Lock vs Synchronized in Java: When and Why to Use Each

Introduction

In Java multithreaded programming, ensuring thread safety often involves using synchronized or the Lock API.

A common interview question is:

When should you use synchronized, and when should you use Lock?

To answer this correctly, we need a clear understanding of how both mechanisms work and what trade-offs they involve.

synchronized

synchronized is Java’s built-in locking mechanism that enforces mutual exclusion and memory visibility.

Each synchronized construct has two parts:

1. A reference to the lock object
   • For synchronized methods, the lock is the current object (this)
2. A block of code protected by the lock

Every Java object can act as a lock. These built-in locks are also known as intrinsic locks or monitor locks.

Usage Examples

// Method-level synchronization
public synchronized void synchronizedMethod() {
    // critical section
}

// Block-level synchronization
synchronized (lockObject) {
    // critical section
}

Characteristics of synchronized

• Implicit lock acquisition and release
• Lock is released automatically when exiting the block or when an exception occurs
• Only one thread can hold the lock at a time (mutual exclusion)
• Simple and safe to use

Lock Interface

The Lock interface provides a more flexible and expressive locking mechanism compared to synchronized.

Its key feature is that lock acquisition and release are explicit, giving developers more control.

Core Methods

public interface Lock {
    void lock();
    void lockInterruptibly() throws InterruptedException;
    boolean tryLock();
    boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
    void unlock();
    Condition newCondition();
}

Typical Usage Pattern

Lock lock = new ReentrantLock();

lock.lock();
try {
    // critical section
} finally {
    lock.unlock();
}

Using finally is critical to avoid deadlocks caused by forgotten unlock calls.

Key Differences: Lock vs synchronized

ReentrantLock

ReentrantLock is the most commonly used implementation of the Lock interface.

Features:

• Supports fair and non-fair locking (non-fair by default)
• Reentrant: the same thread can acquire the lock multiple times
• Built on top of AQS (AbstractQueuedSynchronizer)

Fair locks grant access in FIFO order, while non-fair locks prioritize throughput.

Choosing Between Lock and synchronized

As a general rule:

Prefer synchronized for simplicity and safety
Use Lock only when you need advanced features

Use Lock when you need:

• Interruptible lock acquisition
• Timed lock attempts
• Fair locking
• Multiple condition queues
• Locking across non-structured blocks

Otherwise, synchronized is usually the better choice.

Interview Tip

If asked:

What synchronization mechanisms does Java provide?

A solid answer would be:

Java provides built-in synchronization via synchronized and more advanced locking mechanisms through the Lock API, with ReentrantLock being the most commonly used implementation.

Conclusion

Both synchronized and Lock solve the same core problem: thread safety.

The difference lies in control vs simplicity.

Understanding their internal behavior and trade-offs allows you to make informed choices in both interviews and production systems.