Thread basics: interview questions & scenarios

Model answer

A process is a running program with its own address space, given to it by the OS. java Main creates one process holding one JVM, and nothing in it is visible to another process unless you go through a socket, a pipe, a file or shared memory. A thread is a path of execution inside a process, and it's the unit the OS scheduler puts on a core. Every Java program starts with at least the main thread plus a handful of JVM threads (GC, JIT compiler, Reference Handler, Finalizer and so on).

What threads in one JVM share and keep private:

Static fields live on the heap. In HotSpot they're stored with the class's java.lang.Class object (they moved out of PermGen in JDK 7), so every thread sees the same static field. Metaspace holds the class's metadata, not its static values. Local variables are the only thing guaranteed to be thread-confined, because they sit in the thread's own stack frame. The objects they point to are still on the shared heap.

Model answer

public class StartVsRun {
    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> System.out.println("running in " + Thread.currentThread().getName());
        Thread t = new Thread(task, "worker");
        t.run();
        t.start();
        t.join();
        t.start();
    }
}

Real output on Java 21 (java StartVsRun.java):

running in main
running in worker
Exception in thread "main" java.lang.IllegalThreadStateException
	at java.base/java.lang.Thread.start(Thread.java:1525)
	at StartVsRun.main(StartVsRun.java:8)

Notice three things:

• run() is a plain method call. No new thread is created; the task runs on main, synchronously.
• start() asks the JVM to create a new OS thread, which then calls run() on its own stack. start() returns right away.
• A thread can be started at most once, even after it has terminated. The second start() throws IllegalThreadStateException (an unchecked exception). If you need to run the task again, create a new Thread or, better, submit the Runnable to an executor.