Singleton object per user in java web app

My Enterprise application requires an Singleton ShoppingCart object(per user),this cart object should not be technically static(which may lead to memory leak).

do i need to make changes to the code (or) apply any pattern to the web application, to achieve singleton per user?

public final class ShoppingCartSingleTon {

    private static ShoppingCartSingleTon instance = null;

    private ShoppingCartSingleTon() {}

    public static ShoppingCartSingleTon getInstance() {
        if (instance == null) {
            synchronized(ShoppingCartSingleTon.class) {
                if (instance == null) {
                    instance = new ShoppingCartSingleTon();
                }
            }
        }
        return instance;
    }
}