How does thread-based shared memory variable reading occur?

In Java, reading a variable from shared memory by threads occurs considering the Java Memory Model. Each thread can have its own local copy of the variable in the processor cache, so to guarantee reading the current value from shared memory, synchronization must be considered.

If a variable is declared with the volatile keyword, then reading and writing this variable happens directly from main memory, which ensures visibility of changes between threads without additional synchronization.

If volatile is not used, then to correctly read a variable from shared memory, synchronized blocks (synchronized) should be used, which ensure memory consistency and visibility of changes between threads.

Example with volatile:

volatile int sharedVar;

// In one thread
sharedVar = 10;

// In another thread
int localCopy = sharedVar; // will guaranteedly see the updated value

Without volatile or synchronized, a thread may see a stale value of the variable due to caching.