#46 | Object-level locking vs Class-level locking | Java Interview

In this video, we explain object-level locking vs class-level locking in Java — a very important Core Java & Multithreading interview question that often decides whether a candidate truly understands synchronization or not.

We break it down using simple definitions, clear differences, and practical examples so you can confidently explain it in interviews.

👨‍💻 What you’ll learn:

Object-Level Locking (Instance-Level Lock)

• Lock is applied on a specific object instance
• Only one thread can execute synchronized code per object
• Lock is taken on:
– this (current object)
– or any instance lock object (new Object())
• Different objects → different locks → threads can run in parallel

Class-Level Locking (Static-Level Lock)

• Lock is applied on the Class itself, not on objects
• Only one thread in the entire JVM can execute that synchronized code at a time
• Lock is taken on:
– ClassName.class
• All objects of that class share the same lock

Key Differences (Interview Gold)

• Lock Scope: Object vs Class
• Parallelism:
– Object-level → more parallel execution
– Class-level → single global lock
• Used With:
– Instance synchronized methods/blocks
– Static synchronized methods / synchronized(ClassName.class)

Example Explained

Object-Level Locking
• Thread T1 locks object c1
• Thread T2 locks object c2
• Both run at the same time ✅

Class-Level Locking
• Both threads lock Counter.class
• One thread waits until the other finishes ❌
• Only one thread executes at a time

🎯 What interviewers expect:
• You understand monitor locks
• You know the difference between this and ClassName.class
• You can explain concurrency vs safety trade-offs
• You can give a real example

🧠 How you should answer in interviews:
Object-level locking synchronizes on a specific object instance, allowing parallel execution across different objects. Class-level locking synchronizes on the class itself, enforcing a single lock across all instances and allowing only one thread to execute at a time.

This video is perfect for Core Java interviews, multithreading concepts, backend preparation, and service-based company interviews.

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Видео #46 | Object-level locking vs Class-level locking | Java Interview канала Sudhanshu Jaiswal