what are read barriers and write barriers in synchronized block

Question

I am looking into how a synchronized and volatile variable works in java and i came across a concept called read and write barrier . Can anyone help me to understand the mea

Answer 1

( the answers above are quite complete), I just want to demonstrate the concept with a simple scheme

 Thread 1                                                             Thread 2


   |          
   |                                                                      
   |                                                                      |
   |                                                                      |
   |  Everything Thread 1                                                 |
   |   wrote before here                                                  |
   |                                                                      |
   |                                                                      |
    _ _ _ _ _ _ _ _ _ _                                                   |
      ( write  barrier)         (happens before)         (read barrier)   |
   |                                                     _ _  _ _ _ _ _ _  
   |
   |                                                   is guaranteed      |
   |                                                  to be visible to    |
   |                                                  Thread 2            |
   |                                                                      |