Explain Java Virtual Threads (Project Loom) and their impact.

Virtual threads (Java 21): lightweight threads managed by JVM, not OS. Millions of virtual threads possible vs thousands of platform threads. Syntax: Thread.startVirtualThread(), Executors.newVirtualThreadPerTaskExecutor(). Continuation-based: virtual thread yields on blocking operations, carrier thread runs other virtual threads. Benefits: simple blocking code at scale, no reactive complexity. Use cases: high-concurrency I/O (web servers, database access). Considerations: don't pool virtual threads, avoid pinning (synchronized, native code), thread-local usage. Structured concurrency: StructuredTaskScope for managing related threads. Game-changer for concurrent Java applications.