Question

I know that singleton classes are inherently not thread-safe. But is there any way to make it so? I have a singleton in a java web application, and I want one class to exist per user of this program.

This is an issue because the singleton holds state, otherwise it’d be fine.

Answer 1

I have a singleton in a java web application, and I want one class to exist per user of this program.

It makes no sense to make it a singleton. Just make it a session attribute. The HttpSession is tied to the browser session and specific for every visitor. For example, the shopping cart:

Cart cart = (Cart) session.getAttribute("cart");

if (cart == null) {
    cart = new Cart();
    session.setAttribute("cart", cart);
}

// ...

It can just be a simple Javabean.

See also:

• How do servlets work? Instantiation, sessions, shared variables and multithreading