java code for Block Filter

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

public class BlockFilter implements Filter {

private FilterConfig config;

/** Creates new BlockFilter */
public BlockFilter() {
}

public void init(FilterConfig filterConfig) throws ServletException {

this.config = filterConfig;
}

public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest req = null;
boolean authenticated = false;
PrintWriter out = null;

if (request instanceof HttpServletRequest) {

req = (HttpServletRequest) request;
String user = req.getParameter("user");
authenticated = authenticateUser(user);
}

if (authenticated) {
chain.doFilter(request, response);
} else {

response.setContentType("text/html");
out = response.getWriter();
out
.println("");
out.println("

Sorry your authentication attempt failed

");

out.println("");
out.close();

}
}// doFilter

public void destroy() {
/*
* called before the Filter instance is removed from service by the web
* container
*/
}

private boolean authenticateUser(String userName) {

//authenticate the user using JNDI and a database, for instance
return false;

}
}