Both Kotlin and Java are languages that are heavily used in Distributed Systems, handling multiple asynchronous operations simultanouesely. Blocking is not an option in such an environment, but let’s be honest, we crave for easier synchronous world. That’s why JVM as a platform have inherent focus on providing easier option of multithreaded programming. Today, I’d like to compare two of such options - Kotlin Coroutines and Project Loom, that want to provide fibers and continuations to core Java.

1. TO BE CONTINUED
Multithreading with Project
Loom and Kotlin's Coroutines
ARTUR SKOWROŃSKI
VIRTUSLAB

2. WON’T TEACH YOU
COROUTINES.

4. SYNCHRONOUS
fun main(args : Array<String>) {
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
fun getLocationNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK”).first()
}
fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours
}
fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

5. SYNCHRONOUS
fun main(args : Array<String>) {
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
fun getLocationNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK”).first()
}
fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours
}
fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

6. SYNCHRONOUS
fun main(args : Array<String>) {
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
fun getLocationNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK”).first()
}
fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours
}
fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

7. SYNCHRONOUS
fun main(args : Array<String>) {
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
fun getLocationNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK”).first()
}
fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours
}
fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

8. SYNCHRONOUS
fun main(args : Array<String>) {
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
fun getLocationNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK”).first()
}
fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours
}
fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

9. CALLBACKS
fun main(args: Array<String>) {
getLocationsNearCoordinates("SW8 5YY, UK")
.success {location ->
val category = location.classification.category
if (category == "Store") {
getStoreDetails(location.id)
.success { locationWrapper ->
val openingHours = locationWrapper
.openingHours
.first()
.standardOpeningHours
println("Store Opening Hours: $openingHours")
}
}
if (category == "Office") {
payOfficeRent(location.id)
.success {
println("Invoice paid")
}
}
}
}

10. CALLBACKS
fun main(args: Array<String>) {
getLocationsNearCoordinates("SW8 5YY, UK")
.success {location ->
val category = location.classification.category
if (category == "Store") {
getStoreDetails(location.id)
.success { locationWrapper ->
val openingHours = locationWrapper
.openingHours
.first()
.standardOpeningHours
println("Store Opening Hours: $openingHours")
}
}
if (category == "Office") {
payOfficeRent(location.id)
.success {
println("Invoice paid")
}
}
}
}

11. CALLBACKS
fun main(args: Array<String>) {
getLocationsNearCoordinates("SW8 5YY, UK")
.success {location ->
val category = location.classification.category
if (category == "Store") {
getStoreDetails(location.id)
.success { locationWrapper ->
val openingHours = locationWrapper
.openingHours
.first()
.standardOpeningHours
println("Store Opening Hours: $openingHours")
}
}
if (category == "Office") {
payOfficeRent(location.id)
.success {
println("Invoice paid")
}
}
}
}

12. CALLBACKS
fun main(args: Array<String>) {
getLocationsNearCoordinates("SW8 5YY, UK")
.success {location ->
val category = location.classification.category
if (category == "Store") {
getStoreDetails(location.id)
.success { locationWrapper ->
val openingHours = locationWrapper
.openingHours
.first()
.standardOpeningHours
println("Store Opening Hours: $openingHours")
}
}
if (category == "Office") {
payOfficeRent(location.id)
.success {
println("Invoice paid")
}
}
}
}

13. CALLBACKS
fun main(args: Array<String>) {
getLocationsNearCoordinates("SW8 5YY, UK")
.success {location ->
val category = location.classification.category
if (category == "Store") {
getStoreDetails(location.id)
.success { locationWrapper ->
val openingHours = locationWrapper
.openingHours
.first()
.standardOpeningHours
println("Store Opening Hours: $openingHours")
}
}
if (category == "Office") {
payOfficeRent(location.id)
.success {
println("Invoice paid")
}
}
}
}

14. CALLBACKS
fun main(args: Array<String>) {
getLocationsNearCoordinates("SW8 5YY, UK")
.success {location ->
val category = location.classification.category
if (category == "Store") {
getStoreDetails(location.id)
.success { locationWrapper ->
val openingHours = locationWrapper
.openingHours
.first()
.standardOpeningHours
println("Store Opening Hours: $openingHours")
}
}
if (category == "Office") {
payOfficeRent(location.id)
.success {
println("Invoice paid")
}
}
}
}

15. PROMISESRX

16. BRANCHING IS HARD
fun combineAsync(name1: String, name2: String): CompletableFuture<Image>{
val future1 = CompletableFuture.supplyAsync(name1-> ...)
val future2 = CompletableFuture.supplyAsync(name2-> ...)
return future1.thenCombine(future2){i1,i2 -> ...}
}

17. EXCEPTION HANDLING
IS HARD

18. OVERALL IT’S HARD…

19. IMPERATIVE
fun main(args : Array<String>) {
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
fun getLocationNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK”).first()
}
fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours
}
fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

20. COROUTINES
Współprogram

21. 1958

22. 1963

23. NATIVE SUPPORT

24. NATIVE?

25. IMPERATIVE
fun main(args : Array<String>) {
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
fun getLocationNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK”).first()
}
fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours
}
fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

26. COROUTINE
fun main() {
GlobalScope.launch {
val location = getLocationsNearKnownPostcode().first()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
runBlocking {
delay(2000L)
}
}
suspend fun getLocationsNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK")
}
suspend fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours.first().standardOpeningHours
}
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

27. COROUTINE
fun main() {
GlobalScope.launch {
val location = getLocationsNearKnownPostcode().first()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
runBlocking {
delay(2000L)
}
}
suspend fun getLocationsNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK")
}
suspend fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours.first().standardOpeningHours
}
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

28. COROUTINE
fun main() {
GlobalScope.launch {
val location = getLocationsNearKnownPostcode().first()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
runBlocking {
delay(2000L)
}
}
suspend fun getLocationsNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK")
}
suspend fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours.first().standardOpeningHours
}
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

29. COROUTINE
fun main() {
GlobalScope.launch {
val location = getLocationsNearKnownPostcode().first()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
runBlocking {
delay(2000L)
}
}
suspend fun getLocationsNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK")
}
suspend fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours.first().standardOpeningHours
}
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

30. COROUTINE
fun main() {
GlobalScope.launch {
val location = getLocationsNearKnownPostcode().first()
val category = location.classification.category
if (category == "Store") {
val openingHours = getOpeningHours(location.id)
println("Store Opening Hours: $openingHours")
}
if (category == "Office") {
payInvoice(location.id)
println("Invoice paid")
}
}
runBlocking {
delay(2000L)
}
}
suspend fun getLocationsNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK")
}
suspend fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours.first().standardOpeningHours
}
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

31. THREAD
BLOCKED
Function 1
Function 2

32. SUSPEND
Function 1
Function 2
THREAD
Suspension Point

33. SUSPEND
Coroutine 1
Function 2
getLocationsNearKnownPostcode getOpeningHours
Function 3
THREAD

34. CONTINUATIONS

35. DIRECT STYLE CODE
Action + Result and Continuation
val location = getLocationsNearKnownPostcode()
val category = location.classification.category
ActionResult
Continuation

36. CONTINUATION PASSING STYLE
getLocationsNearKnownPostcode()
.success {location ->
val category = location.classification.category
}
Action
Result

37. CONTINUATION PASSING STYLE
getLocationsNearKnownPostcode()
.success {location ->
val category = location.classification.category
}
Action
Result
Fancy Name for Callback
Continuation

38. FROM KOTLIN FILE
suspend fun getLocationsNearKnownPostcode(): List<LocationWrapper> {
return getLocationsNearCoordinates("SW8 5YY, UK")
}
suspend fun getOpeningHours(id: String): Any {
return getStoreDetails(id).openingHours.first().standardOpeningHours
}
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

39. public final static getLocationsNearKnownPostcode(Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
@Lorg/jetbrains/annotations/Nullable;() // invisible
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
L0
LINENUMBER 33 L0
LDC "SW8 5YY, UK"
INVOKESTATIC TescoLocationAPIKt.getLocationsNearCoordinates (Ljava/lang/String;)Ljava/util/List;
ARETURN
L1
MAXSTACK = 1
MAXLOCALS = 1
// access flags 0x19
// signature (Ljava/lang/String;Lkotlin/coroutines/Continuation<Ljava/lang/Object;>;)Ljava/lang/Object;
// declaration: getOpeningHours(java.lang.String, kotlin.coroutines.Continuation<java.lang.Object>)
public final static getOpeningHours(Ljava/lang/String;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
@Lorg/jetbrains/annotations/Nullable;() // invisible
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 1
L0
LINENUMBER 37 L0
ALOAD 0
INVOKESTATIC TescoLocationAPIKt.getStoreDetails (Ljava/lang/String;)LLocationWrapper;
INVOKEVIRTUAL LocationWrapper.getOpeningHours ()[LLocationOpeningHours;
INVOKESTATIC kotlin/collections/ArraysKt.first ([Ljava/lang/Object;)Ljava/lang/Object;
CHECKCAST LocationOpeningHours
INVOKEVIRTUAL LocationOpeningHours.getStandardOpeningHours ()Ljava/util/HashMap;
ARETURN
L1
LOCALVARIABLE id Ljava/lang/String; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 2
// access flags 0x19
// signature (Ljava/lang/String;Lkotlin/coroutines/Continuation<Ljava/lang/Object;>;)Ljava/lang/Object;
// declaration: payInvoice(java.lang.String, kotlin.coroutines.Continuation<java.lang.Object>)
public final static payInvoice(Ljava/lang/String;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
@Lorg/jetbrains/annotations/Nullable;() // invisible
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 1
L0
LINENUMBER 41 L0
ALOAD 0
INVOKESTATIC TescoLocationAPIKt.payOfficeRent (Ljava/lang/String;)LLocationResult;
ARETURN
L1
LOCALVARIABLE id Ljava/lang/String; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 2
JVM BYTECODE

40. TAKE A LOOK
public final static payInvoice(Ljava/lang/String;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
@Lorg/jetbrains/annotations/Nullable;() // invisible
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 1
L0
LINENUMBER 41 L0
ALOAD 0
INVOKESTATIC TescoLocationAPIKt.payOfficeRent (Ljava/lang/String;)LLocationResult;
ARETURN
L1
LOCALVARIABLE id Ljava/lang/String; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 2

41. TAKE A LOOK
public final static payInvoice(Ljava/lang/String;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
@Lorg/jetbrains/annotations/Nullable;() // invisible
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 1
L0
LINENUMBER 41 L0
ALOAD 0
INVOKESTATIC TescoLocationAPIKt.payOfficeRent (Ljava/lang/String;)LLocationResult;
ARETURN
L1
LOCALVARIABLE id Ljava/lang/String; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 2
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

42. TAKE A LOOK
public final static payInvoice(Ljava/lang/String;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
@Lorg/jetbrains/annotations/Nullable;() // invisible
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 1
L0
LINENUMBER 41 L0
ALOAD 0
INVOKESTATIC TescoLocationAPIKt.payOfficeRent (Ljava/lang/String;)LLocationResult;
ARETURN
L1
LOCALVARIABLE id Ljava/lang/String; L0 L1 0
MAXSTACK = 1
MAXLOCALS = 2
suspend fun payInvoice(id: String): Any {
return payOfficeRent(id)
}

43. public interface Continuation<in T> {
public val context: CoroutineContext
public fun resumeWith(result: Result<T>)
}

44. TAKE A LOOK
public final static main()V
L0
LINENUMBER 13 L0
GETSTATIC kotlinx/coroutines/GlobalScope.INSTANCE : Lkotlinx/coroutines/GlobalScope;
CHECKCAST kotlinx/coroutines/CoroutineScope
ACONST_NULL
ACONST_NULL
NEW kotlinx/coroutines/guide/compose01/CoroutineCodeKt$main$1
DUP
ACONST_NULL
INVOKESPECIAL kotlinx/coroutines/guide/compose01/CoroutineCodeKt$main$1.<init> (Lkotlin/coroutines/
Continuation;)V
CHECKCAST kotlin/jvm/functions/Function2
ICONST_3
ACONST_NULL
INVOKESTATIC kotlinx/coroutines/BuildersKt.launch$default (Lkotlinx/coroutines/
CoroutineScope;Lkotlin/coroutines/CoroutineContext;Lkotlinx/coroutines/CoroutineStart;Lkotlin/jvm/
functions/Function2;ILjava/lang/Object;)Lkotlinx/coroutines/Job;
POP
L1

45. COROUTINE
Coroutine
Stack
Operation 1
Operation 2
Operation 3
SUSPEND FUNCTION
Operation 1
Operation 2
Operation 3
Operation 4
Operation 5
SUSPEND FUNCTION
Operation 4
Operation 5
State
Operation 1
Operation 2
Operation 3

46. PROJECT LOOM

47. KERNEL THREADS
PROCESSESOS
PROCESS
PROCESS
THREAD
THREAD
THREAD
THREAD
KERNEL SPACE THREADS

48. THREADS
CPU SHEDULER
THREAD
Instruction 1
Instruction 2
Instruction 3
Instruction 4
THREAD
Continuation

49. OS
CPU SHEDULER
THREAD
Instruction 1
Instruction 2
Instruction 3
Instruction 4
THREAD

50. USER/KERNEL SPACE BORDER
CPU SHEDULER JVM
CONTEXT SWITCH

51. PROJECT METROPOLIS
PROJECT VALHALLA
PROJECT AMBER
PROJECT PANAMA

52. PROJECT LOOM

53. THREAD
USER/KERNEL SPACE BORDER
CPU SCHEDULER JVM
FORKJOINSCHEDULER
THREAD
THREAD
THREAD

54. THREAD
USER/KERNEL SPACE BORDER
CPU SCHEDULER JVM
FORKJOINSCHEDULER
THREAD
Instruction 1
Instruction 2
Instruction 3
Instruction 4
Coroutines

55. public class Continuation implements Runnable {
public Continuation(ContinuationScope s, Runnable r)
public final void run()
public static void yield(ContinuationScope s)
public boolean isDone()
}

56. Thread Fiber
Strand
FIBER
Scheduler + Continuation

57. var scope = new ContinuationScope("scheduler");
private final Fiber<Void> execution;
execution.apply(() -> {
var location = getLocationsNearKnownPostcode.yield(scope);
var category = location.classification.category
if (category == "Store") {
var openingHours = getOpeningHours(location.id).yield(scope)
System.out.println("Store Opening Hours:"+ openingHours)
}
if (category == "Office")
payInvoice(location.id).yield()
println("Invoice paid")
}
});

58. NO MORE THREAD POOLS
Thread: ~1MB
Fiber: ~1KB

59. HTTP://DOCS.PARALLELUNIVERSE.CO/QUASAR/

60. LACK OF COMMON
ABSTRACTION

61. RESTRICTIONS
Lack of support for JNI
Cannot suspend under lock
Partial (emulated) support for
ThreadLocal

62. hg clone http://hg.openjdk.java.net/loom/loom
cd loom
hg update -r fibers
sh configure
make images
JAVA 11

63. DONE WHEN IT’S DONE

64. SOURCES
http://cr.openjdk.java.net/~rpressler/loom/Loom-
Proposal.html
https://www.youtube.com/watch?v=YrrUCSi72E8
KOTLINCONF 2017 - DEEP DIVE INTO COROUTINES
ON JVM BY ROMAN ELIZAROV
https://www.youtube.com/watch?v=vbGbXUjlRyQ
PROJECT LOOM: FIBERS AND CONTINUATIONS FOR
JAVA BY ALAN BATEMAN
PROJECT LOOM: FIBERS AND CONTINUATIONS FOR
THE JAVA VIRTUAL MACHINE

65. THANK YOU