Virtual Threads, Structured Concurrency and Scoped Values: Putting it all together

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Balkrishna Rawool Virtual Threads, Structured Concurrency and Scoped Values: Putting it all together 22 May 2025

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3 Balkrishna Rawool IT Chapter Lead, ING Bank @BalaRawool

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4 Project Loom Purpose: Ø Supporting easy-to-use, high-throughput lightweight concurrency and new programming models for it Features: Ø Virtual Threads – Final in JDK 21 Ø Structured Concurrency – Preview Ø Scoped Values – Final in JDK 25

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Virtual Threads

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6 Platform Threads and Virtual Threads JVM OS Platform Threads Platform Threads: Ø Thin wrapper over OS threads Ø One-to-one mapping with OS threads Ø Thread pools Virtual Threads Virtual Threads: Ø Lightweight user threads Ø Highly scalable Ø No need for pooling

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7 Virtual Thread Scheduler Platform/ Carrier Thread Virtual Threads

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A simple web-app with virtual threads

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9 A simple web-app with virtual threads Ø SpringBoot App Ø SpringBoot 3.4 Ø JDK 25 (Project Loom early access build) Ø Domain: Banking Ø Name: A Bank

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Structured Concurrency

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11 Virtual threads enable Structured Concurrency

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12 Programs à Structured Programming Concurrent programs à Structured Concurrency

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13 Structured Concurrency: Examples All these examples can be implemented using StructuredTaskScope API.

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14 Structured Concurrency: Continued Principle: Ø When flow of execution splits into multiple concurrent flows, they rejoin in the same code block Benefits: Ø Error handling with short-circuiting Ø Cancellation propagation Ø Clarity Ø Observability

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Structured Concurrency API

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16 Feature: Loan application Ø Customer applies for loan Ø Get customer details Ø Using customer details Ø Get accounts Ø Get loans Ø Get external credit scores Ø Two services provide this score Ø Response from one is enough Ø Aggregate all this info and send to offer-calculation service

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17 Feature: Loan application Get Customer Details Get Account Details Get Loan Details Get Credit Score Calculate Offer Get Credit Score Get Credit Score 1 Get Credit Score 2

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Scoped Values

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ThreadLocal 19 Ø Each thread gets its own copy of the object. Ø Useful for Ø Request/session-specific data Ø Resource management (of non-thread-safe objects) Ø Caching Ø Observability (e.g. CorrelationID) Ø VirtualThread-s support ThreadLocal-s. Ø Problems Ø Unconstrained mutability Ø Unbounded lifetime Ø Expensive inheritance (with InheritableThreadLocal)

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Scoped Values 20 Ø Immutable Ø All (allowed) threads use the same copy Ø Inherited through the StructuredTaskScope to the child-virtual-threads Ø Bounded by the scope ScopedValue VALUE = ScopedValue.newInstance(); ScopedValue.where(VALUE, someValue) .run(() -> {...});

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Scoped Values API

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22 Source code and contact @balarawool.bsky.social /balkrishnarawool Please rate the talk :)