Threads Basics

A thread is the smallest unit of execution within a program. In Java, multithreading allows a single process to spawn multiple concurrent execution paths, maximizing CPU utilization and enabling responsive, parallel application behavior.

Process vs. Thread Memory Layout

A Process is an isolating execution container allocated its own private memory address space by the operating system. If one process crashes, others are unaffected.

A Thread is an execution path inside a process. Threads are lightweight because they share process resources, leading to:

• Shared Memory: Heap, static fields, class definitions (Metaspace), and open file descriptors. This permits fast data sharing but introduces concurrency risks.
• Private Memory: Each thread gets a private Call Stack (storing local variables and method invocation frames) and a Program Counter (tracking the current instruction index).

The Thread Scheduler and CPU Mapping

Java threads map directly to native operating system threads on modern JVMs (1-to-1 model). The OS Thread Scheduler decides which threads run, when they get context-switched out, and which CPU cores they run on. Scheduling can be preemptive or cooperative; modern systems use preemptive scheduling, allocating time slices to threads.

Code Walkthrough

The following code shows a basic thread setup where a worker executes concurrently alongside the main thread.

public class ThreadBasicsDemo {
    public static void main(String[] args) {
        System.out.println("Main starts: " + Thread.currentThread().getName());
Thread worker = new Thread(() -> {
            System.out.println("Worker executes: " + Thread.currentThread().getName());
        }, "Worker-Thread");
worker.start(); // Spawns execution path
try {
            worker.join(); // Main thread blocks until worker finishes
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
System.out.println("Main ends");
    }
}

Interview-Relevant Information