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javaclassvolatile

Question regarding Java volatile keyword for reference types

I understand the volatile keyword in Java can make the read/write operations of reference variables and all primitives except long and double, atomic in nature.

I also know compound statements such as incrementing an integer, var++, are not atomic and should not be used in place of synchronized statements.

But what about this class?

public class Setter{

    private int num = 0;

    public int setNum(int numIn){
        num = numIn;
        return num;
    }
}

Now say you declare an instance of Setter as volatile.

public class Main {
     private volatile Setter s;

     public static void main(String[] args){
         s = new Setter();
         s.setNum(5);
     }
}

Is a call to the method setNum atomic? Is this statement safe if multiple threads are calling it at once, each with different values?

What would be an example of using a volatile class in a proper way, and using it in an unsafe compound statement?

Solution

  • Is a call to the method setNum atomic?

    No, it's not. It is only the reads / writes to s that would be volatile.

    This can be compared with letting a List be final. This is not sufficient to make the list immutable, only the list-reference itself.