Java Memory Model in few words

This article is mostly consist of quotes from The Well-Grounded Java Developer by Benjamin J. Evans and Martijn Verburg. I like simplicity and brevity of their explanation approach.

And that’s how Happens-Before and Synchronizes-With relationships are explained.

• Happens-Before — This relationship indicates that one block of code fully completes before the other can start.
• Synchronizes-With — This means that an action will synchronize its view of an object with main memory before continuing.

The JMM has these main rules:

• An unlock operation on a monitor Synchronizes-With later lock operations.
• A write to a volatile variable Synchronizes-With later reads of the variable.
• If an action A Synchronizes-With action B, then A Happens-Before B.
• If A comes before B in program order, within a thread, then A Happens-Before B.

The general statement of the first two rules is that “releases happen before acquires”. In other words, the locks that a thread holds when writing are released before the locks can be acquired by other operations (including reads).

There are additional rules, which are really about sensible behavior:

• The completion of a constructor Happens-Before the finalizer for that object starts to run (an object has to be fully constructed before it can be finalized).
• An action that starts a thread Synchronizes-With the first action of the new thread.
• Thread.join() Synchronizes-With the last (and all other) actions in the thread being joined.*
• If X Happens-Before Y and Y Happens-Before Z then X Happens-Before Z (transitivity).

They even have granted free access to entire Chapter 4. Modern concurrency. Although I’m not relish with the code style (using trailing underscore to emphasize method parameters - why?) this chapter definitely deserves to read it.

Footnotes:

* JLS. Chapter 17. Threads and Locks assures that:

All actions in a thread happen-before any other thread successfully returns from a join() on that thread.

So, the truth is that thread actions happens-before Thread.join().