Is thread safe calling outer class variable?

Question

public class Status {

    private Scheduler scheduler;
    private volatile Boolean lastResult;

    public Status() {
        scheduler = new Scheduler();
        scheduler.everyTenSeconds(new Action());
    }


    public boolean isSth()  {
        if (lastResult != null && lastResult) {
            return lastResult;
        } else {
            return checkSth();
        }
    }


    private boolean checkSth() throws SomeException {
        // checking sth
    }

    private class Action implements Runnable {

        @Override
        public void run() {
            try {
                lastResult = checkSth();
            } catch (SomeException e) {
                lastResult = false;

            }

        }
    }
}

Is calling outer class variable is thread safe? Last result is lastResult but while calling isSth() lastResult is always null. Despite run was called twice.

Can you share the example code you're running to produce the problem you describe? — Mick Mnemonic, Sep 18 '17 at 12:26
Sorry but i didnt understand, are you trying to call thread which has an outer class as variable or thread which has a variable from outer class or calling a thread from outer class — Jeredriq Demas, Sep 18 '17 at 12:36

Stephen C · Answer 1 · 2017-09-19T03:39:35.797

Is thread safe calling outer class variable?

In general, it is neither thread-safe or not thread-safe. The thread safety depends on what you actually do.

And ... to be strictly correct ... you cannot call a variable:

You can read the value of a variable.
If the value of the variable is an object, then you can call an instance method of that object.

In your example, the code is doing sequences of operations involving a volatile. A single read or write operation on a volatile is atomic and has clearly defined properties with regards to the Java Memory Model. However, a sequence of operations is NOT atomic, and the JMM properties are complicated.

To cut a long story short, your code is performing sequences of read and write operations on the lastResult and is NOT thread-safe.

However, despite that, I don't see how / why your code is consistently seeing lastResult set to null. I suspect that it is something to do with the way that you are using this code. We really need an MCVE; i.e. a complete and executable example that we can run to see the behavior that you are seeing.

Is thread safe calling outer class variable?

1 Answers1