private static final long serialVersionUID = 1L;

Question

What is private static final long serialVersionUID = 1L in the below code?

public class Authenticator extends HttpServlet {

    private static final long serialVersionUID = 1L;
    
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
        String username = request.getParameter("username");
        String password = request.getParameter("mypassword");
        if(authenticate(username, password)) {
            
            HttpSession session = request.getSession();
            session.setAttribute("username",username);
            response.sendRedirect("product.jsp");
            return;
        }
        else {
            response.sendRedirect("index.jsp");
            return;
        }
    }
}

Answer 1

First, few quick pointers regarding this.

1. It plays a role during serialization and deserialization. So for the classes e.g. MyCustomRuntimeException where you don't care about serialization, you could ignore it.
2. If you are using it on classes for serialization purposes but would not store them for any longer durations - more specifically if you would never create a backward incompatible version of that class during this storage, you would not need to change this.
3. For the use cases where you know you would permanently store it and would create new incompatible version, make sure to increment(uniquely change) it.

From java.io.Serializable

The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender's class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named "serialVersionUID" that must be static, final, and of type long: ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;

If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class--serialVersionUID fields are not useful as inherited members. Array classes cannot declare an explicit serialVersionUID, so they always have the default computed value, but the requirement for matching serialVersionUID values is waived for array classes.

From Joshua Bloch in Effective Java

the automatically generated UID is generated based on a class name, implemented interfaces, and all public and protected members. Changing any of these in any way will change the serialVersionUID. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations and can produce different serialVersionUID in different environments. This can result in unexpected InvalidClassException during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier in serialVersionUID where possible, since such declarations apply only to the immediately declaring class.