The Java Language Specification, Third Edition

THREADS AND LOCKS Executions 17.4.6
DISCUSSION
In a happens-before consistent set of actions, each read sees a write that it is allowed to
see by the happens-before ordering.
For example, the behavior shown in Trace 17.5 is happens-before consistent, since there
are execution orders that allow each read to see the appropriate write.
Initially, A == B == 0. In this case, since there is no synchronization, each read can see
either the write of the initial value or the write by the other thread. One such execution order
is
1: B = 1;
3: A = 2;
2: r2 = A; // sees initial write of 0
4: r1 = B; // sees initial write of 0
Similarly, the behavior shown in Trace 17.5 is happens-before consistent, since there
is an execution order that allows each read to see the appropriate write. An execution order
that displays that behavior is:
1: r2 = A; // sees write of A = 2
3: r1 = B; // sees write of B = 1
2: B = 1;
4: A = 2;
In this execution, the reads see writes that occur later in the execution order. This may
seem counterintuitive, but is allowed by happens-before consistency. Allowing reads to see
later writes can sometimes produce unacceptable behaviors.
17.4.6 Executions
An execution E is described by a tuple < P, A, po, so, W, V, sw, hb >, comprising:
• P - a program
• A - a set of actions
• po - program order, which for each thread t, is a total order over all actions
performed by t in A
• so - synchronization order, which is a total order over all synchronization
actions in A
• W - a write-seen function, which for each read r in A, givesW(r), the write
action seen by r in E.
DRAFT
• V - a value-written function, which for each write w in A,givesV(w), the value
written by w in E.
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