evercookie-master/assets/EvercookieEtagServlet.java
package evercookie;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * This is a Java Servlet port of evercookie_etag.php, the server-side component
 * of Evercookie that uses the If-None-Match and Etag headers to keep track of
 * persistent values.
 * 
 * Install this servlet at /evercookie_etag.php in your web.xml (or add a @WebServlet
 * annotation) and you won't even need to modify evercookie.js! This assumes
 * that Evercookie's assets are in your web root.
 * 
 * Of course, if you have set $_ec_baseurl to something, you should install this
 * at [$_ec_baseurl]evercookie_etag.php. Remember, $ec_baseurl needs a trailing
 * slash in the evercookie.js.
 * 
 * @author Gabriel Bauman <gabe@codehaus.org>
 * 
 */
public class EvercookieEtagServlet extends HttpServlet {

	private static final long serialVersionUID = 1L;

	public EvercookieEtagServlet() {
		super();
	}

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

		boolean cookieExists = false;
		String cookieValue = null;
		Cookie[] cookies = req.getCookies();

		if (null != cookies) {
			// Iterate over cookies until we find one named evercookie_etag
			for (Cookie cookie : cookies)
			{
				if (cookie.getName().equals("evercookie_etag")) {
					cookieExists = true;
					cookieValue = cookie.getValue();
					break;
				}
			}
		}

		ServletOutputStream body = resp.getOutputStream();

		try {

			if (cookieExists) {
				// Cookie set; send cookie value as Etag header/response body.
				resp.addHeader("Etag", cookieValue);
				body.print(cookieValue);
			}
			else
			{
				// No cookie; set the body to the request's If-None-Match value.
				body.print(req.getHeader("If-None-Match"));
			}

		} finally {
			// close the output stream.
			body.close();
		}

		resp.setStatus(200);

	}
}
