What does \'happens-before consistency\' mean in the context of the Java Language Specification?

Understanding Happens-Before Consistency

In chapter 17 of the Java Language Specification (JLS), "happens-before consistency" is defined as follows:

"A set of actions A is happens-before consistent if for all reads r in A, where W(r) is the write action seen by r, it is not the case that either hb(r, W(r)) or that there exists a write w in A such that w.v = r.v and hb(W(r), w) and hb(w, r)."

This definition can be simplified as: for a set of actions to be happens-before consistent, no read should see a write that occurs later, and no write should be followed by a read that sees an earlier value.

Your Questions

• Your first two questions:

  • Yes, your understanding of the definition is correct.
  • "w.v = r.v" means that the value written by write action w is the same as the value read by read action r.

• Your third question:

  The numbers to the left of the actions in the execution orders represent the sequential order in which the actions were executed.

• Your fourth question:

  Yes, your understanding that both r2 and r1 saw the initial write of 0 because A and B are not volatile fields is correct. Non-volatile fields can be stored in registers, which can lead to temporary inconsistencies.

• Your fifth and sixth questions:

  The situation described in the second execution order, where reads see writes that occur later, can occur in real-world multi-threaded scenarios. An example could be:

  • Thread A writes to a shared variable X.
  • Thread B reads the value of X at a later point in time, but due to cache coherence delays, it sees the old value.

  To prevent such inconsistencies, volatile fields can be used to ensure that reads and writes are performed in a happens-before consistent manner.

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