lecture1.md

Overview of JVM Memory Model

Introduction

• Process : unit of launch from the viewpoint of OS
  • each process maintains isolated memory space
• Thread : unit of execution within processes
  • All threads share same process => needs concurrency
  • Assume OS threads, as opposed to user-level threads(maintained by user level library ; e.g. Green Threads in early Java)

happens-before-relation in Java Memory Model

• Regarding visibility of write effects (assure write-after-read)

happens-before #1 : Program order

Each action in a thread happens-before every other subsequent action in same thread.

happens-before #2 : Transitivity

if A -> B -> C, then A -> C

happens-before #3 : Volatile fields

Each time the variable must be stored/read from the main memory (invalidates cache!) (ref: link)
Note that, volatie vs atomic vs synchronized is all different. (ref : link)

happens-before #4 : Monitor locking

Lock i = ~
i.lock()
try {

}
finally {
  i.unlock()
}

OR

synchronized block : foo.synchronized { }

This rule ensures that unlock -> lock, otherwise, deadlock.

happens-before #5 : Thread start, thread termination

Each execution barrier located at thread start / join