Problem with jsp

I have a form (with a few fields)..basically a singup form. and i have the username field which should be chosen to be unique by the user. For this i have to connect to database,chec开发者_StackOverflow社区k whether such a username exists and then display an appropriate message in the same form. All this must happen when i click the "check Availabilty" button. How should i handle this

If you want to do it all in one form (not recommended), you can write a single JSP using JSTL <sql> tags.

A better approach would be to break the problem up into parts. Start with a Credential class:

package model;

public class Credential
{
    private String username;
    private String password;

    // Add the rest.
}

Have a DAO interface that lets you get a Credential out of a database:

package persistence;

public interface CredentialDao
{
    List<Credential> find();
    Credential find(String username);
}

Write a servlet that fields requests from your JSP with the form, validates and binds the input, uses a CredentialDao to search for the Credential given a username, and then decides what to send as the response to the JSP by examining what comes back.

1. get username
2. check if it exists using "select username from..."
3. if it exists, redirect user to an error page telling them that user already exists (or use Ajax to tell them the same thing on one page)
4. if it doesn't exist, register a new user using "insert username to..." or whatever mechanism you're using and then tell user registration was sucessfull