Happens-before relates to multi-threading applications where logic has happened before a sequence of thread logic has occurred.
Questions tagged [happens-before]
108 questions
42
votes
4 answers
Memory Consistency - happens-before relationship in Java
While reading Java docs on Memory Consistency errors. I find points related to two actions that creates happen - before relationship:
When a statement invokes Thread.start(), every statement that has a
happens-before relationship with that…
Prateek
- 12,014
- 12
- 60
- 81
39
votes
3 answers
Java memory model: volatile variables and happens-before
I'd like to clarify how happens-before relation works with volatile variables. Let we have the following variables:
public static int i, iDst, vDst;
public static volatile int v;
and thread A:
i = 1;
v = 2;
and thread B:
vDst = v;
iDst = i;
Are…
Alexey
- 9,197
- 5
- 64
- 76
17
votes
1 answer
What does "strongly happens before" mean?
The phrase "strongly happens before" is used several times in the C++ draft standard.
For example: Termination [basic.start.term]/5
If the completion of the initialization of an object with static
storage duration strongly happens before a call to…
curiousguy
- 8,038
- 2
- 40
- 58
16
votes
2 answers
Kotlin coroutines "happens-before" guarantees?
Do Kotlin coroutines provide any "happens-before" guarantees?
For example, is there "happens-before" guarantee between write to mutableVar and subsequent read on (potentially) other thread in this case:
suspend fun doSomething() {
var mutableVar…
Vasiliy
- 16,221
- 11
- 71
- 127
15
votes
3 answers
Does Java allow a volatile read to be optimized away if the value isn't needed, also removing the happens-before synchronization?
The following code sample shows a common way to demonstrate concurrency issues caused by a missing happens-before relationship.
private static /*volatile*/ boolean running = true;
public static void main(String[] args) throws…
stonar96
- 1,359
- 2
- 11
- 39
12
votes
2 answers
Happens-before for direct ByteBuffer
I have a direct ByteBuffer (off-heap) in one thread and safely publish it to a different thread using one of the mechanisms given to me by JMM. Does the happens-before relationship extend to the native (off-heap) memory wrapped by the ByteBuffer? If…
Philippe Marschall
- 4,452
- 1
- 34
- 52
10
votes
2 answers
Do I have reordering issue and is it due to reference escape?
I have this class where I cache instances and clone them (Data is mutable) when I use them.
I wonder if I can face a reordering issue with this.
I've had a look at this answer and JLS but I am still not confident.
public class DataWrapper {
…
Gurwinder Singh
- 38,557
- 6
- 51
- 76
10
votes
2 answers
Does .awaitTermination() establish happens-before with work done in the executor?
Question I've had for years:
In this pseudocode,
ExecutorService svc = Executors.newFixedThreadPool(3);
svc.submit(new Runnable() { /* code A */ });
svc.shutdown();
if(svc.awaitTermination(...)) {
// code B
.awaitTermination() is not documented…
tariksbl
- 1,069
- 6
- 20
10
votes
4 answers
Java Happens-Before and Thread Safety
Suppose I have a class which wraps a HashMap as follows:
public final class MyClass{
private final Map map;
//Called by Thread1
public MyClass( int size ){
this.map = new HashMap( size );
…
CaptainHastings
- 1,557
- 1
- 15
- 32
8
votes
3 answers
How to understand the channel communication rules in golang memory model?
Learning golang on the way, I got a little confused when trying to understand the channel communications described in the memory model spec as below:
A send on a channel happens before the corresponding receive from that channel completes.
The…
Roy Ling
- 1,592
- 1
- 14
- 31
8
votes
1 answer
How deep volatile publication guarantees?
As is known guarant that if we have some object reference and this reference has final field - we will see all reachable fields from final field(at least when constructor was finished)
example 1:
class Foo{
private final Map map;
Foo(){
…
gstackoverflow
- 36,709
- 117
- 359
- 710
7
votes
1 answer
How are the final multi-threading guarantees and the memory model related in Java?
The memory model is defined in 17.4. Memory Model.
The final field multi-threading guarantees are given in 17.5. final Field Semantics.
I don't understand why these are separate sections.
AFAIK both final and the memory model provide some…
poq
- 71
- 3
7
votes
5 answers
Happens-before rules in Java Memory Model
I am currently studying for a concurrent programming exam and don't understand why the output of this program is 43. Why is x = y + 1 executed before t.start()? I also should explain which happens-before rules I used.
If I understand the program…
lmaonuts
- 125
- 9
7
votes
1 answer
Does this non-standard Java synchronization pattern work?
Let's say I have two threads running like this:
Thread A which performs computation while updating pixels of a shared image
Thread B periodically reads the image and copies it to the screen
Thread A performs work quickly, say 1 million updates per…
Nayuki
- 17,911
- 6
- 53
- 80
7
votes
2 answers
Does a Java Lock object enforce a happens-before relationship?
Java provides a Lock object in the concurrency package that according to the documentation provides more extensive locking operations than can be obtained using synchronized methods and statements.
The synchronized methods/blocks besides mutual…
Petrakeas
- 1,521
- 15
- 28