Java Web App中每个用户的单例对象

Question

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;
    }
}

Answer 1

A cart is usually stored in the session of the user. So the cart should be SessionScoped or ViewScoped.

You can include it via injection (cdi, autowired, ...). Spring and JavaEE have these well documented.

In a Web Application where multiple users can access the backend nothing should be static except explicitly wanted. It may cause massive synchronization and concurrency problems.