Java.Multithreading.When to use what ?

🔎 Key Criteria to Decide:

1. Do you need mutual exclusion?
2. Do you need only visibility or atomicity?
3. Do you need fine-grained locking or advanced features like try-lock, fairness, etc.?
4. Are threads coordinating around a condition (like a signal or phase)?

Let’s look at each scenario 👇

🧵 1. Use synchronized when:

✅ You need:

• Basic mutual exclusion
• Simplicity and safety
• Minimal code (Java handles lock acquire/release automatically)

❌ Avoid if:

• You need interruption, timeout, or non-blocking attempts
• You need complex coordination

🔧 Example:

public synchronized void increment() {
    count++;
}

📌 Use it for most simple locking needs, especially when guarding an entire method or small critical section.

🔒 2. Use ReentrantLock when:

✅ You need:

• Advanced control over locking, like:
  • tryLock()
  • lockInterruptibly()
  • Fairness policy
  • Manual unlocking
• Condition variables (newCondition()), which are more powerful than wait()/notify()

❌ Avoid if:

• You don’t need that control — synchronized is cleaner and safer

🔧 Example:

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

📌 Use for low-level, performance-critical, or condition-driven logic.

⚡ 3. Use Atomic Variables (AtomicInteger, etc.) when:

✅ You need:

• Lock-free atomic operations
• Simple counters, flags, or compare-and-set logic
• High performance

❌ Not suitable for:

• Complex logic involving multiple shared variables
• Coordinating across threads with blocking

🔧 Example:

AtomicInteger counter = new AtomicInteger();
counter.incrementAndGet();

📌 Great for lightweight concurrent counters or flags.

👁️ 4. Use volatile when:

✅ You need:

• Visibility guarantee only
• One thread writes, others read
• You want to avoid locking

❌ Don’t use for compound actions (e.g. ++, check-then-act)

🔧 Example:

volatile boolean running = true;

📌 Use it for simple flags or safe publication (e.g., singleton ready flags).

🛏️ 5. Use wait()/notify() when:

✅ You need:

• A thread to pause until a condition is met
• Classic producer-consumer logic

❌ Avoid if:

• You can use Condition, CountDownLatch, or higher-level tools

📌 It’s powerful but tricky — must always be used inside synchronized.

🧰 6. Use java.util.concurrent utilities when:

Tool	Use Case
CountDownLatch	Wait until N threads complete
CyclicBarrier	Wait until all threads reach a point
Semaphore	Limit concurrent access to a resource
Phaser	Dynamic coordination across phases
Exchanger	Swap data between two threads

📌 Use for coordinating multiple threads with clear rules.

🧰 6. Use java.util.concurrent utilities when:

Tool	Use Case
CountDownLatch	Wait until N threads complete
CyclicBarrier	Wait until all threads reach a point
Semaphore	Limit concurrent access to a resource
Phaser	Dynamic coordination across phases
Exchanger	Swap data between two threads

📌 Use for coordinating multiple threads with clear rules.