Using Structured Concurrency and Scoped Values to Create a Virtual Threads Based Asynchronous Application

The Loom project is about making asynchronous programming simpler. The promise is to enable the "simple thread-per-request style to scale with near-optimal hardware utilization". Before virtual threads, this was only achievable with callback-based reactive programming. Loom first brought virtual thread in JDK 21 that makes the blocking of threads almost free. Along with it, it published the Scoped Values API in JDK 25, as a replacement of the Thread Local API. The last element is the Structured Concurrency API, which is still a preview feature in JDK 26 and 27. This hands-on lab guides you through writing a virtual thread-based web application that utilizes structured concurrency to parallelize requests, and implements different strategies to process them. Then it shows you how you can use scoped values to pass sensitive information from one module of your application to another one, without relying on method parameters. It also uses the principles of Data Oriented Programming to organize your application in a simple way. You will use StructuredTaskScopes, the implementations of the Joiner interface that the API gives you, and you will create your own implementations as well. In the end, you will see the patterns of code that the Loom project gives you to achieve the same throughput as a classical reactive application, and you will then be able to compare both approaches to choose which one you prefer.