Question

package Data_Project;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

import java.io.IOException;

public class Main extends Application {
    public Stage window;
    @Override
    public void start(Stage primaryStage) throws Exception{
        window = primaryStage;
        //Scene1
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        Parent root1 = FXMLLoader.load(getClass().getResource("Admin.fxml"));
        Controller a1 = new Controller();
        a1.getSubmitButton().setOnAction(e -> {
            window.setScene(new Scene(root1,500,500));
        });
        window.setTitle("Log in");
        window.setScene(new Scene(root,500,500));
        window.show();
    }


    public static void main(String[] args) throws IOException {
       launch(args);

    }
}

嘿那里，我犯这个代码的错误，我无法弄清楚是什么错误。

应用程序启动方法中的异常线程中的异常＆＃34; main＆＃34; java.lang.RuntimeException：Application start方法中的异常 com.sun.javafx.application.LauncherImpl.launchApplication1（LauncherImpl.java:875）在 com.sun.javafx.application.LauncherImpl.lambda $ launchApplication $ 147（LauncherImpl.java:157）在 com.sun.javafx.application.LauncherImpl $$ LAMBDA $ 1 / 989110044.run（未知来自java.lang.Thread.run（Thread.java:745）引起的： java.lang.NullPointerException at Data_Project.Main.start（Main.java:20）

XML：

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

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

<GridPane alignment="center" hgap="10" vgap="10" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="Data_Project.Controller">
   <children>

          <Label text="Username" GridPane.columnIndex="1" GridPane.rowIndex="1" />
          <TextField fx:id="userField" GridPane.columnIndex="2" GridPane.rowIndex="1" />
          <Label text="Password" GridPane.columnIndex="1" GridPane.rowIndex="3" />
          <PasswordField fx:id="passField" GridPane.columnIndex="2" GridPane.columnSpan="1" GridPane.rowIndex="3" />
          <Button fx:id="submitButton" onAction="#submitForm" text="Log in" GridPane.columnIndex="2" GridPane.columnSpan="2" GridPane.rowIndex="4" />
          <Label fx:id="errorLabel" textFill="#c30808" GridPane.columnIndex="2" GridPane.columnSpan="2" GridPane.rowIndex="6" />
   </children>

</GridPane>






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

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

<GridPane alignment="center" hgap="10" vgap="10" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="Data_Project.Admin">
    <children>

        <Label text="Username" GridPane.columnIndex="1" GridPane.rowIndex="1" />
        <TextField fx:id="userField" GridPane.columnIndex="2" GridPane.rowIndex="1" />
        <Label text="Password" GridPane.columnIndex="1" GridPane.rowIndex="3" />
        <PasswordField fx:id="passField" GridPane.columnIndex="2" GridPane.columnSpan="1" GridPane.rowIndex="3" />
        <Button fx:id="submitButton" text="Log in" GridPane.columnIndex="2" GridPane.columnSpan="2" GridPane.rowIndex="4" />
        <Label fx:id="errorLabel" textFill="#c30808" GridPane.columnIndex="2" GridPane.columnSpan="2" GridPane.rowIndex="6" />
    </children>

</GridPane>

XML既是不同的文件，但在这里显示为1个文件，它们具有相同的属性，因为它用于测试场景切换器，但由于某种原因，我无法使其工作！

控制器类：

package Data_Project;


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


public class Controller{
    Scene window;
    @FXML Button submitButton;
    @FXML TextField userField;
    @FXML PasswordField passField;
    @FXML Label errorLabel;

    public void submitForm(ActionEvent actionEvent) {
       authorizedUser user = new authorizedUser();
        if(!user.checkCredentials(userField.getText(), passField.getText()))
        {
            errorLabel.setText("Invalid credentials");
        }
    }


    public Button getSubmitButton() {
        return submitButton;
    }
}

管理：

package Data_Project;


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


public class Admin{
    Scene window;
    @FXML Button submitButton;
    @FXML TextField userField;
    @FXML PasswordField passField;
    @FXML Label errorLabel;

    public Button getSubmitButton() {
        return submitButton;
    }
}

Answer 1

FXMLLoader解析FXML文件，创建控制器类的实例（如果指定了一个）并将任何@FXML - 注释字段注入该控制器实例。

如果您创建自己的控制器实例，就像使用

一样

Controller a1 = new Controller();

然后FXMLLoader对该实例一无所知（它将如何？），因此@FXML - 注释字段未初始化。因此，当你打电话

a1.getSubmitButton()

它返回null，因此您获得NullPointerException。

要解决此问题，请替换

Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));

和

Controller a1 = new Controller();

与

FXMLLoader loader = new FXMLLoader(getClass().getResource("sample.fxml"));
Parent root = loader.load();
Controller a1 = loader.getController();

这为您提供了对FXMLLoader创建的的控制器实例的引用，因此它已正确初始化了其@FXML个注释字段。

JavaFx不会改变场景

1 个答案: