Using public variables in a servlet

Question

Hi, I have a public variable "CustID" in my servlet. Note that is a public member variable and not a static variable.
In the doPost() method, I am trying to use the custId field.
Pls. look at the code below. I want to know that in a multi-user environment since the custId field is defined at Servlet level as a member variable, is there a chance for this variable to be corrupted and I fire a wrong query ???
Awaiting your answers
public class QWCustomerServlet extends HttpServlet
public boolean debug = false;
public QWFieldList fieldList = new QWFieldList();
public String custID = null;
public doPost(HttpServletRequest req, HttpServletResponse res)
HttpSession session = null;
if ( (session = request.getSession(false))!= null){ 
custId = session.getAttribute("cid");
String query = "select * from customer where cust_id=" + custId;
Statement stmt = conn.createStatement();
stmt.executeQuery(query);
}

Answer 1

Yes. There is only one instance of a servlet and all requests thread through it. Don't use instance variables unless you want all threads to share (usually in a read-only manner such as a reference to a resource). In your example, move the custId declaration inside doPost and it will be thread-safe.