Loom in JDK 25: Virtual Threads, Structured Concurrency, and Scoped Values

The Loom project is about making asynchronous programming simpler by allowing IO calls to block your threads, without hurting the performance of your application. To do that, it first redefined what a thread is by introducing the concept of virtual thread. Virtual threads are lightweight threads that are separated from OS threads. Virtual threads were made final in JDK 21, but with a performance hiccup when they execute an IO call inside a synchronized block. Along with Virtual Threads, JDK 21 came with two preview features: the Scoped Value API as a replacement of the old crufty ThreadLocal API, and the Structured Concurrency API as a way to organize concurrent asynchronous tasks. In JDK 25, Virtual Threads work well with synchronized blocks, the Scope Value API is final, and the Structured Concurrency API, still in preview, has been revamped to be more flexible. This presentation covers these three aspects of the Loom project, with in depth details about how things are working under the hood, patterns, and non-trivial examples that you will be able to use in your applications.