Java / Java 21 Virtual Threads Interview questions
What is the difference between virtual threads and reactive programming?
Reactive frameworks like Project Reactor or RxJava achieve high concurrency by never blocking at all - I/O completion triggers callbacks composed through operator chains (map, flatMap, and so on), which avoids tying up any thread while waiting.
The trade-off is a different programming model: sequential logic gets restructured into chained operators, stack traces become harder to follow across asynchronous boundaries, and debugging tools built around ordinary call stacks are less effective.
Virtual threads achieve comparable scalability while keeping ordinary, sequential, blocking-looking code - a debugger can step through it, and a stack trace reads like a normal one. What reactive libraries still offer that virtual threads alone don't is a rich, built-in vocabulary for backpressure and stream composition; structured concurrency narrows that gap for task orchestration but isn't a full replacement for reactive streams.
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