Java - How to check an existing user in the database (for a signup interface)?

Question

So here's what I've done so far. it is also already connected to the database with the username as the primary key.

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("text/html;charset=UTF-8");
    try {
        Accounts bean = new Accounts();
        bean.setUsername(request.getParameter("uname"));
        bean.setName(request.getParameter("fname"));
        bean.setEmail(request.getParameter("email"));
        bean.setPassword(request.getParameter("password"));
        AccountsDAO aO = new AccountsDAO();
        aO.addAccount(bean);
        response.sendRedirect("Stream.jsp");
    }
    finally{

    }
}

what i want is, when i try to register an account with the same username/email of any existing user in the database, it will redirect to another with another page also using a response.sendRedireect("file.jsp"); that will require the user to register again with another username/email. Thank you in advance!

here is my AccountsDAO

public void addAccount(Accounts bean) {
    try {
        bConnector = new DBConnector();
        Connection c = bConnector.getConnection();
        String query = "insert into account (username, name, email, password) values (?,?,?,?)";
        PreparedStatement preparedstatement = c.prepareStatement(query);
        preparedstatement.setString(1, bean.getUsername());
        preparedstatement.setString(2, bean.getName());
        preparedstatement.setString(3, bean.getEmail());
        preparedstatement.setString(4, bean.getPassword());
        preparedstatement.executeUpdate();
    } catch (SQLException ex) {
        Logger.getLogger(AccountsDAO.class.getName()).log(Level.SEVERE, null, ex);
    }
}

Answer 1

Once you have username as primary it will not allow to insert duplicates . so try like this

try
{
//query to insert your username which you get from user
}

If it is duplicate it will throw an exception as ora- so handle the exception using page redirect using catch block.

catch(Exception e)
{
system.out.print(e);
response.sendRedirect("file.jsp");
}

Update

Declare an integer variable to check whether you have inserted the value . Also change the return type of the method addAccount(Accounts bean) to int

public int addAccount(Accounts bean) {

    int count=0; 
    try {
        bConnector = new DBConnector();
        Connection c = bConnector.getConnection();
        String query = "insert into account (username, name, email, password) values (?,?,?,?)";
        PreparedStatement preparedstatement = c.prepareStatement(query);
        preparedstatement.setString(1, bean.getUsername());
        preparedstatement.setString(2, bean.getName());
        preparedstatement.setString(3, bean.getEmail());
        preparedstatement.setString(4, bean.getPassword());
        count=preparedstatement.executeUpdate();
    } catch (SQLException ex) {
        Logger.getLogger(AccountsDAO.class.getName()).log(Level.SEVERE, null, ex);
    }
 return count;
}

In your servlet,

int count=0;
try {
        Accounts bean = new Accounts();
        bean.setUsername(request.getParameter("uname"));
        bean.setName(request.getParameter("fname"));
        bean.setEmail(request.getParameter("email"));
        bean.setPassword(request.getParameter("password"));
        AccountsDAO aO = new AccountsDAO();
        count=aO.addAccount(bean);
        if(count>0){
        response.sendRedirect("Stream.jsp"); \\Success condition here
        }
        else{
         response.sendRedirect("file.jsp");   \\ failure condition here
       }
    }
    catch(Exception e)
{
system.out.print(e);
}

Hope this helps!