CallbackServlet.java

package getAccessToken;

import java.io.IOException;
import java.util.logging.Logger;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import twitter4j.Twitter;
import twitter4j.TwitterException;
import twitter4j.auth.AccessToken;
import twitter4j.auth.RequestToken;

@SuppressWarnings("serial")
public class CallbackServlet extends HttpServlet {
	private static final Logger log = Logger.getLogger(CallbackServlet.class.getName());

	public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {

		RequestToken requestToken = (RequestToken) req.getSession().getAttribute("requestToken");
		Twitter twitter = (Twitter) req.getSession().getAttribute("twitter");
		String verifier = req.getParameter("oauth_verifier");

		AccessToken accessToken = null;
		try {
			accessToken = twitter.getOAuthAccessToken(requestToken, verifier);
		} catch (TwitterException e) {
			log.warning(e.getMessage());
		}

		if (accessToken != null) {
			// アクセストークンを永続化する
		}

		req.getSession().removeAttribute("requestToken");

		resp.sendRedirect(req.getContextPath() + "/");
	}
}

error text

getAccessToken.GetAccessTokenServlet doPost: 401:Authentication credentials (http://dev.twitter.com/pages/auth) were missing or incorrect. Ensure that you have set valid conumer key/secret, access token/secret, and the system clock in in sync.
<?xml version="1.0" encoding="UTF-8"?>
<hash>
  <request>/oauth/request_token</request>
  <error>Desktop applications only support the oauth_callback value 'oob'</error>
</hash>

GetAccessTokenServlet.java

package getAccessToken;

import java.io.IOException;
import java.util.logging.Logger;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import twitter4j.Twitter;
import twitter4j.TwitterException;
import twitter4j.TwitterFactory;
import twitter4j.auth.RequestToken;

@SuppressWarnings("serial")
public class GetAccessTokenServlet extends HttpServlet {
	private static final Logger log = Logger.getLogger(GetAccessTokenServlet.class.getName());

	@Override
	public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {

		StringBuffer callbackURL = req.getRequestURL();
		int index = callbackURL.lastIndexOf("/");
		callbackURL.replace(index, callbackURL.length(), "").append("/callback");

		try {
			Twitter twitter = new TwitterFactory().getInstance();

			// ここでtwitter.setOAuthConsumer("consumerKey", "consumerSecret")を呼ぶとIllegalStateExceptionになるから
			// この二つはちゃんとセットされているっぽい

			RequestToken requestToken = twitter.getOAuthRequestToken(callbackURL.toString());

                        // たぶん、↑ここで失敗してる。このページから遷移しない。

			req.getSession().setAttribute("requestToken", requestToken);
			req.getSession().setAttribute("twitter", twitter);
			resp.sendRedirect(requestToken.getAuthenticationURL());

		} catch (TwitterException e) {
			log.warning(e.getMessage());
		}
	}
}