How can I fix a NullPointerException when interacting with FXML elements in the start method?

问题

I'm writing some code where I interact (Change text) with FXML elements from the start method. However, I find that when I call the method from my controller class I get a NullPointerException. I've found that the issue has something to do with threads, but I have been able to get anything to work. I have included some sample code that generates the same problem.

Main class:

public class Main extends Application {
    Controller C = new Controller();

    @Override
    public void start(Stage stage) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        Scene scene = new Scene(root, 300, 275);
        stage.setTitle("FXML Welcome");
        stage.setScene(scene);
        stage.show();
        C.handleSubmitButtonAction();//Error happens here.
    }

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

Controller class:

public class Controller{
    @FXML public Text actiontarget;

    @FXML protected void handleSubmitButtonAction() {
        actiontarget.setText("Sign in button pressed");
    }
}

FXML code:

<GridPane fx:id="GridPane" fx:controller="sample.Controller" xmlns:fx="http://javafx.com/fxml" alignment="center" hgap="10" vgap="10">
    <children>
        <Text text="Welcome" GridPane.columnIndex="0" GridPane.rowIndex="0" GridPane.columnSpan="2"/>
        <Label text="User Name:" GridPane.columnIndex="0" GridPane.rowIndex="1"/>
        <TextField GridPane.columnIndex="1" GridPane.rowIndex="1"/>
        <Label text="Password:" GridPane.columnIndex="0" GridPane.rowIndex="2"/>
        <PasswordField fx:id="passwordField" GridPane.columnIndex="1" GridPane.rowIndex="2"/>
        <HBox spacing="10" alignment="bottom_right" GridPane.columnIndex="1" GridPane.rowIndex="4">
            <Button text="Sign In" onAction="#handleSubmitButtonAction"/>
        </HBox>
        <Text fx:id="actiontarget" GridPane.columnIndex="1" GridPane.rowIndex="6"/>
    </children>
</GridPane>

回答1:

The FXMLLoader creates a instance of the controller itself, if you specify the controller class using the fx:controller attribute in the fxml.

To get access to that instance, you need to create a FXMLLoader instance and use getController() after loading the fxml. Otherwise the controller instance where the values are injected is different to the one you create yourself and store in the C field:

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

来源：https://stackoverflow.com/questions/37164835/how-can-i-fix-a-nullpointerexception-when-interacting-with-fxml-elements-in-the

标签

java

javafx

fxml