code: public final void open(AccessToken accessToken, StatusCallback callback) {
          synchronized (this.lock) {
              if (pendingAuthorizationRequest != null) {
                  throw new UnsupportedOperationException(
                          "Session: an attempt was made to open a session that has a pending request.");
              }
  
              if (state.isClosed()) {
                  throw new UnsupportedOperationException(
                          "Session: an attempt was made to open a previously-closed session.");
              } else if (state != SessionState.CREATED && state != SessionState.CREATED_TOKEN_LOADED) {
                  throw new UnsupportedOperationException(
                          "Session: an attempt was made to open an already opened session.");
              }
  
              if (callback != null) {
                  addCallback(callback);
              }
  
              this.tokenInfo = accessToken;
  
              if (this.tokenCachingStrategy != null) {
                  this.tokenCachingStrategy.save(accessToken.toCacheBundle());
              }
  
              final SessionState oldState = state;
              state = SessionState.OPENED;
              this.postStateChange(oldState, state, null);
          }
  
          autoPublishAsync();
      }
docs: Opens a session based on an existing Facebook access token. This method should be used
  only in instances where an application has previously obtained an access token and wishes
  to import it into the Session/TokenCachingStrategy-based session-management system. An
  example would be an application which previously did not use the Facebook SDK for Android
  and implemented its own session-management scheme, but wishes to implement an upgrade path
  for existing users so they do not need to log in again when upgrading to a version of
  the app that uses the SDK.
  <p/>
  No validation is done that the token, token source, or permissions are actually valid.
  It is the caller's responsibility to ensure that these accurately reflect the state of
  the token that has been passed in, or calls to the Facebook API may fail.
  
  @param accessToken the access token obtained from Facebook
  @param callback    a callback that will be called when the session status changes; may be null