java - 通过另一个文件中的Controller Instance Reference在MainController中设置文本字段

我试图从另一个文件在我的MainController中设置文本字段的文本。
我读了this：

FXMLLoader loader = new FXMLLoader(getClass().getResource("main.fxml"));
MainController mc = (MainController) loader.getController();

是您应该如何获得Javafx中的Controller Instance的引用。我尝试实现它，但是在尝试设置TextField时，它给了我NullPointerException。
这是我的代码：

Main.java：

package application;

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


public class Main extends Application {

    static MainController mc;

    @Override
    public void start(Stage primaryStage) {
        try {
            Parent root = FXMLLoader.load(getClass().getResource("main.fxml"));
            Scene scene = new Scene(root,348,212);
            scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
            primaryStage.setScene(scene);
            primaryStage.setTitle("Flextime calculator");
            primaryStage.getIcons().add(new Image(getClass().getResourceAsStream("assets/icon.png")));
            primaryStage.setResizable(false);

            FXMLLoader loader = new FXMLLoader(getClass().getResource("main.fxml"));
            mc = (MainController) loader.getController();

            primaryStage.show();
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

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

    public static MainController getMc() {
        return mc;
    }
}

MainController.java：

public class MainController {

    @FXML Label fxFlextime;
    @FXML Label fxTotalFlextime;
    @FXML TextField fxStarttime;
    @FXML TextField fxEndtime;
    @FXML TextField fxDailyWorktime;
    @FXML TextField fxBreaktime;
    @FXML TextField fxAddBreaktime;
    @FXML TextField fxBreakGap;
    @FXML CheckBox fxAddBreak;

    //Create Model, Userconfig
    Model model = new Model();
    UserConfig config = new UserConfig();

    public void initialize() {
        config.initialize();
    }
}

UserConfig.java：

public class UserConfig {

    public void initialize() {
        insertDefault();
    }

    void insertDefault() {
        List<String> prefValues = getPreference();
        //FXMLLoader loader = new FXMLLoader(getClass().getResource("main.fxml"));
        //MainController mc = (MainController) loader.getController();
        // ^Tried also putting it here to see wether im referencing the variable wrong or something,
        // so maybe im getting the Controller wrong? With mc.fxStarttime.setText("") it also didnt work here
        Main.mc.fxStarttime.setText("");
        Main.mc.fxEndtime.setText(prefValues.get(2));
        Main.mc.fxDailyWorktime.setText(prefValues.get(3));
        Main.mc.fxBreaktime.setText(prefValues.get(4));
        Main.mc.fxAddBreaktime.setText(prefValues.get(5));
        Main.mc.fxBreakGap.setText(prefValues.get(6));

    }

    static List<String> getPreference() {
        // This will define a node in which the preferences can be stored
        Preferences userPrefs = Preferences.userNodeForPackage(Main.class);

        try {
            String[] keys = userPrefs.keys();

            if (keys == null) {
                userPrefs.put("DEFStarttime"    , "1");
                userPrefs.put("DEFEndtime"      , "");
                userPrefs.put("DEFDailyWorktime", "");
                userPrefs.put("DEFBreaktime"    , "");
                userPrefs.put("DEFAddBreaktime" , "");
                userPrefs.put("DEFBreakGap"     , "");
            }
        } catch (BackingStoreException ex) {
            System.err.println(ex);
        }

        List<String> prefValues = new ArrayList<String>();
        prefValues.add(userPrefs.get("DEFStarttime" , ""));
        prefValues.add(userPrefs.get("DEFEndtime"   , ""));
        prefValues.add(userPrefs.get("DEFDailyWorktime" , ""));
        prefValues.add(userPrefs.get("DEFBreaktime" , ""));
        prefValues.add(userPrefs.get("DEFAddBreaktime"  , ""));
        prefValues.add(userPrefs.get("DEFBreakGap" , ""));
        /*
        for(String string : prefValues) {
            System.out.println(string);
        }
        */
        return prefValues;
    }
}

错误：

javafx.fxml.LoadException:
/C:/eclipse-workspace/project/bin/application/main.fxml

    at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2625)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2595)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2466)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3237)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3194)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3163)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3136)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3113)
    at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:3106)
    at application.Main.start(Main.java:18)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    at java.base/java.lang.Thread.run(Thread.java:830)
Caused by: java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
    at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:273)
    at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2591)
    ... 17 more
Caused by: java.lang.NullPointerException
    at application.UserConfig.insertDefault(UserConfig.java:25)
    at application.UserConfig.initialize(UserConfig.java:18)
    at application.MainController.initialize(MainController.java:39)
    ... 28 more

我试图将其缩减为最少的代码。

编辑：
这里是Main.fxml：

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

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.CheckBox?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.Menu?>
<?import javafx.scene.control.MenuBar?>
<?import javafx.scene.control.MenuItem?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.FlowPane?>
<?import javafx.scene.layout.VBox?>

<VBox maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="212.0" prefWidth="348.0" xmlns="http://javafx.com/javafx/11.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.MainController">
   <children>
      <MenuBar fx:id="fxMenuBar">
        <menus>
          <Menu fx:id="fxFileMenu" mnemonicParsing="false" text="File">
            <items>
              <MenuItem fx:id="fxExitItem" mnemonicParsing="false" onAction="#exitProgram" text="Exit" />
            </items>
          </Menu>
          <Menu mnemonicParsing="false" text="Settings">
            <items>
              <MenuItem mnemonicParsing="false" text="About" />
            </items>
          </Menu>
        </menus>
      </MenuBar>
      <FlowPane alignment="CENTER_LEFT" prefHeight="25.0" prefWidth="403.0">
         <children>
            <Label alignment="CENTER" prefHeight="15.0" prefWidth="65.0" text="Starttime:" underline="true">
               <FlowPane.margin>
                  <Insets left="10.0" top="10.0" />
               </FlowPane.margin>
            </Label>
            <TextField fx:id="fxStarttime" alignment="CENTER" prefHeight="25.0" prefWidth="45.0" promptText="00:00">
               <FlowPane.margin>
                  <Insets top="10.0" />
               </FlowPane.margin></TextField>
            <Label alignment="CENTER" prefHeight="17.0" prefWidth="112.0" text="Required daily hours:" underline="true">
               <FlowPane.margin>
                  <Insets left="10.0" right="5.0" top="10.0" />
               </FlowPane.margin>
            </Label>
            <TextField fx:id="fxDailyWorktime" alignment="CENTER" prefHeight="25.0" prefWidth="45.0" promptText="00.00">
               <FlowPane.margin>
                  <Insets top="10.0" />
               </FlowPane.margin></TextField>
         </children>
      </FlowPane>
      <FlowPane alignment="CENTER_LEFT" prefHeight="36.0" prefWidth="300.0">
         <children>
            <Label alignment="CENTER" prefHeight="15.0" prefWidth="65.0" text="Endtime:" underline="true">
               <FlowPane.margin>
                  <Insets left="10.0" />
               </FlowPane.margin>
            </Label>
            <TextField fx:id="fxEndtime" alignment="CENTER" prefHeight="25.0" prefWidth="45.0" promptText="00:00" />
            <Label alignment="CENTER" prefHeight="15.0" prefWidth="65.0" text="Breaktime:" underline="true">
               <FlowPane.margin>
                  <Insets left="57.0" right="5.0" />
               </FlowPane.margin>
            </Label>
            <TextField fx:id="fxBreaktime" alignment="CENTER" prefHeight="25.0" prefWidth="45.0" promptText="00" />
         </children>
      </FlowPane>
      <FlowPane prefHeight="38.0" prefWidth="300.0">
         <children>
            <CheckBox fx:id="fxAddBreak" mnemonicParsing="false" text="Add:">
               <FlowPane.margin>
                  <Insets left="20.0" top="5.0" />
               </FlowPane.margin>
            </CheckBox>
            <TextField fx:id="fxAddBreaktime" alignment="CENTER" prefHeight="25.0" prefWidth="45.0" promptText="00">
               <FlowPane.margin>
                  <Insets left="10.0" right="5.0" top="5.0" />
               </FlowPane.margin>
            </TextField>
            <Label text="breaktime from ">
               <FlowPane.margin>
                  <Insets top="5.0" />
               </FlowPane.margin>
            </Label>
            <TextField fx:id="fxBreakGap" alignment="CENTER" prefHeight="25.0" prefWidth="45.0" promptText="00.00">
               <FlowPane.margin>
                  <Insets top="5.0" />
               </FlowPane.margin>
            </TextField>
            <Label text="working hours">
               <FlowPane.margin>
                  <Insets left="5.0" top="5.0" />
               </FlowPane.margin>
            </Label>
         </children>
      </FlowPane>
      <FlowPane alignment="CENTER_RIGHT" prefHeight="32.0" prefWidth="300.0">
         <children>
            <Label text="Flextime:">
               <FlowPane.margin>
                  <Insets right="10.0" />
               </FlowPane.margin>
            </Label>
            <Label fx:id="fxFlextime" alignment="CENTER" prefHeight="10.0" prefWidth="35.0" text="0.00" textAlignment="CENTER">
               <FlowPane.margin>
                  <Insets />
               </FlowPane.margin>
            </Label>
            <Label text="h">
               <FlowPane.margin>
                  <Insets right="10.0" />
               </FlowPane.margin>
            </Label>
         </children>
      </FlowPane>
      <FlowPane alignment="CENTER_RIGHT" prefHeight="32.0" prefWidth="300.0">
         <children>
            <Label text="Total flextime:" underline="true">
               <FlowPane.margin>
                  <Insets right="10.0" />
               </FlowPane.margin>
            </Label>
            <Label fx:id="fxTotalFlextime" alignment="CENTER" prefHeight="10.0" prefWidth="35.0" text="0.00" textAlignment="CENTER">
               <FlowPane.margin>
                  <Insets />
               </FlowPane.margin>
            </Label>
            <Label text="h">
               <FlowPane.margin>
                  <Insets right="10.0" />
               </FlowPane.margin>
            </Label>
         </children>
      </FlowPane>
      <FlowPane alignment="CENTER_RIGHT" prefHeight="38.0" prefWidth="300.0">
         <children>
            <Button mnemonicParsing="false" onAction="#calculate" text="Calculate" underline="true">
               <FlowPane.margin>
                  <Insets right="10.0" />
               </FlowPane.margin>
            </Button>
            <Button mnemonicParsing="false" onAction="#addToTotal" text="Add to total" />
            <Button mnemonicParsing="false" onAction="#clear" text="Clear">
               <FlowPane.margin>
                  <Insets left="10.0" right="10.0" />
               </FlowPane.margin>
            </Button>
         </children>
      </FlowPane>
   </children>
</VBox>

最佳答案

之所以得到NullPointerException是因为Main.mc在尝试访问UserConfig中的字段时为null。

在加载main.fxml Parent root = FXMLLoader.load(getClass().getResource("main.fxml"));的行中，调用了控制器initialize()中的MainController方法，此时您正在调用userConfig.initialize()，该引用也向后引用Main.mc是null。

您可以在MainController类中将配置公开，并在Main类中设置mc字段后初始化用户配置。或者，您可以将MainController作为参数注入到UserConfig初始化方法中。

Main.java

FXMLLoader loader = new FXMLLoader(getClass().getResource("main.fxml"));
Parent root = (Parent) loader.load();
Scene scene = new Scene(root,348,212);
scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
primaryStage.setScene(scene);
primaryStage.setTitle("Flextime calculator");
primaryStage.getIcons().add(new Image(getClass().getResourceAsStream("assets/icon.png")));
primaryStage.setResizable(false);

mc = (MainController) loader.getController();
mc.config.initialize();

MainController.java

public UserConfig config = new UserConfig();

@FXML
public void initialize() {
    //remove the config.initialize(); from here
}