JavaFX sample for an fxml based Login screen to main application transition with login session data transfer

login.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.paint.*?>

<StackPane prefWidth="231.0" xmlns:fx="http://javafx.com/fxml" fx:controller="login.LoginController">
  <children>
    <StackPane>
      <children>
        <VBox spacing="10.0">
          <children>
            <GridPane>
              <children>
                <Label text="Username:" GridPane.columnIndex="0" GridPane.rowIndex="0" />
                <Label text="Password:" GridPane.columnIndex="0" GridPane.rowIndex="1" />
                <TextField fx:id="user" promptText="Use &quot;open&quot; to login" text="open" GridPane.columnIndex="1" GridPane.rowIndex="0" />
                <TextField fx:id="password" promptText="Use &quot;sesame&quot; to login" text="sesame" GridPane.columnIndex="1" GridPane.rowIndex="1" />
              </children>
              <columnConstraints>
                <ColumnConstraints hgrow="SOMETIMES" maxWidth="148.0" minWidth="10.0" prefWidth="109.0" />
                <ColumnConstraints hgrow="SOMETIMES" maxWidth="228.0" minWidth="10.0" prefWidth="189.0" />
              </columnConstraints>
              <rowConstraints>
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
              </rowConstraints>
            </GridPane>
            <StackPane prefHeight="-1.0" prefWidth="-1.0">
              <children>
                <Button fx:id="loginButton" alignment="CENTER" defaultButton="true" mnemonicParsing="false" text="Login" StackPane.alignment="CENTER_RIGHT" />
              </children>
            </StackPane>
          </children>
        </VBox>
      </children>
    </StackPane>
  </children>
  <padding>
    <Insets bottom="10.0" left="10.0" right="10.0" top="10.0" />
  </padding>
</StackPane>

LoginController.java

package login;

import javafx.event.*;
import javafx.fxml.FXML;
import javafx.scene.control.*;

/** Controls the login screen */
public class LoginController {
  @FXML private TextField user;
  @FXML private TextField password;
  @FXML private Button loginButton;
  
  public void initialize() {}
  
  public void initManager(final LoginManager loginManager) {
    loginButton.setOnAction(new EventHandler<ActionEvent>() {
      @Override public void handle(ActionEvent event) {
        String sessionID = authorize();
        if (sessionID != null) {
          loginManager.authenticated(sessionID);
        }
      }
    });
  }

  /**
   * Check authorization credentials.
   * 
   * If accepted, return a sessionID for the authorized session
   * otherwise, return null.
   */   
  private String authorize() {
    return 
      "open".equals(user.getText()) && "sesame".equals(password.getText()) 
            ? generateSessionID() 
            : null;
  }
  
  private static int sessionID = 0;

  private String generateSessionID() {
    sessionID++;
    return "xyzzy - session " + sessionID;
  }
}

LoginDemoApplication.java

package login;

import java.io.IOException;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.*;
import javafx.stage.Stage;
import login.LoginManager;

/** Main application class for the login demo application */
public class LoginDemoApplication extends Application {
  public static void main(String[] args) { launch(args); }
  @Override public void start(Stage stage) throws IOException {
    Scene scene = new Scene(new StackPane());
    
    LoginManager loginManager = new LoginManager(scene);
    loginManager.showLoginScreen();

    stage.setScene(scene);
    stage.show();
  }
}

LoginManager.java

package login;

import java.io.IOException;
import java.util.logging.*;
import javafx.fxml.FXMLLoader;
import javafx.scene.*;

/** Manages control flow for logins */
public class LoginManager {
  private Scene scene;

  public LoginManager(Scene scene) {
    this.scene = scene;
  }

  /**
   * Callback method invoked to notify that a user has been authenticated.
   * Will show the main application screen.
   */ 
  public void authenticated(String sessionID) {
    showMainView(sessionID);
  }

  /**
   * Callback method invoked to notify that a user has logged out of the main application.
   * Will show the login application screen.
   */ 
  public void logout() {
    showLoginScreen();
  }
  
  public void showLoginScreen() {
    try {
      FXMLLoader loader = new FXMLLoader(
        getClass().getResource("login.fxml")
      );
      scene.setRoot((Parent) loader.load());
      LoginController controller = 
        loader.<LoginController>getController();
      controller.initManager(this);
    } catch (IOException ex) {
      Logger.getLogger(LoginManager.class.getName()).log(Level.SEVERE, null, ex);
    }
  }

  private void showMainView(String sessionID) {
    try {
      FXMLLoader loader = new FXMLLoader(
        getClass().getResource("mainview.fxml")
      );
      scene.setRoot((Parent) loader.load());
      MainViewController controller = 
        loader.<MainViewController>getController();
      controller.initSessionID(this, sessionID);
    } catch (IOException ex) {
      Logger.getLogger(LoginManager.class.getName()).log(Level.SEVERE, null, ex);
    }
  }
}

mainview.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.paint.*?>

<StackPane prefWidth="231.0" xmlns:fx="http://javafx.com/fxml" fx:controller="login.MainViewController">
  <children>
    <StackPane>
      <children>
        <VBox spacing="10.0">
          <children>
            <GridPane>
              <children>
                <Label text="Session ID:" GridPane.columnIndex="0" GridPane.rowIndex="0" />
                <Label fx:id="sessionLabel" text="Undefined" GridPane.columnIndex="1" GridPane.rowIndex="0" />
              </children>
              <columnConstraints>
                <ColumnConstraints hgrow="SOMETIMES" maxWidth="148.0" minWidth="10.0" prefWidth="109.0" />
                <ColumnConstraints hgrow="SOMETIMES" maxWidth="228.0" minWidth="10.0" prefWidth="189.0" />
              </columnConstraints>
              <rowConstraints>
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
              </rowConstraints>
            </GridPane>
            <StackPane prefHeight="-1.0" prefWidth="-1.0">
              <children>
                <Button id="loginButton" fx:id="logoutButton" alignment="CENTER" defaultButton="true" mnemonicParsing="false" text="Logout" StackPane.alignment="CENTER_RIGHT" />
              </children>
            </StackPane>
          </children>
        </VBox>
      </children>
    </StackPane>
  </children>
  <padding>
    <Insets bottom="10.0" left="10.0" right="10.0" top="10.0" />
  </padding>
</StackPane>

MainViewController.java

package login;

import javafx.event.*;
import javafx.fxml.FXML;
import javafx.scene.control.*;

/** Controls the main application screen */
public class MainViewController {
  @FXML private Button logoutButton;
  @FXML private Label  sessionLabel;
  
  public void initialize() {}
  
  public void initSessionID(final LoginManager loginManager, String sessionID) {
    sessionLabel.setText(sessionID);
    logoutButton.setOnAction(new EventHandler<ActionEvent>() {
      @Override public void handle(ActionEvent event) {
        loginManager.logout();
      }
    });
  }
}

Thanks for this .You should make a change at
stage.setScene(scene);
stage.show();
to
primarystage.setScene(scene);
primarystage.show();

depend on what ur variable

You are correct. Thanks for the correction.