What are the differences between stack and heap from the perspective of multithreading?

In a multithreaded application, each thread has its own independent stack. The stack stores:

• Local variables of primitive types.
• References to objects in the heap.
• Method arguments.
• Method return values.

When a method is called, a new frame (stack frame) is created at the top of the thread's stack. When the method finishes, this frame is removed. Working with the stack is very fast because it is a LIFO (Last-In, First-Out) structure, and access to elements occurs from the top. The thread's stack is private and inaccessible to other threads.

Heap is a shared memory area for all threads in the application. The heap stores:

• Object instances.
• Arrays.

Objects in the heap are created using the new operator. Access to objects in the heap can be performed from any thread if the thread has a reference to that object. Memory management in the heap is handled by the garbage collector. This makes working with the heap slower compared to the stack because it requires searching for free space for new objects and collecting unused ones. Since the heap is shared, synchronization mechanisms (e.g., synchronized blocks, Lock) are necessary when working with shared objects to prevent race conditions and ensure thread-safe access.

Main differences:

Attribute	Stack	Heap
Memory area	Private to each thread	Shared among all threads
Stored data	Primitives, references, method calls	Objects, arrays
Memory management	Automatic (method calls)	Garbage collector
Speed	Faster	Slower
Access	Only from its own thread	From any thread if a reference exists
Synchronization	Not required	Required for shared object access

// Example:
public class MultithreadingMemory {
    private static int sharedVar = 0; // Variable in the heap (class object)

    public void methodA() {
        int localVarA = 10; // Local variable in the current thread's stack
        Object obj = new Object(); // Object in the heap, localVarA references it in the stack

        synchronized (this) { // Synchronization for accessing shared resources (e.g., sharedVar)
            sharedVar++;
        }
    }

    public void methodB() {
        int localVarB = 20; // Local variable in the current thread's stack
        // Access to sharedVar from another thread is possible if methodB is called in another thread
        // and there is a reference to the MultithreadingMemory object.
    }
}

Thus, in multithreading, the stack provides isolation of local data for each thread, while the heap serves as a common storage for objects, which requires special attention to synchronization during parallel access.