Synchronized Methods

In Java, tagging an instance method as synchronized forces a thread to acquire the monitor lock of the object instance before executing the method body.

Locking Mechanics

When a thread invokes a synchronized instance method:

• It automatically acquires the monitor lock of the calling object (this).
• If another thread already holds that lock, the invoking thread is suspended and enters the BLOCKED state.
• Non-synchronized methods do NOT require a lock and can be executed concurrently by other threads without blocking.

Code Walkthrough

The following example demonstrates that while a thread holds the lock, other threads are blocked from entering other synchronized methods on the same instance.

public class SynchronizedMethodsDemo {
    public synchronized void methodA() {
        System.out.println(Thread.currentThread().getName() + " inside methodA");
        try {
            Thread.sleep(2000); // Hold lock for 2 seconds
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        System.out.println(Thread.currentThread().getName() + " exiting methodA");
    }
public synchronized void methodB() {
        System.out.println(Thread.currentThread().getName() + " inside methodB");
    }
public static void main(String[] args) {
        SynchronizedMethodsDemo demo = new SynchronizedMethodsDemo();
// Thread 1 enters methodA and acquires lock
        new Thread(demo::methodA, "Thread-1").start();
// Thread 2 tries to enter methodB on the same instance, but blocks
        new Thread(demo::methodB, "Thread-2").start();
    }
}

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