Understanding Class and Object-Level Locks in Java

In the world of multithreading, synchronization is a crucial concept to ensure that threads don’t interfere with each other while accessing shared resources. Class and object-level locks are two mechanisms used in Java to achieve this synchronization. In this blog, we’ll explore what class and object-level locks are, when to use them, and their advantages and disadvantages.

What Are Object-Level and Class-Level Locks?

Object-Level Lock:

  • An object-level lock, also known as an instance lock, is associated with an instance (object) of a class.
  • It is used to synchronize access to instance-specific data to prevent multiple threads from interfering with each other.
  • When a thread acquires an object-level lock, it prevents other threads from acquiring the lock for the same object until it releases it.
  • Here, Acquiring a lock means now the thread have the pass to access synchronized methods and block and no other thread can execute those blocks or method.

Class-Level Lock:

  • A class-level lock, often referred to as a static lock, is associated with a class rather than an instance of that class.
  • It is used to prevent multiple threads from accessing static synchronized methods of the same class simultaneously.
  • When a thread acquires a class-level lock, it prevents other threads from entering any static synchronized methods of that class until it releases the lock.

When Do We Use Them?

Object-Level Lock:

A thread acquires an object level lock when it have to access synchronized block or method. And we generally wrap a logic or method under synchronize keyword when there is some data that needs to be protected from concurrent access.

Use object-level locks when you want to synchronize access to instance-specific data.

Class-Level Lock:

A thread requires a class level lock when it want to execute static synchronized method. They are suitable for scenarios where you need to coordinate access to shared resources that are common across all instances of a class.

Advantages of Locks

  1. Mutual Exclusion: The primary benefit of holding an object/class lock is that it enforces mutual exclusion. When a thread acquires a lock on an object/class, it gains exclusive access to the synchronized block or method associated with that lock. This ensures that only one thread can execute the synchronized code at a time, preventing concurrent access and potential data corruption.
  2. Thread Safety:Object locks are a fundamental tool for achieving thread safety in Java. By synchronizing critical sections of your code with locks, you can protect shared data from being accessed or modified by multiple threads simultaneously, thus avoiding data races and ensuring data consistency.

Disadvantage of Locks

  1. Risk of Deadlocks: One of the most significant disadvantages of locking is the potential for deadlocks. Deadlocks occur when multiple threads each hold one or more locks and are waiting for another thread to release a lock. This can result in a situation where none of the threads can proceed, effectively causing a program to freeze.
  2. Performance Bottlenecks: Lock contention, where multiple threads are competing for the same lock, can lead to performance bottlenecks. If many threads frequently contend for the same lock, it can cause delays and impact the overall performance of the program.

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