Example of using coroutines with Spring MVC

This is an example of using coroutines with Spring MVC (not webflux!)

DemoController uses https://github.com/joost-de-vries/spring-coroutine, but since that library was published to bintray, I copied the class SpringCoroutineScope and dependencies here.

The /boomerang/{x} endpoint waits for x seconds and then returns x back. If x is 0 then it throws an exception.

Looking at the implementation I suspected that, by reusing the same scope instance in a Spring bean, if one of the requests fails it would cancel the parent job and other parallel or subsequent requests would also fail.

This can be reproduced by changing back:

@Suppress("FunctionName")
fun SpringScope(dispatcher: CoroutineDispatcher = Dispatchers.Default, job: Job = SupervisorJob()): SpringScope =
    SpringScope(dispatcher + job)

to

@Suppress("FunctionName")
fun SpringScope(dispatcher: CoroutineDispatcher = Dispatchers.Default, job: Job = Job()): SpringScope =
    SpringScope(dispatcher + job)

Obviously, using a SupervisorJob fixes this.

Here is a use case that reproduces the aforementioned problem:

#running the app:
./gradlew clean run

#let's do some requests
curl http://localhost:8080/boomerang/1

curl http://localhost:8080/boomerang/10

#if we call it in less than 10 seconds, the previous request will also fail
curl http://localhost:8080/boomerang/0
#this also fails
curl http://localhost:8080/boomerang/10