Java.Multithreding.What is a ThreadLocal variable?

🧵🔐 What is a ThreadLocal Variable?

A ThreadLocal is a special Java class that provides thread-local storage.

📌 Each thread that accesses a ThreadLocal variable gets its own independent copy of the variable.

So even though many threads use the same ThreadLocal instance, each thread interacts only with its own isolated value.

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✅ Why Use It?

• Avoid shared mutable state
• Thread-safe without synchronization
• Commonly used for:
  • User sessions in web apps
  • Date formatters (which are not thread-safe)
  • Per-thread context like trace IDs or DB connections

🔧 Example:

public class ThreadLocalExample {
    private static final ThreadLocal<Integer> threadId = ThreadLocal.withInitial(() -> 0);

    public static void main(String[] args) {
        Runnable task = () -> {
            threadId.set((int) (Math.random() * 1000));
            System.out.println(Thread.currentThread().getName() + ": " + threadId.get());
        };

        new Thread(task, "Thread-A").start();
        new Thread(task, "Thread-B").start();
    }
}

✅ Output (sample):

Thread-A: 713
Thread-B: 154

Each thread gets its own copy of threadId.

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🔁 Methods

Method	Description
get()	Gets the value for the current thread
set(value)	Sets the value for the current thread
remove()	Clears the value (useful to prevent memory leaks)
withInitial(Supplier)	Sets an initial value lazily

❗ Memory Leak Warning (Especially in Thread Pools)

If you forget to call remove(), and your thread is reused (like in a thread pool), the value may leak across tasks.

threadLocal.remove(); // ✅ Do this after the task finishes

🧠 Summary

Feature	ThreadLocal
Shared across threads?	❌ No — each thread has its own value
Thread-safe?	✅ Yes — by design
Used for	Context storage per thread
Common risks	Memory leaks in long-lived threads (e.g., pools)