main end")
statement you will see an exception as well. Without the sleep
the application ends before the exception is thrown:
Exception in thread "DefaultDispatcher-worker-3" java.lang.Exception: aaa
at de.e2.app$Companion$main$job$1$1.invokeSuspend(AsyncProblem2.kt:26)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:32)
at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:236)
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:594)
at kotlinx.coroutines.scheduling.CoroutineScheduler.access$runSafely(CoroutineScheduler.kt:60)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:742)
In both cases you run into a standard behavior of structured concurrency introduced with Kotlin 1.3 (see https://medium.com/@elizarov/structured-concurrency-722d765aa952).
If in an async
block an exception is thrown, the own coroutine is cancelled and the parent coroutines as well: see Kotlin coroutine can't handle exception
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