Process vs Thread

// Process: launching a separate OS process from Java
ProcessBuilder pb = new ProcessBuilder("python3", "script.py");
pb.redirectErrorStream(true);
Process process = pb.start();
int exitCode = process.waitFor();  // blocks until child process finishes
System.out.println("Exit: " + exitCode);

// Thread: lightweight execution unit within the same JVM process
Thread thread = new Thread(() -> {
    System.out.println("Thread id: " + Thread.currentThread().getId());
    // shares heap with parent — can read/write same objects
});
thread.start();
thread.join();  // wait for thread to finish

// Demonstrating shared heap between threads
class SharedCounter {
    int count = 0;  // lives on heap — visible to ALL threads
}

SharedCounter counter = new SharedCounter();

Thread t1 = new Thread(() -> counter.count++);
Thread t2 = new Thread(() -> counter.count++);
t1.start(); t2.start();
t1.join();  t2.join();
// count may be 1 (race condition!) not 2
// Fix: use AtomicInteger or synchronized

// Each thread has its own stack:
Thread t3 = new Thread(() -> {
    int localVar = 42;  // on t3's stack — NOT shared
    System.out.println(localVar);
});

// Java 21 Virtual Threads (Project Loom) — millions of threads cheaply
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (int i = 0; i < 1_000_000; i++) {
        executor.submit(() -> {
            Thread.sleep(Duration.ofMillis(100));  // parks, not blocks OS thread
            return "done";
        });
    }
}  // auto-shutdown

// Check if current thread is virtual
System.out.println(Thread.currentThread().isVirtual());