volatile keyword doesnt synchronize the values - multithreading

Question

Below piece of code gives me result as follows:
Change listener
Change listener
Change listener
Incrementing MY_INT to:1
Incrementing MY_INT to:2
Got Change for MY_INT :1loc:0
Change listener
Change listener
Change listener
Change listener
Change listener
Change listener
Change listener
Change listener
Change listener
Incrementing MY_INT to:3
Change listener
Incrementing MY_INT to:4
Incrementing MY_INT to:5
Got Change for MY_INT :3loc:2
Change listener
Here my question is, since i used volatile, MY_INT value should be reflect as either 4 or 5. Why it is not reflecting. Soemtimes, even for that value we are getting 1. Please someone let me know where i am going wrong.
public class VolatileTest {
private static volatile int MY_INT = 0;
public static void main(String[] args) {
new ChangeListener().start();
new ChangeMaker().start();
}
static class ChangeListener extends Thread {
#Override
public void run() {
int local_value = MY_INT;
while ( local_value < 5){
if( local_value!= MY_INT){
System.out.println("Got Change for MY_INT :"+ MY_INT+"loc:"+local_value);
local_value= MY_INT;
}
System.out.println("Change listener");
}
}
}
static class ChangeMaker extends Thread{
#Override
public void run() {
int local_value = MY_INT;
while (MY_INT <5){
System.out.println("Incrementing MY_INT to:"+(local_value+1));
MY_INT = ++local_value;
}
}
}
}

Answer 1

Here's one thing that can happen:
ChangeMaker ChangeListener
... ...
MY_INT = 3
Sees MY_INT != 2, calls println(...)
MY_INT = 4
MY_INT = 5
local_value = MY_INT; // 5
all done (MY_INT==5) all done (MY_INT==5)
The change listener can miss seeing changes because the test local_value!=MY_INT is not atomic with the assignment local_value=MY_INT;

Answer 2

You did alright except one thing, you did not add synchronisation while accessing you volatile variable. Unlike atomics volatile variables guarantee you getting actual value but not changing. I guess it's better to use atomics as AtomicInteger in your case. Also you can change your int to Integer add extra synchronizedto your code as at example below.
public class VolatileTest {
private static volatile Integer MY_INT = 0;
public static void main(String[] args) {
new ChangeListener().start();
new ChangeMaker().start();
}
static class ChangeListener extends Thread {
#Override
public void run() {
int local_value = MY_INT;
while (local_value < 5) {
// ! Here
synchronized (MY_INT) {
if (local_value != MY_INT) {
System.out.println("Got Change for MY_INT :" + MY_INT + "loc:" + local_value);
local_value = MY_INT;
}
}
System.out.println("Change listener");
}
}
}
static class ChangeMaker extends Thread {
#Override
public void run() {
int local_value = MY_INT;
while (MY_INT < 5) {
System.out.println("Incrementing MY_INT to:" + (local_value + 1));
// ! And here
synchronized (MY_INT) {
MY_INT = ++local_value;
}
}
}
}
}