Java multithreading ConcurrentModificationException

Question

I have several tests that run in parallel, and uses the method below. Please see the code below it throws ConcurrentModificationException occasionally. I cannot figure out how it can happen?


private static MyObject myObject; 

public void setupMyObject{
    syncronized(this){
       myObject = Optional.ofNullable(myObject).orElse(SomeConfig.ofDefaults());
    }
}

Could you show more code? What you've shown is where you think the problem lies. However that may not be the case and furthermore, even if it turned out to be, there is no context to form an answer. — GetSet
– GetSet, Commented Jan 17, 2020 at 23:14
That code cannot throw ConcurrentModificationException. --- Besides, that code is really badly flawed: ofNullable(myObject) means that myObject can be null, and by it's very nature, that orElse(...) can never be effectual, because can only have an effect when myObject is null, which would make myObject.ofDefaults() throw a NullPointerException. Unless of course ofDefaults() is a static method, in which case the code should have been MyObject.ofDefaults(), using the class, not the instance, to qualify the method call, i.e. the code is flawed. — Andreas
– Andreas, Commented Jan 17, 2020 at 23:27

Salim · Accepted Answer · 2020-01-17 23:48:37Z

2

myObject is a static variable, to lock it you need to put class object in synchronized. Your present implementation doesn't lock it properly.

import java.util.Optional;

public class Test {

    private static String myObject;

    public void setupMyObject(){
        synchronized(Test.class){
            myObject = Optional.ofNullable(myObject).orElse(SomeConfig.ofDefaults());
        }
    }

}

answered Jan 17, 2020 at 23:48

Salim

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