Does a lock on class, locks class variables too? - java

Question

I have the below class

public class Example{

    public static List<String> list = new ArrayList<String>();

    public static void  addElement(String val){
        synchronized(list){
            list.add(val);
        }
    }

    public static synchronized void printElement(){
        Iterator<String> it = list.iterator();
        while(it.hasNext()){
            //print element
        }
    }
}

Will the iterator() call in the printElement method throw ConcurrentModificationException? The basic question is if the lock on class object is acquired(as done in printElement method), will it lock the class members/ variables too? please help me with the answer.

Answer 1

Does a lock on class, locks class variables too? - java

Your lock is on your instance, not your class. And no, it only locks the instance.

Will the iterator() call in the printElement method throw ConcurrentModificationException?

It will if the code in that method modifies the list during the iteration. But if all of your code in that class also synchronizes, and you haven't given a reference to that list to anything outside your class, then you know that only the code in that method is running.

You'd probably be better off, though, synchronizing on the list itself. That way, even if you've given out a reference to the list, assuming all code that uses it synchronizes on it, you'll be safe from concurrent mods:

public static void printElement(){
//            ^--- No `synchronized ` here unless you REALLY need it for other reasons

    synchronized (list) {
        Iterator<String> it = list.iterator();
        while(it.hasNext()){
            //print element
        }
    }
}

If you are giving out references and want to be really sure, either use a list returned by Collections.synchronizedList or something from the java.util.concurrent package.

Answer 2

No, a synchronized method does not lock the object variables, a synchronized method will lock only this.

Your code is not thread safe, since you are locking on different objects on addElement and printElement. There is nothing preventing the insertion to occur while iterating the list, if both method are called concurrently.