code: public void doFinish(HttpServletRequest request, HttpServletResponse response)
              throws IOException, ServletException {
          if (!common.checkGet(request, response)) return;
  
          User user = common.requireLoggedInUser(request, response);
          if (user == null) {
              common.pageSoftError(response, "Can't do /dropbox-auth-finish.  Nobody is logged in.");
              return;
          }
  
          DbxAuthFinish authFinish;
          try {
              authFinish = getWebAuth(request).finishFromRedirect(
                  getRedirectUri(request),
                  getSessionStore(request),
                  request.getParameterMap()
              );
          } catch (DbxWebAuth.BadRequestException e) {
              common.log.println("On /dropbox-auth-finish: Bad request: " + e.getMessage());
              response.sendError(400);
              return;
          } catch (DbxWebAuth.BadStateException e) {
              // Send them back to the start of the auth flow.
              response.sendRedirect(common.getUrl(request, "/dropbox-auth-start"));
              return;
          } catch (DbxWebAuth.CsrfException e) {
              common.log.println("On /dropbox-auth-finish: CSRF mismatch: " + e.getMessage());
              response.sendError(403);
              return;
          } catch (DbxWebAuth.NotApprovedException e) {
              common.page(response, 200, "Not approved?", "Why not, bro?");
              return;
          } catch (DbxWebAuth.ProviderException e) {
              common.log.println("On /dropbox-auth-finish: Auth failed: " + e.getMessage());
              response.sendError(503, "Error communicating with Dropbox.");
              return;
          } catch (DbxException e) {
              common.log.println("On /dropbox-auth-finish: Error getting token: " + e);
              response.sendError(503, "Error communicating with Dropbox.");
              return;
          }
  
          // We have an Dropbox API access token now.  This is what will let us make Dropbox API
          // calls.  Save it in the database entry for the current user.
          user.dropboxAccessToken = authFinish.getAccessToken();
          common.saveUserDb();
  
          response.sendRedirect("/");
      }
docs: an HTTP POST.