What is the Java Memory Model? How does it affect multithreading?

Java Memory Model (JMM) is a specification that describes how threads in Java interact through memory and how changes made by one thread become visible to others. JMM defines the rules of visibility and the order of read and write operations in a multithreaded environment.

Impact on multithreading:

• Ensures correct synchronization between threads.
• Determines when variable changes become visible to other threads.
• Guarantees that operations within synchronized blocks are executed in a specific order.

Without understanding JMM, errors related to caching values in registers or CPU may occur, leading to unpredictable behavior of multithreaded programs.

Example: using the volatile keyword guarantees that a write to a variable is immediately visible to all threads.

volatile boolean flag = false;

// Thread 1
flag = true;

// Thread 2
if (flag) {
    // will definitely see the update
}