Thread vs Runnable in Java: Key Differences & When to Use
Thread is the actual worker; you create it, then start it. Runnable is just a job description—an interface with a single run() method you hand to any executor or Thread constructor.
Devs often say “extend Thread” when they really mean “implement Runnable.” The mix-up happens because new coders see Thread first, and IDEs auto-complete the longer pattern, making the simpler Runnable feel hidden in plain sight.
Key Differences
Thread is a class you instantiate; you can only extend it once. Runnable is an interface you can implement many times, letting one object submit tasks to pools, executors, or lambda expressions without wasting inheritance.
Which One Should You Choose?
Favor Runnable. It keeps your class hierarchy free, works inside thread pools, and plays nicely with modern concurrency utilities. Use Thread only when you need to tweak low-level behavior like priority or daemon status.
Can I start Runnable directly?
No. Wrap it in a Thread or submit to an Executor; Runnable has no start() method.
Why prefer Runnable in Spring apps?
Spring’s @Async and TaskExecutor expect Runnable or Callable, making Runnable the path of least resistance.
Does Runnable affect performance?
Not directly; the overhead lies in thread creation, not the choice of Runnable versus Thread subclass.