Java.Multithreading.What is the difference between stack and heap in terms of multithreading?

🧠 Stack vs Heap in Multithreading

Feature	Stack	Heap
📌 Scope	Thread-local (private per thread)	Shared among all threads
🧵 Thread-safety	✅ Safe — each thread has its own stack	❌ Not safe — needs synchronization
🧠 Use	Stores local variables, method calls	Stores objects, instance data
♻️ Lifetime	Lives while method is running	Lives until GC collects it
🧹 Clean-up	Automatic when method returns	Garbage Collected

🔍 Stack (Per Thread)

Each thread has its own stack where it stores:

• Method call frames
• Local variables (primitives and references)
• Return addresses

No two threads share the same stack, so: ✔️ No synchronization needed ✔️ Very fast access

🔍 Heap (Shared Across Threads)

The heap is where:

• All objects (new SomeClass()) live
• Instance variables of objects are stored

❗ Multiple threads can access the same object, so:

• You must use synchronization (synchronized, ReentrantLock, volatile, etc.)
• Otherwise, you’ll face race conditions or visibility issues

🔧 Example

public class Example {
    private int shared = 0; // Lives in the heap

    public void run() {
        int local = 0; // Lives in the stack (per thread)
        local++;       // Thread-safe
        shared++;      // Not thread-safe!
    }
}

🛡️ Thread Safety Summary

Variable Type	Lives In	Thread-Safe?
Local variable	Stack	✅ Yes (per-thread)
Object instance var	Heap	❌ No (needs sync)
Static shared data	Heap	❌ No (needs sync)