Project Loom is a huge step forward in the way the JVM implements concurrency. It introduces a concurrency model that is straight forward to use without sacrificing scalability. It gets a lot of traction since it was released in preview mode in Java 19, however, it is not a silver bullet that will magically resolve everything. In this talk we will go through the essentials of the project and help understand when and where applying it will make your life as a Java developer significantly better.

2. Loom
Haim Yadid @ Next Insurance

3. Loom me up Scotty!
Haim Yadid @ Next Insurance

4. Loom me up Scotty!
Project Loom - What's in it for Me?
Haim Yadid @ Next Insurance

5. Loom me up Scotty!
Project Loom - What's in it for Me?
Haim Yadid @ Next Insurance
Dall E - Star Trek spaceship Enterprise flying inside a loom, Van gogh style

10. @lifeyx

11. Pro t L o es . 2017
JE -425
JE -428
JE -429

13. CONCURRENCY

14. request (task)

15. getBid(user)

16. getBid(user)
Start End
Duration

17. Running Blocked

18. getBid(user)
resolve user country Query bidding service 1
for user country
Query bidding service 2
for user country

19. Example - Single Threaded
val country = userClient.
findUserByEmail(userEmail).country
t1
https://github.com/lifey/loom-playground

20. Example - Single Threaded
val country = userClient.
findUserByEmail(userEmail).country
val resultA = serverA.getBid(country)
val resultB = serverB.getBid(country)
t1

21. Example - Single Threaded
val country = userClient.
findUserByEmail(userEmail).country
val resultA = serverA.getBid(country)
val resultB = serverB.getBid(country)
val lowestBid = if (resultA.price < resultB.price)
resultA
else
resultB
t1

22. getBid
resolve user country
Query bidding service 1
for user country
Query bidding service 2
for user country fan out is 3

23. getBid(John)
getBid(Robb)

24. CONCURRENCY
CONCURRENCY
CONCURRENCY CONCURRENCY
CONCURRENCY

25. threads
Immutability
Deadlocks
synchronization
Thread pools
race conditions
contention
shared mutable state
message passing
channels
Cache Coherence
callbacks
semaphores

26. Concurrency
is
Hard

27. Java and Concurrency / Parallelism
Java 1.0-1.1
(1995)
Threads
wait /notify
synchronized

28. Java and Concurrency / Parallelism
Java 1.0-1.1
(1995)
Threads
wait /notify
synchronized
Java 1.4
(2002)
Non blocking IO
SocketChannel
Selector

29. Java and Concurrency / Parallelism
Java 1.0-1.1
(1995)
Threads
wait /notify
synchronized
Java 1.4
(2002)
Non blocking IO
SocketChannel
Selector
Java 1.5
(2004)
j.u.c
Executors
Future
Reentrant Locks
Semaphores
Latches

30. Java and Concurrency / Parallelism
Java 1.0-1.1
(1995)
Threads
wait /notify
synchronized
Java 1.4
(2002)
Non blocking IO
SocketChannel
Selector
Java 1.5
(2004)
j.u.c
Executors
Future
Reentrant Locks
Semaphores
Latches
Java 1.7
(2011)
ForkJoin Pool
Java 1.8
(2014)
ParallelStreams
CompletableFuture

31. Java and Concurrency / Parallelism
Java 1.0-1.1
(1995)
Threads
wait /notify
synchronized
Java 1.4
(2002)
Non blocking IO
SocketChannel
Selector
Java 1.5
(2004)
j.u.c
Executors
Future
Reentrant Locks
Semaphores
Latches
Java 1.7
(2011)
ForkJoin Pool
Java 1.8
(2014)
ParallelStreams
CompletableFuture
Java 19
Loom
(2022)
VirtualThreads
Structured Concurrency

32. traditional Java Concurrency Model
● Thread per request (task) style
● Java thread is a wrapper of OS thread
● 1 or more thread to serve the request using Futures
● Executors and thread pools to control concurrency level

33. getBid
resolve user country
Query bidding service 1
for user country
Query bidding service 2
for user country fan out is 3

34. Example (Kotlin)
val userCountry =
userClient.findUserByEmail(userEmail).country
t1

35. Example (Kotlin)
val userCountry =
userClient.findUserByEmail(userEmail).country
executor.invokeAny (listOf(
))
t1

36. Example (Kotlin)
val userCountry =
userClient.findUserByEmail(userEmail).country
executor.invokeAny (listOf(
Callable { clientA.getBid(userCountry) } ,
Callable { clientB.getBid(userCountry) }
))
t1
t2
t3

37. Example (Kotlin)
val userCountry =
userClient.findUserByEmail(userEmail).country
val price = executor.invokeAny (listOf(
Callable { clientA.getBid(userCountry) } ,
Callable { clientB.getBid(userCountry) }
))
.price
t1
t2
t3

38. SCALABILITY &
HARDWARE UTILIZATION

39. So How much concurrency ? Little’s Law
● λ - throughput (arrival rate)
● L - average concurrency
● W- average latency /duration
L= λ * W

40. E= M * C2

41. getBid(user)
resolve user country Query bidding service 1
for user country
Query bidding service 2
for user country

42. 50% 50%
32 CPU cores
64 (32*2) concurrent tasks

43. 5
%
99%
32 CPU cores
3200 (32*100) concurrent tasks

44. 5
%
99.9 %
32 CPU cores
32000 (32*1000) concurrent tasks

45. getBid
resolve user country
Query bidding service 1
for user country
Query bidding service 2
for user country fan out is 3

46. 5
%
99.9 %
32 CPU cores
Fan out of 5
150K (32*1000*5) concurrent tasks

47. C1M Problem
What is needed for a server to hold
1M concurrent connections

48. So what is the Problem With OS Threads
● stack: ~1 MB
● scheduling >1µs
● launch a new thread ~0.1ms
● context switch involves kernel meaning returning from future
~5µs
● Usually Linux OS can support thousands of threads

49. Asynchronous Programming Style,
Callback / Non Blocking / Async Model
Event Loop

50. Event Loop
Asynchronous Programming Style,
Callback / Non Blocking / Async Model
#
of
cores

51. Async
● Less readable (for most developers )
● Callback hell anyone ?
a diagram from Ron Pressler’s presentation
CompletableFuture.supplyAsync(info::getUrl, pool)
.thenCompose(url -> getBodyAsync(url,
HttpResponse.BodyHandlers.ofString()))
.thenApply(info::findImage)
.thenCompose(url -> getBodyAsync(url,
HttpResponse.BodyHandlers.ofByteArray()))
.thenApply(info::setImageData)
.thenAccept(this::process)
.exceptionally(t -> { t.printStackTrace(); return null; });

52. Async is Tooling - Nightmare!
● Where is my call stack ?
● debug
● profile
● troubleshoot
● understanding number of actively
running task

53. ●

54. a slide from Ron Pressler’s presentation

55. Project Loom is
about
Developer Productivity (& Sanity)

56. VIRTUAL THREADS
JEP-425 (preview)

57. Virtual Thread is:
A unit of execution (a task) which can run on
on carrier Platform(OS) thread

58. Hello World
Thread.ofVirtual()
.name("virtual-", 1)

59. Hello World
Thread.ofVirtual()
.name("virtual-", 1)
.start {
}.join()
Runnable

60. Hello World
Thread.ofVirtual()
.name("virtual-", 1)
.start {
Thread.sleep(2000)
println("Hello World:"+ Thread.currentThread())
}.join()
Output :
Hello World: VirtualThread[#23,virtual-1]/runnable@ForkJoinPool-1-worker-1

61. Example: Executor
val executor = Executors.
newVirtualThreadPerTaskExecutor()

62. Example: Executor
val executor = Executors.
newVirtualThreadPerTaskExecutor()
executor.use {
}

63. Example: Executor
val executor = Executors.
newVirtualThreadPerTaskExecutor()
executor.use {
val future = executor.submit {
}
}
Callable

64. Example: Executor
val executor = Executors.
newVirtualThreadPerTaskExecutor()
executor.use {
val future = executor.submit {
Thread.sleep(2000)
println("Hello World: " + Thread.currentThread())
}
}

65. Virtual thread

66. Scheduler
Scheduler
Virtual thread

67. Common Pool
Scheduler
Virtual thread
Platform Threads

68. Mounting
Scheduler
Virtual thread
Platform Threads
Continuation.run

69. Yield
Scheduler
Virtual thread
Platform Threads
Continuation.yield

70. Yield -> Unmounting
Scheduler
Virtual thread
Platform Threads

71. Virtual Threads stack frames
● Mounted -> On the carrier thread stack
● UnMounted -> in the Heap
○ pay as you go stack can be very small

72. Loom Is
JVM
Copying Stack frames
Changes in GC root scanning
JRE
Continuation
Changes in all IO APIs

73. Thread Locals
Context (e.g.MDC)
TransactionID
Shared Heavy Data Structures
❌
✅
Non thread safe Objects (XML Parser)
❌
Inheritable

74. You can prevent this from happening by setting
inheritInheritableThreadLocals(false)

75. STRUCTURED CONCURRENCY
JEP-428 (incubating)

76. Structured Concurrency
● Makes writing correct concurrent code easier
● Cancellation, error propagation, timeouts
● Observe hierarchy of virtual threads at runtime

77. Example: Structured Concurrency
val country = userRClient.findUserByEmail(userEmail).country

78. Example: Structured Concurrency
val country = userRClient.findUserByEmail(userEmail).country
val scope = StructuredTaskScope.
ShutdownOnSuccess<BiddingResponse>()
scope.use {
}

79. Example: Structured Concurrency
val country = userRClient.findUserByEmail(userEmail).country
val scope = StructuredTaskScope.
ShutdownOnSuccess<BiddingResponse>()
scope.use {
scope.fork { serverA.getBid(country) }
scope.fork { serverB.getBid(country) }
}

80. Example: Structured Concurrency
val country = userRClient.findUserByEmail(userEmail).country
val scope = StructuredTaskScope.
ShutdownOnSuccess<BiddingResponse>()
scope.use {
scope.fork { serverA.getBid(country) }
scope.fork { serverB.getBid(country) }
val firstSuccessfulFuture = scope.join()
return firstSuccessfulFuture.result().price
}

81. Example: Structured Concurrency
val country = userRClient.findUserByEmail(userEmail).country
val scope = StructuredTaskScope.
ShutdownOnSuccess<BiddingResponse>()
scope.use {
scope.fork { serverA.getBid(country) }
scope.fork { serverB.getBid(country) }
val firstSuccessfulFuture = scope.joinUntil(
Instant.now().plusMillis(3000))
return firstSuccessfulFuture.result().price
}

82. Scoped Values
JEP-429 (candidate)

83. Tooling

84. Debugger Breakpoint

85. Debugger Stack Trace

86. Debugger Virtual Threads

87. Thread dumps
jstack -l <pid>
Shows only platform threads

88. Thread dumps
jcmd <pid> Thread.dump_to_file -format=json <File name>
Thread.ofVirtual
executor.submit
scope.fork
Platform Threads
{
"tid": "5604",
"name": "",
"stack": [
"java.base/jdk.internal.vm.Continuation.yield(Continuation.java:357)",
"java.base/java.lang.VirtualThread.yieldContinuation(VirtualThread.java:370)",
"java.base/java.lang.VirtualThread.parkNanos(VirtualThread.java:532)",
"java.base/java.lang.VirtualThread.doSleepNanos(VirtualThread.java:713)",
"java.base/java.lang.VirtualThread.sleepNanos(VirtualThread.java:686)",
"java.base/java.lang.Thread.sleep(Thread.java:451)",
"playground.ExecutorTest$vthreadExecutorTest$1$1$1.invoke(ExecutorTest.kt:30)",
"playground.ExecutorTest$vthreadExecutorTest$1$1$1.invoke(ExecutorTest.kt:27)",
"playground.ExecutorTest.vthreadExecutorTest$lambda-1$lambda-0(ExecutorTest.kt:27)",
"java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:577)",
"java.base/java.util.concurrent.ThreadPerTaskExecutor$ThreadBoundFuture.run(ThreadPerTaskExecutor.ja
va:352)",
"java.base/java.lang.VirtualThread.run(VirtualThread.java:287)",
"java.base/java.lang.VirtualThread$VThreadContinuation.lambda$new$0(VirtualThread.java:174)",
"java.base/jdk.internal.vm.Continuation.enter0(Continuation.java:327)",
"java.base/jdk.internal.vm.Continuation.enter(Continuation.java:320)"
]
},
❌
✅
✅
✅

89. Java Flight Recorder (JFR)
● jdk.VirtualThreadStart
● jdk.VirtualThreadEnd
● jdk.VirtualThreadPinned
● jdk.VirtualThreadSubmitFailed

90. Summary

91. Should I use Project Loom in
Production?

92. Should I use Project Loom in
Production?
Of course not

93. Not Yet

94. But Maybe
● If you are starting to write a new service that is IO bound and
requires high level of concurrency (> 10K concurrent
connections)
● If you want to play around
○ https://github.com/lifey/loom-playground
○ https://github.com/cmpxchg16/kotlin-loom
● If you want to provide feedback

95. supports Virtual Threads
Since: v10.0.12
Enable by:
QueuedThreadPool.setUseVirtualThread(true)

96. https://github.com/oracle/graal/pull/4802

97. https://github.com/quarkusio/quarkus/pull/24942

98. Helidon Nima
With “dumb”
synchronous code on
Loom and some
simple tuning, quickly
got similar or better
performance to Netty
— Tomas Langer
(Helidon)

99. --enable-preview
JDK 19

100. --enable-preview
--add-modules jdk.incubator.concurrent
JDK 19

101. Pinned Threads
● When a virtual thread is in a synchronized block
(or in JNI call)
● It cannot be unmounted
● Even when blocked on IO
-Djdk.tracePinnedThreads=full

102. Don’t Po
Vir Th e s