javafx.fxml.LoadException: Root hasn't been set. Use method setRoot() before load.

一、问题描述：

二、代码：

<!-- PersonOverview.fxml -->
<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.SplitPane?>
<?import javafx.scene.control.TableColumn?>
<?import javafx.scene.control.TableView?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.ColumnConstraints?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.layout.RowConstraints?>

<!--告诉 PersonOverview.fxml使用的是哪个控制器以及元素与控制器中的属性的对应关系.fx:controller="ch.makery.address.view.PersonOverviewController"-->

<fx:root prefHeight="400.0" prefWidth="600.0" type="AnchorPane" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="ch.makery.address.view.PersonOverviewController">
   <children>
      <SplitPane dividerPositions="0.29797979797979796" layoutX="44.0" layoutY="14.0" prefHeight="400.0" prefWidth="600.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
        <items>
          <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="160.0" prefWidth="100.0">
               <children>
                  <TableView fx:id="personTable" layoutX="-12.0" prefHeight="200.0" prefWidth="200.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
                    <columns>
                      <TableColumn fx:id="firstNameColumn" prefWidth="75.0" text="FirstName" />
                      <TableColumn fx:id="lastNameColumn" prefWidth="75.0" text="LastName" />
                    </columns>
                     <columnResizePolicy>
                        <TableView fx:constant="CONSTRAINED_RESIZE_POLICY" />
                     </columnResizePolicy>
                  </TableView>
               </children>
            </AnchorPane>
          <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="160.0" prefWidth="100.0">
               <children>
                  <Label layoutX="14.0" layoutY="7.0" text="Person Details" AnchorPane.leftAnchor="5.0" AnchorPane.topAnchor="5.0" />
                  <GridPane layoutX="46.0" layoutY="109.0" AnchorPane.leftAnchor="5.0" AnchorPane.rightAnchor="5.0" AnchorPane.topAnchor="30.0">
                    <columnConstraints>
                      <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
                      <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
                    </columnConstraints>
                    <rowConstraints>
                      <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                      <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                      <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                        <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                        <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                        <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                    </rowConstraints>
                     <children>
                        <Label fx:id="firstNameLabel" text="First Name" />
                        <Label text="Label" GridPane.columnIndex="1" />
                        <Label fx:id="lastNameLabel" text="Last Name" GridPane.rowIndex="1" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="1" />
                        <Label fx:id="streetLabel" text="Street" GridPane.rowIndex="2" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="2" />
                        <Label fx:id="cityLabel" text="City" GridPane.rowIndex="3" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="3" />
                        <Label fx:id="postalCodeLabel" text="Postal code" GridPane.rowIndex="4" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="4" />
                        <Label fx:id="birthdayLabel" text="Birthday" GridPane.rowIndex="5" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="5" />
                     </children>
                  </GridPane>
                  <HBox layoutX="209.0" layoutY="328.0" spacing="8.0">
                     <children>
                        <Button mnemonicParsing="false" text="New" />
                        <Button mnemonicParsing="false" text="Edit" />
                        <Button mnemonicParsing="false" text="Delete" />
                     </children>
                  </HBox>
               </children>
            </AnchorPane>
        </items>
      </SplitPane>
   </children>
</fx:root>

// mainApp
package ch.makery.address;


//    这里的 Stage 是一个主容器，它就是我们通常所认为的窗口（有边，高和宽，还有关闭按钮）。
//    在这个 Stage 里面，你可以放置一个 Scene，当然你可以切换别的 Scene，而在这个 Scene 里面，我们就可以放置各种各样的控件
import java.io.IOException;
import ch.makery.address.model.Person;
import ch.makery.address.view.PersonOverviewController;
import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;


public class mainApp extends Application {

    private Stage primaryStage;
    private BorderPane rootLayout;
    /**
     * The data as an observable list of Persons.
     */

    /**
     * The data as an observable list of Persons.
     */
    private ObservableList<Person> personData = FXCollections.observableArrayList();

    /**
     * Constructor
     */
    public mainApp() {
        // Add some sample data
        personData.add(new Person("Hans", "Muster"));
        personData.add(new Person("Ruth", "Mueller"));
        personData.add(new Person("Heinz", "Kurz"));
        personData.add(new Person("Cornelia", "Meier"));
        personData.add(new Person("Werner", "Meyer"));
        personData.add(new Person("Lydia", "Kunz"));
        personData.add(new Person("Anna", "Best"));
        personData.add(new Person("Stefan", "Meier"));
        personData.add(new Person("Martin", "Mueller"));
    }

    /**
     * Returns the data as an observable list of Persons.
     * @return
     */
    public ObservableList<Person> getPersonData() {
        return personData;
    }



    @Override
    public void start(Stage primaryStage) {
        this.primaryStage = primaryStage;
        this.primaryStage.setTitle("AddressApp");

        initRootLayout();

        showPersonOverview();
    }

    /**
     * Initializes the root layout.
     */
    public void initRootLayout() {
        try {
            // Load root layout from fxml file.
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(mainApp.class.getResource("view/RootLayout.fxml"));
            rootLayout = (BorderPane) loader.load();

            // Show the scene containing the root layout.
            Scene scene = new Scene(rootLayout);
            primaryStage.setScene(scene);
            primaryStage.show();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * Shows the person overview inside the root layout.
     */
    public void showPersonOverview() {
        try {
            // Load person overview.
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(mainApp.class.getResource("view/PersonOverview.fxml"));

            AnchorPane personOverview = (AnchorPane) loader.load();

            // Set person overview into the center of root layout.
            rootLayout.setCenter(personOverview);

            // Give the controller access to the main app.
            PersonOverviewController controller = loader.getController();
            controller.setMainApp(this);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * Returns the main stage.
     * @return
     */
    public Stage getPrimaryStage() {
        return primaryStage;
    }

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

三、原因：（截图来自 https://stackoverflow.com/questions/23729277/javafx-fxml-load-file-issues-with-setting-root）

四、解决：用 Scene Builder 打开 PersonOverview.fxml（根据你自己的文件名），找到Document下的Controller组（一般在左下角），取消对的选择

保存，然后回到 IDEA中，运行mainApp，就可以正常运行啦！下面是修改过后的PersonOverview.fxml中的代码（注意：在Scene Builder中修改并保存，那么它的代码也是会自动修改的）

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

<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.SplitPane?>
<?import javafx.scene.control.TableColumn?>
<?import javafx.scene.control.TableView?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.ColumnConstraints?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.layout.RowConstraints?>

<!--告诉 PersonOverview.fxml使用的是哪个控制器以及元素与控制器中的属性的对应关系.fx:controller="ch.makery.address.view.PersonOverviewController"-->

<AnchorPane prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="ch.makery.address.view.PersonOverviewController">
   <children>
      <SplitPane dividerPositions="0.29797979797979796" layoutX="44.0" layoutY="14.0" prefHeight="400.0" prefWidth="600.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
        <items>
          <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="160.0" prefWidth="100.0">
               <children>
                  <TableView fx:id="personTable" layoutX="-12.0" prefHeight="200.0" prefWidth="200.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
                    <columns>
                      <TableColumn fx:id="firstNameColumn" prefWidth="75.0" text="FirstName" />
                      <TableColumn fx:id="lastNameColumn" prefWidth="75.0" text="LastName" />
                    </columns>
                     <columnResizePolicy>
                        <TableView fx:constant="CONSTRAINED_RESIZE_POLICY" />
                     </columnResizePolicy>
                  </TableView>
               </children>
            </AnchorPane>
          <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="160.0" prefWidth="100.0">
               <children>
                  <Label layoutX="14.0" layoutY="7.0" text="Person Details" AnchorPane.leftAnchor="5.0" AnchorPane.topAnchor="5.0" />
                  <GridPane layoutX="46.0" layoutY="109.0" AnchorPane.leftAnchor="5.0" AnchorPane.rightAnchor="5.0" AnchorPane.topAnchor="30.0">
                    <columnConstraints>
                      <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
                      <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
                    </columnConstraints>
                    <rowConstraints>
                      <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                      <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                      <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                        <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                        <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                        <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                    </rowConstraints>
                     <children>
                        <Label fx:id="firstNameLabel" text="First Name" />
                        <Label text="Label" GridPane.columnIndex="1" />
                        <Label fx:id="lastNameLabel" text="Last Name" GridPane.rowIndex="1" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="1" />
                        <Label fx:id="streetLabel" text="Street" GridPane.rowIndex="2" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="2" />
                        <Label fx:id="cityLabel" text="City" GridPane.rowIndex="3" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="3" />
                        <Label fx:id="postalCodeLabel" text="Postal code" GridPane.rowIndex="4" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="4" />
                        <Label fx:id="birthdayLabel" text="Birthday" GridPane.rowIndex="5" />
                        <Label text="Label" GridPane.columnIndex="1" GridPane.rowIndex="5" />
                     </children>
                  </GridPane>
                  <HBox layoutX="209.0" layoutY="328.0" spacing="8.0">
                     <children>
                        <Button mnemonicParsing="false" text="New" />
                        <Button mnemonicParsing="false" text="Edit" />
                        <Button mnemonicParsing="false" text="Delete" />
                     </children>
                  </HBox>
               </children>
            </AnchorPane>
        </items>
      </SplitPane>
   </children>
</AnchorPane>