Thread Lock

About

A lock is a synchronization mechanism that controls access to shared resources in a multithreaded environment. Locks ensure that only one thread can access a critical section at a time, preventing race conditions, data inconsistency, and thread interference.

Why Are Locks Needed?

In Java, multiple threads can execute concurrently, which can lead to unexpected behavior when accessing shared resources. Locks are required to:

1. Prevent Data Corruption – Ensures that two threads don’t modify shared data simultaneously.

2. Maintain Data Consistency – Prevents inconsistent reads and writes.

3. Avoid Race Conditions – Ensures proper sequencing of operations.

4. Ensure Thread Safety – Provides a controlled environment for resource access.

5. Control Execution Order – Ensures critical sections execute in the correct order.

Types of Locks in Java

Locks can be classified based on their behavior, scope, and implementation. Below are the major categories:

1. Intrinsic Locks (Monitor Locks)

• Implicitly used by synchronized methods and blocks.

• Each object in Java has a built-in monitor lock.

• Only one thread can acquire the lock at a time.

2. Explicit Locks

• Provided by the java.util.concurrent.locks package.

• Require manual locking and unlocking.

• Examples:

  • ReentrantLock

  • ReadWriteLock

  • StampedLock

Lock Scopes

• Method-Level Lock – Applied to an entire method using synchronized.

• Block-Level Lock – Applied to a specific block of code.

• Object-Level Lock – Applied to an instance of a class.

• Class-Level Lock – Applied to static members.

Locking Strategies

• Fair vs. Unfair Locks – Determines the order in which threads acquire locks.

• Reentrant Locks – Allows the same thread to acquire the lock multiple times.

• Spin Locks – Actively wait for a lock instead of blocking.

• Optimistic Locks – Assume no conflicts and validate before committing changes.

Intrinsic vs. Explicit Locks

Feature	Intrinsic (Monitor) Lock	Explicit (ReentrantLock)
Implementation	synchronized keyword	ReentrantLock class
Locking & Unlocking	Handled by JVM	Requires manual lock() and unlock()
Fairness	Always unfair	Can be fair (new ReentrantLock(true))
Interruptibility	Cannot be interrupted	Supports lockInterruptibly()
Condition Variables	Uses wait()/notify()	Supports Condition objects
Performance	Simpler but less flexible	More control, can be optimized

Problems With Locks

Locks can introduce several issues if not managed properly:

1. Deadlocks – Occur when two or more threads wait indefinitely for each other’s locks.

2. Starvation – Low-priority threads may never get a chance to execute.

3. Livelocks – Threads keep changing states without making progress.

4. Performance Overhead – Excessive locking can reduce concurrency.