Who in Java is responsible for updating variables with the volatile modifier in a multithreaded environment?

In Java, the JVM and hardware platform are responsible for updating variables with the volatile modifier in a multithreaded environment. The volatile modifier guarantees that reading and writing this variable will occur directly from main memory, not from the processor cache, ensuring visibility of changes between threads.

That is, when one thread changes the value of a volatile variable, this change becomes immediately visible to other threads. The JVM inserts special memory instructions (memory barriers) that prevent reordering of operations and ensure correct synchronization.

Example:

volatile boolean flag = false;

// Thread 1
flag = true; // updating the volatile variable

// Thread 2
if (flag) {
    // will see the updated value
}

Thus, the responsibility for correct updating and visibility of volatile lies with the JVM and hardware, and the developer just needs to use this modifier correctly.