Loom is more than virtual threads: Structured Concurrency and Scoped Values

Most of the talk about Project Loom as been about JEP 425: Virtual Threads. That's great because virtual threads are incredibly useful and will revolutionize how we write high-throughput applications. But did you know there are two more JEPs associated with Project Loom that are also very useful? First we'll learn about JEP 453: Structured Concurrency and how it will simplify multi-threaded programming on the JVM. Next, we'll learn about JEP 487: Scoped Values, which aims to solve some problems that ThreadLocals have had since their introduction way back in Java 1.2! Come to this talk and we'll learn why these projects are useful additions to the JVM, and write some code to illustrate where you'll be able to use them effectively.

Transcript

1. Loom Is More Than Virtual Threads! KCDC 2025 2025-08-14 Todd Ginsberg Structured Concurrency and Scoped Values @ToddGinsberg Lead Engineer - Payments Deutsche Bank
2. Gold Sponsors Titanium Sponsors Other Awesome Organizations Platinum Sponsors
3. Hello! Todd Ginsberg Raleigh, NC TriJUG organizer 30 years of professional experience Currently: Director, Lead Engineer - Payments Deutsche Bank Cary, NC Photo Credit: Andrew Byala @todd.ginsberg.com
4. @todd.ginsberg.com Agenda Project Loom Overview Virtual Threads Review Structured Concurrency Scoped Values Q & A
5. Project Loom
6. @todd.ginsberg.com Project Loom is to intended to explore, incubate and deliver Java VM features and APIs built on top of them for the purpose of supporting easy-to-use, high-throughput lightweight concurrency and new programming models on the Java platform. https://wiki.openjdk.org/display/loom/Main
7. @todd.ginsberg.com Project Loom is to intended to explore, incubate and deliver Java VM features and APIs built on top of them for the purpose of supporting easy-to-use, high-throughput lightweight concurrency and new programming models on the Java platform. https://wiki.openjdk.org/display/loom/Main
8. @todd.ginsberg.com Project Loom is to intended to explore, incubate and deliver Java VM features and APIs built on top of them for the purpose of supporting easy-to-use, high-throughput lightweight concurrency and new programming models on the Java platform. https://wiki.openjdk.org/display/loom/Main
9. @todd.ginsberg.com VIRTUAL THREADS
10. @todd.ginsberg.com READS
11. @todd.ginsberg.com Structured Concurrency Scoped Values
12. @todd.ginsberg.com Structured Concurrency Scoped Values
13. @todd.ginsberg.com Structured Concurrency Scoped Values
14. @todd.ginsberg.com Structured Concurrency Scoped Values
15. @todd.ginsberg.com Structured Concurrency Scoped Values .
16. @todd.ginsberg.com VIRTUAL THREADS
17. @todd.ginsberg.com VIRTUAL THREADS
18. @todd.ginsberg.com Project Loom: Timeline 9 10 18 19 20 21 22 23 Loom 11 … 17 24 25
19. @todd.ginsberg.com Project Loom: Timeline 9 10 18 19 20 21 22 23 Loom Virtual Threads Released Preview Incubating 11 … 17 24 25 +
20. @todd.ginsberg.com Project Loom: Timeline 9 10 18 19 20 21 22 23 Loom Virtual Threads Structured Concurrency Released Preview Incubating 11 … 17 24 25 +
21. @todd.ginsberg.com Project Loom: Timeline 9 10 18 19 20 21 22 23 Loom Virtual Threads Structured Concurrency Scoped Values Released Preview Incubating 11 … 17 24 25 +
22. @todd.ginsberg.com Project Loom: Timeline 9 10 18 19 20 21 22 23 Loom Virtual Threads Structured Concurrency Scoped Values Released Preview Incubating 11 … 17 24 25 +
23. Review: Virtual Threads
24. @todd.ginsberg.com Platform Threads JVM Platform Thread Platform Thread Platform Thread Platform Thread
25. @todd.ginsberg.com Platform Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Platform Thread Platform Thread Platform Thread Platform Thread
26. @todd.ginsberg.com Platform Thread Usage Platform Thread
27. @todd.ginsberg.com Platform Thread Usage Platform Thread
28. @todd.ginsberg.com Platform Thread Usage Platform Thread
29. @todd.ginsberg.com Platform Thread Usage Platform Thread
30. @todd.ginsberg.com Platform Thread Usage Platform Thread Not Optimal
31. @todd.ginsberg.com Java Threads: The Downside Memory hungry
32. @todd.ginsberg.com Java Threads: The Downside Memory hungry Slow creation time
33. @todd.ginsberg.com Java Threads: The Downside Memory hungry Slow creation time 0% useful when blocked
34. @todd.ginsberg.com Java Threads: The Downside Memory hungry Slow creation time Overhead for context switching 0% useful when blocked
35. @todd.ginsberg.com Java Threads: The Downside Memory hungry Limited number by OS Slow creation time Overhead for context switching 0% useful when blocked
36. @todd.ginsberg.com Platform Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Platform Thread Platform Thread Platform Thread Platform Thread
37. @todd.ginsberg.com Platform Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Platform Thread Platform Thread Platform Thread Platform Thread
38. @todd.ginsberg.com Virtual Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread
39. @todd.ginsberg.com Virtual Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread
40. @todd.ginsberg.com Virtual Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread
41. @todd.ginsberg.com Virtual Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread
42. @todd.ginsberg.com Virtual Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread
43. @todd.ginsberg.com Carrier Thread Usage 1 2 1 1 Carrier Virtual 1 Virtual 2 Runnable Running Waiting 2
44. @todd.ginsberg.com Virtual Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread
45. @todd.ginsberg.com Virtual Threads Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Virtual Thread Platform Thread
46. @todd.ginsberg.com Virtual Threads as Tasks Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Task Task Task Task Task Task Task Task Task Task Task Task Task Task Platform Thread
47. @todd.ginsberg.com Virtual Threads as Tasks Operating System JVM OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread OS Thread Carrier Thread Carrier Thread Carrier Thread Carrier Thread Task Task Task Task Task Task Task Platform Thread Task Task Task Task Task Task Task Task Task Task Task Task Task Task Task Task Task Task Task Task Task
48. @todd.ginsberg.com What Virtual Threads Give Us Same API: java.lang.Thread
49. @todd.ginsberg.com What Virtual Threads Give Us Same API: java.lang.Thread Much lower memory requirements
50. @todd.ginsberg.com What Virtual Threads Give Us Same API: java.lang.Thread Much lower memory requirements Number limited by heap memory
51. @todd.ginsberg.com What Virtual Threads Give Us Same API: java.lang.Thread Much lower memory requirements Number limited by heap memory Much quicker creation time
52. @todd.ginsberg.com What Virtual Threads Give Us Same API: java.lang.Thread Much lower memory requirements Number limited by heap memory Much quicker creation time Make better use of system resources
53. Demo
54. @todd.ginsberg.com Project Loom is to intended to explore, incubate and deliver Java VM features and APIs built on top of them for the purpose of supporting easy-to-use, high-throughput lightweight concurrency and new programming models on the Java platform. https://wiki.openjdk.org/display/loom/Main
55. @todd.ginsberg.com Project Loom is to intended to explore, incubate and deliver Java VM features and APIs built on top of them for the purpose of supporting easy-to-use, high-throughput lightweight concurrency and new programming models on the Java platform. https://wiki.openjdk.org/display/loom/Main
56. @todd.ginsberg.com Better Throughput Little’s Law: concurrency = arrival rate * latency
57. @todd.ginsberg.com Better Throughput Little’s Law: concurrency = arrival rate * latency 10
58. @todd.ginsberg.com Better Throughput Little’s Law: concurrency = arrival rate * latency 10 .5
59. @todd.ginsberg.com Better Throughput Little’s Law: concurrency = arrival rate * latency 10 .5 5
60. @todd.ginsberg.com Better Throughput Little’s Law: concurrency = arrival rate * latency
61. @todd.ginsberg.com Better Throughput Little’s Law: concurrency = arrival rate * latency throughput
62. @todd.ginsberg.com Better Throughput Little’s Law: throughput = concurrency / latency
63. @todd.ginsberg.com Better Throughput Little’s Law: throughput = concurrency / latency
64. @todd.ginsberg.com Better Throughput Little’s Law: throughput = concurrency / latency
65. @todd.ginsberg.com Better Throughput Little’s Law: throughput = concurrency / latency Virtual Threads!
66. @todd.ginsberg.com Final Thoughts on Virtual Threads Do Not Pool Virtual Threads
67. @todd.ginsberg.com Final Thoughts on Virtual Threads Do Not Pool Virtual Threads
68. @todd.ginsberg.com Final Thoughts on Virtual Threads Do Not Pool Virtual Threads
69. @todd.ginsberg.com Final Thoughts on Virtual Threads Do Not Pool Virtual Threads Create
70. @todd.ginsberg.com Final Thoughts on Virtual Threads Do Not Pool Virtual Threads Create Use
71. @todd.ginsberg.com Final Thoughts on Virtual Threads Do Not Pool Virtual Threads Create Dispose Use
72. @todd.ginsberg.com Final Thoughts on Virtual Threads Do Not Pool Virtual Threads Be open to new ways of writing concurrent code
73. Structured Concurrency
74. Structured Concurrency Un-
75. Demo
76. @todd.ginsberg.com Three Problems With Unstructured Concurrency
77. @todd.ginsberg.com Three Problems With Unstructured Concurrency Relationships between task and subtask don’t really exist
78. @todd.ginsberg.com Three Problems With Unstructured Concurrency Relationships between task and subtask don’t really exist Observability is difficult
79. @todd.ginsberg.com Three Problems With Unstructured Concurrency Relationships between task and subtask don’t really exist Managing work is difficult, making the true purpose of code opaque Observability is difficult
80. Structured Concurrency
81. @todd.ginsberg.com Structured Concurrency: History • 2016 - Conceived by Martin Sústrik (golang) • 2017 - Nathaniel J. Smith “Nursery Pattern” in Python • Now shipping in Kotlin, Swift, and Java (Preview!)
82. @todd.ginsberg.com Structured Concurrency: What Is It?
83. @todd.ginsberg.com Structured Concurrency: What Is It? Subtasks have well-defined entry and exit points
84. @todd.ginsberg.com Structured Concurrency: What Is It? Subtasks have well-defined entry and exit points Strict nesting of task lifetimes, like they are presented in code
85. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario public String doSomething() { String first = firstMethod(); String second = secondMethod(); return first + second; }
86. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario public String doSomething() { String first = firstMethod(); String second = secondMethod(); return first + second; }
87. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario at Example.firstMethod(Example.java:26) at Example.doSomething(Example.java:22) at Example.main(Example.java:42)
88. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario at Example.firstMethod(Example.java:26) at Example.doSomething(Example.java:22) at Example.main(Example.java:42)
89. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario at Example.firstMethod(Example.java:26) at Example.doSomething(Example.java:22) at Example.main(Example.java:42)
90. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario public String doSomething() { String first = firstMethod(); String second = secondMethod(); return first + second; }
91. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario public String doSomething() { String first = firstMethod(); String second = secondMethod(); return first + second; }
92. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario public String doSomething() { String first = firstMethod(); String second = secondMethod(); return first + second; }
93. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario public String doSomething() { String first = firstMethod(); String second = secondMethod(); return first + second; }
94. @todd.ginsberg.com Subtask Lifetime: Ideal Scenario public String doSomething() { String first = firstMethod(); String second = secondMethod(); return first + second; }
95. @todd.ginsberg.com Java’s Structured Concurrency: Goals
96. @todd.ginsberg.com Java’s Structured Concurrency: Goals Promote a style of concurrent programming that can eliminate common risks arising from cancellation and shutdown, such as thread leaks and cancellation delays https://openjdk.org/jeps/505
97. @todd.ginsberg.com Java’s Structured Concurrency: Goals Promote a style of concurrent programming that can eliminate common risks arising from cancellation and shutdown, such as thread leaks and cancellation delays https://openjdk.org/jeps/505
98. @todd.ginsberg.com Java’s Structured Concurrency: Goals Promote a style of concurrent programming that can eliminate common risks arising from cancellation and shutdown, such as thread leaks and cancellation delays Improve the observability of concurrent code https://openjdk.org/jeps/505
99. @todd.ginsberg.com StructuredTaskScope
100. @todd.ginsberg.com StructuredTaskScope Scope
101. @todd.ginsberg.com StructuredTaskScope Scope Task Task Task Task
102. @todd.ginsberg.com StructuredTaskScope Scope Task Task Task Task join()
103. @todd.ginsberg.com StructuredTaskScope Scope Task Task Task Task join()
104. @todd.ginsberg.com StructuredTaskScope Scope Task Task Task Task join() Interpret Results
105. @todd.ginsberg.com StructuredTaskScope Scope Task Task Task Task join() Interpret Results
106. @todd.ginsberg.com StructuredTaskScope Scope Task Task Task Task join() Throw Exception?
107. @todd.ginsberg.com This Feature Is In Preview java --enable-preview
108. Demo
109. @todd.ginsberg.com Remember: Structured Concurrency
110. @todd.ginsberg.com Remember: Structured Concurrency Explicit relationship between tasks and subtasks
111. @todd.ginsberg.com Remember: Structured Concurrency Explicit relationship between tasks and subtasks Observability is easier
112. @todd.ginsberg.com Remember: Structured Concurrency Explicit relationship between tasks and subtasks Managing work is easier (or automatic) making code intent clearer Observability is easier
113. Scoped Values
114. @todd.ginsberg.com Quick Refresher: ThreadLocal
115. @todd.ginsberg.com Quick Refresher: ThreadLocal Think of it as… Map<Thread,T>
116. @todd.ginsberg.com Quick Refresher: ThreadLocal Think of it as… static Map<Thread,T>
117. @todd.ginsberg.com Quick Refresher: ThreadLocal
118. @todd.ginsberg.com Quick Refresher: ThreadLocal ThreadLocal<String> accessLevel = new ThreadLocal<>();
119. @todd.ginsberg.com Quick Refresher: ThreadLocal ThreadLocal<String> accessLevel = ... void doSomething() { }
120. @todd.ginsberg.com Quick Refresher: ThreadLocal ThreadLocal<String> accessLevel = ... void doSomething() { accessLevel.set("GUEST"); }
121. @todd.ginsberg.com Quick Refresher: ThreadLocal ThreadLocal<String> accessLevel = ... void someOtherPlace() { }
122. @todd.ginsberg.com Quick Refresher: ThreadLocal ThreadLocal<String> accessLevel = ... void someOtherPlace() { switch(accessLevel.get()) { case "GUEST" -> guestLevelWork(); case "ADMIN" -> adminLevelWork(); } }
123. @todd.ginsberg.com Quick Refresher: ThreadLocal ThreadLocal<String> accessLevel = ... void someOtherPlace() { switch(accessLevel.get()) { case "GUEST" -> guestLevelWork(); case "ADMIN" -> adminLevelWork(); } }
124. @todd.ginsberg.com Three Problems with ThreadLocal
125. @todd.ginsberg.com Three Problems with ThreadLocal Unconstrained Mutability
126. @todd.ginsberg.com Problem 1: Unconstrained Mutability void quickHack() { var original = accessLevel.get(); accessLevel.set("ADMIN"); doTheThing(); accessLevel.set(original); }
127. @todd.ginsberg.com Problem 1: Unconstrained Mutability void quickHack() { var original = accessLevel.get(); accessLevel.set("ADMIN"); doTheThing(); accessLevel.set(original); }
128. @todd.ginsberg.com Problem 1: Unconstrained Mutability void quickHack() { var original = accessLevel.get(); accessLevel.set("ADMIN"); doTheThing(); accessLevel.set(original); }
129. @todd.ginsberg.com Three Problems with ThreadLocal Unconstrained Mutability Unbounded Lifetime
130. @todd.ginsberg.com Problem 2: Unbounded Lifecycle accessLevel.set("ADMIN"); doTheThing();
131. @todd.ginsberg.com Problem 2: Unbounded Lifecycle try { accessLevel.set("ADMIN"); doTheThing(); } finally { accessLevel.remove(); }
132. @todd.ginsberg.com Problem 2: Unbounded Lifecycle try { accessLevel.set("ADMIN"); doTheThing(); } finally { accessLevel.remove(); }
133. @todd.ginsberg.com Three Problems with ThreadLocal Unconstrained Mutability Unbounded Lifetime Expensive Inheritance
134. @todd.ginsberg.com Problem 3 : Expensive Inheritance ThreadLocal<GiantCache> cache = new ThreadLocal<>();
135. @todd.ginsberg.com Problem 3 : Expensive Inheritance ThreadLocal<GiantCache> cache = new InheritableThreadLocal<>();
136. @todd.ginsberg.com How Do Scoped Values Help?
137. @todd.ginsberg.com How Do Scoped Values Help? Unconstrained Mutability Values are Immutable
138. @todd.ginsberg.com How Do Scoped Values Help? Unconstrained Mutability Values are Immutable Unbounded Lifetime Defined Lifetime
139. @todd.ginsberg.com How Do Scoped Values Help? Unconstrained Mutability Values are Immutable Unbounded Lifetime Defined Lifetime Expensive Inheritance Cheap/Free Inheritance
140. Demo
141. @todd.ginsberg.com Scoped Values: Remember
142. @todd.ginsberg.com Scoped Values: Remember Useful anywhere you need to pass data one way
143. @todd.ginsberg.com Scoped Values: Remember Useful anywhere you need to pass data one way Try to migrate to Scoped Values when you have structured sharing use cases
144. Remember…
145. @todd.ginsberg.com Structured Concurrency: Preview in Java 25 java --enable-preview
146. @todd.ginsberg.com Nothing Is Going Away It is not a goal to…
147. @todd.ginsberg.com Nothing Is Going Away “... remove the traditional implementation of threads, or to silently migrate existing applications to use virtual threads.” It is not a goal to… https://openjdk.org/jeps/444
148. @todd.ginsberg.com Nothing Is Going Away “... replace any of the concurrency constructs in the java.util.concurrent package, such as ExecutorService and Future.” It is not a goal to… https://openjdk.org/jeps/505
149. @todd.ginsberg.com Nothing Is Going Away “... require migration away from thread-local variables, or to deprecate the existing ThreadLocal API.” It is not a goal to… https://openjdk.org/jeps/506
150. @todd.ginsberg.com What Happens Next? https://openjdk.org/projects/jdk/25/ Follow the Java 25 Release Page
151. @todd.ginsberg.com How To Stay Informed? https://wiki.openjdk.org/display/loom/Main View the Project Loom Wiki
152. Questions? (And Maybe Answers!)
153. @todd.ginsberg.com todd@ginsberg.com https://todd.ginsberg.com Thank You! @todd.ginsberg.com Feedback!