Question

In the below code listings, are Statement 1 and Statement 2 thread safe or not? They are using the VolatileIntWrapper.

If they not thread safe, which statements need to be wrapped in synchronized block?

public class Demo {

    public static void main(String[] args) {

        VolatileIntWrapper volatileIntWrapper = new VolatileIntWrapper() ;

        for(int i = 1 ; i <= 5 ; ++i){
            new ModifyWrapperIntValue(volatileIntWrapper).start() ; 
        }
    }
}

class VolatileIntWrapper{
    public volatile int value = 0 ;
}

class ModifyWrapperIntValue extends Thread{

    private VolatileIntWrapper wrapper ;
    private int counter = 0 ;

    public ModifyWrapperIntValue(VolatileIntWrapper viw) {
        this.wrapper = viw ;
    }

    @Override
    public void run() {

        //randomly increments or decrements VolatileIntWrapper primitive int value

        //we can use below statement also, if value in VolatileIntWrapper is private
        // wrapper.getValue() instead of wrapper.value
        //but, as per my understanding, it will add more complexity to logic(might be requires additional synchronized statements),
        //so, for simplicity, we declared it public


        //Statement 1
        while(wrapper.value > -1500 && wrapper.value < 1500){
            ++counter ;
            int randomValue = (int) (Math.random() * 2) ;

            //Statement 2
            wrapper.value += (randomValue == 0) ?   1       :       -1 ;
        }

        System.out.println("Executed " + counter + " times...");
    }
}

Answer 1

The volatile keyword provides a memory barrier for both reading and writing a field. That means that multiple threads can access the field and be guaranteed to read the most current value and their writes are guaranteed to be seen by other threads.

What volatile does not do is provide any guarantees around the order of operations — especially when you have multiple read and write statements. In your code you are accessing the volatile int a couple of places in your loop:

    while(wrapper.value > -1500 && wrapper.value < 1500){
        ...
        wrapper.value += (randomValue == 0) ?   1       :       -1 ;
    }

There are no guarantees as to the order of operations here. Immediately after thread A tests the value > -1500, another thread might change it before thread A can test value < 1500. Or thread A might do both tests, then thread B might do both tests, then thread A would assign the value, and then thread B would assign the value. That is the nature of multithreading race conditions.

The while loop is the section of code that I suspect would be considered have a bug unless you synchronize around it. You should do something like the following. Once you are synchronizing that section, the synchronized keyword provides the memory barrier itself and so the volatile keyword is unnecessary.

   synchronized (wrapper) {
       while (...) {
         ...
       }
   }