FacebookServlet.java

import java.io.*;
import java.net.*;
import java.util.logging.Logger;

import javax.servlet.ServletException;
import javax.servlet.http.*;

@SuppressWarnings("serial")
public class FacebookServlet extends HttpServlet {

  static final Logger logger = Logger.getLogger(FacebookServlet.class.getName());

  @Override
  protected void doGet(HttpServletRequest request,
      HttpServletResponse response) throws ServletException, IOException {
    String state = request.getParameter("state");
    if ("getAuthCode".equals(state) == false) {
      respondRedirectToGetAuthorizationCode(request, response);
    }
    String error = request.getParameter("error");
    if ("access_denied".equals(error)) {
      respondForShowDeniedByUser(request, response);
    } else {
      respondForAuthorizedByUser(request, response);
    }
  }

  void respondForAuthorizedByUser(HttpServletRequest request,
      HttpServletResponse response) throws IOException {
    String code = request.getParameter("code");
    String accessTokenResponse = retrieveAccessToken(code,
        buildServerNameAndPort(request) + PATH);
    response.setContentType("text/plain");
    response.setCharacterEncoding("utf-8");
    response.getWriter().println("retrieveAccessToken=" + accessTokenResponse);
    String[] splittedResponses = accessTokenResponse.split("\\&");
    String accessToken = null;
    for (String splittedResponse : splittedResponses) {
      if (splittedResponse.startsWith("access_token=")) {
        accessToken = splittedResponse.split("=")[1];
        break;
      }
    }
    response.getWriter().println("accessToken=" + accessToken);
    response.getWriter().println();

    // 試しに取得したアクセストークンを使って、いくつかの情報にアクセスしてみる。
    // アクセストークンの期限が切れていた場合は、それとわかる400が返ってくるので、再取得しなければならない。
    // https://developers.facebook.com/docs/authentication/access-token-expiration/
    PrintWriter writer = response.getWriter();
    HttpURLConnection c;
    BufferedReader reader;
    String line;

    // ユーザ自身の情報
    c = (HttpURLConnection) new URL(
        "https://graph.facebook.com/me?access_token=" + accessToken)
        .openConnection();
    reader = new BufferedReader(new InputStreamReader(c.getInputStream(), "utf-8"));
    while ((line = reader.readLine()) != null) {
      writer.println(line);
    }
    response.getWriter().println();

    // 友達の一覧
    c = (HttpURLConnection) new URL(
        "https://graph.facebook.com/me/friends?limit=5&access_token="
            + accessToken).openConnection();
    reader = new BufferedReader(new InputStreamReader(c.getInputStream(), "utf-8"));
    while ((line = reader.readLine()) != null) {
      writer.println(line);
    }
    response.getWriter().println();

    response.flushBuffer();
  }

  void respondForShowDeniedByUser(HttpServletRequest request,
      HttpServletResponse response) throws IOException {
    response.setContentType("text/plain");
    response.setCharacterEncoding("utf-8");
    response.getWriter().println(request.getQueryString());
    response.flushBuffer();
  }

  void respondRedirectToGetAuthorizationCode(HttpServletRequest request,
      HttpServletResponse response) throws IOException {
    response.sendRedirect(getUrlToGetAutherizationCode(buildServerNameAndPort(request)
        + PATH));
  }

  static CharSequence buildServerNameAndPort(HttpServletRequest request) {
    StringBuilder b = new StringBuilder(request.getScheme()).append("://")
        .append(request.getServerName());
    if (request.getServerPort() != 80 && request.getServerPort() != 443) {
      b.append(":").append(request.getServerPort());
    }
    return b;
  }

  static final String CLIENT_ID = System.getProperty("fb.client_id");

  static final String CLIENT_SECRET = System.getProperty("fb.client_secret");

  static final String PATH = "/fb";

  /**
   * @param callbackURI
   * @return 認可コードを取得するためのURL
   * @throws IOException
   * @author shin1ogawa
   * @see <a href="https://developers.facebook.com/docs/authentication/server-side/">Core Concepts › Authentication › Server-Side Authentication</a>
   * @see <a href="https://developers.facebook.com/docs/authentication/permissions/">Core Concepts › Authentication › Permissions</a>
   */
  static String getUrlToGetAutherizationCode(String callbackURI)
      throws IOException {
    // パラメータを組み立てる
    StringBuilder b = new StringBuilder();
    b.append("response_type=code");
    b.append("&client_id=").append(URLEncoder.encode(CLIENT_ID, "utf-8"));
    b.append("&redirect_uri=").append(URLEncoder.encode(callbackURI, "utf-8"));
    b.append("&scope=").append(URLEncoder.encode("user_about_me,offline_access", "utf-8"));
    b.append("&state=getAuthCode");
    return "https://www.facebook.com/dialog/oauth?" + b.toString();
  }

  /**
   * FBからコールバックされた際に渡される{@code authorizationCode}を使い、アクセストークンを取得し、返す。
   * 
   * @param authorizationCode FBからコールバックされた際に渡される認可コード
   * @param callbackURI
   * @return アクセストークンが含まれた文字列
   * @throws IOException
   * @author shin1ogawa
   * @see <a href="https://developers.facebook.com/docs/authentication/server-side/">Core Concepts › Authentication › Server-Side Authentication</a>
   */
  static String retrieveAccessToken(String authorizationCode,
      String callbackURI) throws IOException {
    // パラメータを組み立てる
    StringBuilder b = new StringBuilder();
    b.append("code=").append(URLEncoder.encode(authorizationCode, "utf-8"));
    b.append("&client_id=").append(URLEncoder.encode(CLIENT_ID, "utf-8"));
    b.append("&client_secret=").append(URLEncoder.encode(CLIENT_SECRET, "utf-8"));
    b.append("&redirect_uri=").append(URLEncoder.encode(callbackURI, "utf-8"));
    b.append("&grant_type=authorization_code");
    byte[] payload = b.toString().getBytes();

    // POST メソッドでリクエストする
    HttpURLConnection c = (HttpURLConnection) new URL(
        "https://graph.facebook.com/oauth/access_token")
        .openConnection();
    c.setRequestMethod("POST");
    c.setDoOutput(true);
    c.setRequestProperty("Content-Length", String.valueOf(payload.length));
    c.getOutputStream().write(payload);
    c.getOutputStream().flush();

    // トークン類が入ったレスポンスボディの内容を返す(JSONで返される)
    StringBuilder respondedJSON = new StringBuilder();
    BufferedReader reader = new BufferedReader(new InputStreamReader(c.getInputStream()));
    String line = null;
    while ((line = reader.readLine()) != null) {
      respondedJSON.append(line).append("\n");
    }
    return respondedJSON.toString();
  }
}