Can Project Loom be used from Kotlin?

Question

I am just getting started at playing with Project Loom...

Given the Java code which seems to work correctly

try (ExecutorService executor = Executors.newVirtualThreadExecutor()) {
    IntStream.range(0, 16).forEach(i -> {
        System.out.println("i = " + i + ", Thread ID = " + Thread.currentThread());
        executor.submit(() -> {
            var thread = Thread.currentThread();
            System.out.println("Thread ID = " + thread);
        });
    });
}

When IntelliJ converts this to Kotlin I get

Executors.newVirtualThreadExecutor().use { executor ->
    IntStream.range(0, 16).forEach(IntConsumer { i: Int ->
        println("i = $i, Thread ID = ${Thread.currentThread()}")
        executor.submit(Runnable {
            println("Thread ID = ${Thread.currentThread()}")
        })
    })
}

private fun ExecutorService.use(block: (ExecutorService) -> Unit) {}

which seems to compile fine, but when executed, nothing is printed on the console?

Is there some latent incompatibility between Kotlin and Project Loom?

Further experiments show that

Executors.newVirtualThreadExecutor()
    .submit(Runnable { println("Thread = ${Thread.currentThread()}") })

does not print anything, but

Executors.newCachedThreadPool()
    .submit(Runnable { println("Thread = ${Thread.currentThread()}") })

does print the expected result, so there is something fundamentally incompatible between Kotlin and Virtual Threads. However,

Executors.newCachedThreadPool().use { executor ->
    IntStream.range(0, 16).forEach(IntConsumer { i: Int ->
        println("i = $i, Thread ID = ${Thread.currentThread()}")
        executor.submit(Runnable {
            println("Thread ID = ${Thread.currentThread()}")
        })
    })
}

does not print anything, so there are other issues at play too... Indeed

Executors.newCachedThreadPool().use { executor ->
    IntStream.range(0, 16).forEach(IntConsumer { i: Int ->
        println("i = $i, Thread ID = ${Thread.currentThread()}")
    })
}

does not print anything, but

IntStream.range(0, 16).forEach(IntConsumer { i: Int ->
    println("i = $i, Thread ID = ${Thread.currentThread()}")
})

does print the expected results? Also,

IntStream.range(0, 16).forEach(IntConsumer { i: Int ->
    println("i = $i, Thread ID = ${Thread.currentThread()}")
    executor.submit(Runnable {
        println("Thread ID = ${Thread.currentThread()}")
    })
})

prints the expected results, so there is some weirdness with use?

This begs the questions

1. Are there some problems with how Project Loom is designed/implemented that is causing problems for Kotlin?
   • Are there some problems with JDK-18 that is not compatible with Kotlin?
2. Are there some problems with how Kotlin is designed/implemented that cannot interoperate with Project Loom?
   • Are there some problems with Kotlin, that are not compatible with JDK-18?

Answer 1

The problem was I was not defining the use extension function properly. The simple fix is

private fun ExecutorService.use(block: (executorService: ExecutorService) -> Unit) = block(this)

So, now

Executors.newVirtualThreadExecutor().use { executorService ->
    IntStream.range(0, 16).forEach(IntConsumer { i: Int ->
        println("i = $i, Thread ID = ${Thread.currentThread()}")
        executorService.submit(Runnable {
            println("Thread ID = ${Thread.currentThread()}")
        })
    })
}

produces output similar to the Java example

i = 1, Thread ID = Thread[#1,main,5,main]
i = 2, Thread ID = Thread[#1,main,5,main]
Thread ID = VirtualThread[#19]/runnable@ForkJoinPool-1-worker-1
Thread ID = VirtualThread[#20]/runnable@ForkJoinPool-1-worker-2
i = 3, Thread ID = Thread[#1,main,5,main]

I often find it frustrating when IntelliJ automatically creates extension functions because it's not immediately obvious to me what to put in the body, and in this case I did nothing, which was wrong. In spite of much Googling around, I could not easily find code that let me to the solutions, so it was kinda hit-and-miss...

Conclusion, it should be possible to use Project Loom from Kotlin, once you know some special black magic...