Java.Multithreading.What is the difference between synchronized and ReentrantLock?

Both synchronized and ReentrantLock are used for mutual exclusion in Java (i.e., only one thread can execute a critical section at a time), but they have key differences in capabilities, flexibility, and usage style.

Let’s break it down:

⚖️ synchronized vs ReentrantLock

Feature	synchronized	ReentrantLock
🧵 Reentrancy	✅ Yes	✅ Yes
🔐 Explicit lock/unlock	❌ No — handled automatically by JVM	✅ Yes — must explicitly lock()/unlock()
⚠️ Must unlock manually	❌ No — released when block/method exits	✅ Yes — always use try-finally
⏱ Timeout support	❌ No	✅ Yes (tryLock(timeout))
🔁 Fairness control	❌ No	✅ Yes (new ReentrantLock(true))
🧪 Condition variables	❌ No — uses wait()/notify()	✅ Yes — via lock.newCondition()
💥 Interruptible lock attempts	❌ No — can’t interrupt waiting thread	✅ Yes — use lockInterruptibly()
🧠 Readability	✅ Very easy & clean	❌ More verbose
🧹 Auto-release on exception	✅ Yes — auto-release	❌ No — must use finally

✅ 1. synchronized (Built-in Monitor Lock)

synchronized(myObject) {
    // Critical section
}

Automatically releases lock when the block exits.

Simpler, cleaner syntax.

Ideal for basic mutual exclusion.

✅ 2. ReentrantLock (From java.util.concurrent.locks)

ReentrantLock lock = new ReentrantLock();

lock.lock();
try {
    // Critical section
} finally {
    lock.unlock(); // Must be called!
}

More powerful and flexible.

Useful when you need:

• Timeouts
• Non-blocking try-locks
• Multiple conditions
• Interruptible lock attempts

🔄 Example: Timeout with ReentrantLock

if (lock.tryLock(500, TimeUnit.MILLISECONDS)) {
    try {
        // critical section
    } finally {
        lock.unlock();
    }
} else {
    System.out.println("Could not acquire lock in time.");
}

🧠 Summary — When to Use What?

Use Case	Recommended Approach
Simple mutual exclusion	✅ synchronized
Need timeout or fairness	✅ ReentrantLock
Need multiple condition variables	✅ ReentrantLock
Want auto-release on exceptions	✅ synchronized
Want interruptible lock attempts	✅ ReentrantLock