Java 8 # synchronized vs Reentrant Lock

“When do we use ReentrantLock in Java?” — perfect for your Java 8 interviews.

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✅ When We Use ReentrantLock

You use ReentrantLock instead of synchronized when you need more advanced locking features that synchronized cannot provide.

Below are the key situations:

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✅ 1. When You Need to Try Lock Without Waiting

synchronized blocks always wait until the lock is free.
But with ReentrantLock, you can use tryLock():

if (lock.tryLock()) {
    try {
        // do work
    } finally {
        lock.unlock();
    }
} else {
    // handle if lock not available
}

Useful for avoiding deadlocks.

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✅ 2. When You Need to Interrupt a Thread Waiting for a Lock

synchronized cannot interrupt a thread waiting for a lock,
but lock.lockInterruptibly() can.

This is needed in high-performance systems like banking, trading, etc.

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✅ 3. When You Need Fairness

You can make a lock fair:

Lock lock = new ReentrantLock(true); // fair lock

Fair lock gives access to next waiting thread, not the latest one.

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✅ 4. When You Need Multiple Condition Variables

With synchronized, you only get one monitor (wait/notify).
ReentrantLock allows multiple conditions:

Condition condition = lock.newCondition();

This helps in producer-consumer or queue systems.

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✅ 5. When You Need Better Performance (Under High Contention)

Under heavy thread loads, ReentrantLock performs better than synchronized.

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⭐ One-Liner Interview Answer

We use ReentrantLock when we need advanced locking features like tryLock, fairness, interruptible waits, and multiple conditions—features that synchronized does not support.

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💡 Want code examples?

I can give you:

✔ Producer–Consumer using ReentrantLock
✔ Deadlock-free example using tryLock
✔ Fair vs unfair locks comparison

Just tell me!