Module 11: Multithreading

A Thread is an independent path of execution running concurrently within a single program process, sharing heap memory while maintaining its own dedicated stack space.

Two Mechanisms to Create Threads

• 1. Extending Thread: Create a subclass of Thread and override its run() method. (Disadvantage: consumes the class's single inheritance slot).
• 2. Implementing Runnable (Best Practice): Pass a Runnable functional interface instance into a Thread constructor. Keeps inheritance open for domain models:

  Runnable task = () -> System.out.println("Running in a new thread: " + Thread.currentThread().getName());
  Thread t = new Thread(task);
  t.start(); // Spawns a new concurrent OS thread!

When multiple concurrent threads read and write shared mutable state without coordination, a race condition occurs — producing corrupted or unpredictable data depending on arbitrary CPU scheduling timing.

The synchronized Keyword

Every Java object contains an intrinsic Monitor Lock. Applying synchronized guarantees mutual exclusion (Mutex): only one thread can hold the lock and execute the protected block or method at any given moment. Other threads must wait their turn in the Blocked state:

class BankAccount {
    private int balance = 1000;

    // Synchronized method locks on the current instance (this)
    public synchronized void withdraw(int amount) {
        if (balance >= amount) {
            balance -= amount;
        }
    }
}