What is \'Happens-Before\' Consistency in Java, and How Does it Work?

Understanding Happens-Before Consistency

The concept of happens-before consistency in the Java Language Specification (JLS) refers to the consistent ordering of reads and writes in a multi-threaded environment.

Definition of Happens-Before Consistency

A set of actions A is happens-before consistent if, for every read r in A where W(r) is the write action seen by r:

• Either hb(r, W(r)) (r happened before W(r))
• Or there exists no write w in A such that w.v = r.v and both hb(W(r), w) and hb(w, r) (there's no inconsistency between r and w)

Questions Answered

1. Is your understanding correct?
Yes, your understanding is correct. "It equals to following words: ..., it is the case that neither ... nor ..." is a logical equivalent of the formal definition.

2. What does "w.v = r.v" mean?
w.v and r.v refer to the values of the variable accessed by the write w and the read r, respectively. The condition w.v = r.v means that the values read in both actions are the same.

3. What do the left numbers in the execution orders represent?
The left numbers represent the order of actions in the original code.

4. Is your understanding about volatile fields correct?
Yes, your understanding is correct. In the first execution order, both r2 and r1 see the initial write of 0 because A and B are non-volatile fields, allowing for delayed or inconsistent reads.

5. Does the situation (reads see writes that occur later) exist in real world?
Yes, it can exist. One example is when two threads access shared data through their own registers and optimizations prevent the reading thread from detecting the change made by the writing thread.

6. Could you give a real example?
Consider the following example:

Thread 1

<code class="java">volatile boolean running = true;</code>

Thread 2

<code class="java">while (running) {
  // Perform task
}</code>

In this example, if the reading thread's optimization prevents it from loading the updated value of running, it will continue to see the outdated value and execute the loop indefinitely, even though Thread 1 has already set running to false.

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