import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import java.sql.*;
import ConnectionPool.*;
/*
This servlet acts as a cache for domain values. In our
Catalog Example it caches the Category Names and Codes.
Preventing excessive database queries.
*/
public class CatalogDomainServlet extends HttpServlet {
// Method Name: init()
// Purpose: This is the default init() method.
public void init(ServletConfig config)
throws ServletException {
super.init(config);
initializeDomains();
}
// Method Name: initializeDomains()
// Purpose: This method gets a reference to the database
// ConnectionPool from the ServletContext. It then performs
// a query getting all of the Category values in the
// database. These values are then stored in a Vector and
// placed in the ServletContext keyed by the string
// DOMAIN_LIST.
private void initializeDomains() throws ServletException {
Connection con = null;
ConnectionPool pool = null;
Vector categories = new Vector(8);
Category category = null;
try {
// Get a reference to the ConnectionPool from the Global
// ServletContext
ServletContext context = getServletContext();
// Get a connection from the ConnectionPool
if ( context == null ) {
throw new ServletException("Could not get a reference" +
" to the ServletContext");
}
pool =(ConnectionPool)
context.getAttribute("CONNECTION_POOL");
// Get a connection from the ConnectionPool
if ( pool == null ) {
// Sleep for 5 seconds and try again
try {
Thread.sleep(5000);
pool =(ConnectionPool)
context.getAttribute("CONNECTION_POOL");
}
catch (Exception e) {
throw new ServletException(e.getMessage());
}
if ( pool == null ) {
// Still could not get reference
throw new ServletException("Could not get reference" +
" to the CONNECTION_POOL");
}
}
con = pool.getConnection();
// Get a connection from the ConnectionPool
if ( con == null ) {
throw new ServletException("Could not get reference" +
" Connection.");
}
if ( con != null ) {
// Create the statement
Statement statement = con.createStatement();
// Use the created statement to SELECT the DATA
// FROM the Titles Table.
ResultSet rs = statement.executeQuery("SELECT * " +
"FROM Categories");
// Iterate over the ResultSet
while ( rs.next() ) {
category = new Category();
category.setId(rs.getInt("category_id"));
// get the name, which is a String
category.setName(rs.getString("category_name"));
// add the resulting category to the Vector
categories.addElement(category);
}
// Close the ResultSet
rs.close();
// Once the domain list is Initailized, add it to the
// Global ServletContext. This makes it available
// To other servlets using the same ServletContext.
context.setAttribute("DOMAIN_LIST", categories);
}
}
catch (SQLException sqle) {
System.err.println(sqle.getMessage());
}
catch (Exception e) {
System.err.println(e.getMessage());
}
finally {
// Release the connection
pool.releaseConnection(con);
}
}
// Method Name: doGet()
// Purpose: This method services GET requests. Its only
// purpose is to prove that the domain values are loading
// properly.
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = new PrintWriter (response.getOutputStream());
out.println("");
out.println("CatalogServlet");
out.println("");
// Get reference to ServletContext
ServletContext context = getServletContext();
// Get reference to the DOMAIN_LIST Vector
Vector categories = (Vector)context.getAttribute("DOMAIN_LIST");
// Itereate over the list of categories
for (int x = 0; x < categories.size(); x++ ) {
Category category = (Category)categories.elementAt(x);
out.println(category.getId() + "-->" + category.getName() + "
");
}
out.println("");
out.close();
}
// Method Name: destroy()
// Purpose: This method performs cleanup on the Vector
// and the ServletContext.
public void destroy() {
// Access the ServletContext using the getAttribute()
// method, which returns a reference to the Categoies.
ServletContext context = getServletContext();
Vector categories =
(Vector)context.getAttribute("DOMAIN_LIST");
if ( categories != null ) {
// Empty List
categories.removeAllElements();
// Remove the Attribute from the ServletContext
context.removeAttribute("DOMAIN_LIST");
}
else {
System.err.println("Could not get a reference to DOMAIN_LIST!");
}
}
//Get Servlet information
public String getServletInfo() {
return "CatalogDomainServlet Information";
}
}