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Ajay Babuu Kunapareddyy
Ajay Babuu Kunapareddyy

Reputation: 11

Java threads not sharing static data

I have two threads which are supposed to share the static variable data(not constant) and they have to execute accordingly. But none of these threads are able to get the updated static variable data, besides each maitaining it's own state for the static variable.

I have placed the static variable in a singleton and declared it as volatile, still the result is same.

Can someone suggest the problem with this approach, and what could be done to resolve this.

Thx, Aj

below is the code

following is the singleton class

***************************************************************************
public class ThreadFinder {

    public static String pageName;//HashMap threadInfo = new HashMap(); //new HashMap();

private static ThreadFinder singletonObject;
/** A private Constructor prevents any other class from instantiating. */
private ThreadFinder () {
    }
public static synchronized ThreadFinder getSingletonObject() {
    if (singletonObject == null) {
        singletonObject = new ThreadFinder();
    }
    return singletonObject;
}
public static synchronized void setPageName(String Name) {

    pageName = Name;
    //return;
}
public static synchronized String getPageName() {

    return pageName;
    //return;
}
public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
}
//public static void main(String args[])
//{
    //ThreadFinder.getSingletonObject().setPageName("IDEN");
    //System.out.println("page name--------->"+ThreadFinder.getSingletonObject().getPageName());
    //ThreadFinder.getSingletonObject().setPageName("THER");
    //System.out.println("page name--------->"+ThreadFinder.getSingletonObject().getPageName());
//}
}

********************************************************************************

from page 1 below code executes and sets the singleton page variable value to "A" and first pollertimer thread uses the page value as A.

m_poller.setModbusMaster(this.m_connection.getModbusMaster());
m_poller.addListener(this);
ThreadFinder.getSingletonObject().setPageName("A");  //Setting the page name here
PollerTimer polerTimerThread = new PollerTimer(period,"A");  // this is thread

*********************************************************************************

from page2 below code executes and sets the singleton page variable value to "B" and second pollertimer thread uses the page value as B.

m_poller.setModbusMaster(this.m_connection.getModbusMaster());
m_poller.addListener(this);
ThreadFinder.getSingletonObject().setPageName("B");  //Setting the page name here
PollerTimer polerTimerThread = new PollerTimer(period,"B");  // this is thread

***********************************************************************************

Now when first pollertimer thread queries page value using getPageName() it is getting value A instead of B, though it was updated to B by the second thread.

Upvotes: 0

Views: 293

Answers (1)

Marko Topolnik
Marko Topolnik

Reputation: 200166

Declaring your reference to the singleton volatile is not going to help if you intend to mutate its state. You need to either use an immutable singleton referred to by a volatile var, or make each individual var in that singleton volatile. And that's only in case you don't have atomicity issues as well. The second approach is pretty much the same as not having a singleton at all, just a number of static volatile global vars.

Upvotes: 3

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