Java Memory Model

Question 1

What is a strong memory model?

Answer 1

Where all processors see exactly the same value for a given memory location at all times

Question 2

What is a weak memory model?

Answer 2

Where memory barriers are required to flush or invalidate the local processor cache to see writes made by other processors or make writes by this processor visible to others.

Question 3

When can a data race occur?

Answer 3

A data race occurs when a variable is read by more than one thread, and written by at least one thread, but the reads and writes are not ordered by happens-before ​ .

Question 4

Explain happens-before

Answer 4

The JMM defines a partial ordering called ​ happens-before ​ on all actions within the program. To guarantee that the thread executing action B can see the results of action A, there must be a ​ happens-before ​ relationship between A and B. In the absence of a happens before ordering between two operations, the JVM is free to reorder them as it pleases.

Question 5

What is a correctly synchronised program

Answer 5

A correctly synchronised program is one with no data races; correctly synchronised
programs exhibit sequential consistency, meaning that all actions within the program appear to happen in a fixed, global order. See the Java Volatile Keywords Deck for more information.

Question 6

What are the rules of JMM (Happens-before)

Answer 6

Program order rule
Monitor lock rule
Volatile Variable rule
Thread Start rule
Thread Termination rule
Interruption rule
Finalizer rule
Transitivity rule