vaadin 10 - Push - Label won't update

Question

MainView include InformationCOmponent:

@Push
@Route
public class MainView extends VerticalLayout {       
    InformationComponent infoComponent;

    public MainView(@Autowired StudentRepository studentRepo, @Autowired Job jobImportCsv, @Autowired JobLauncher jobLauncher, @Value("${file.local-tmp-file}") String inputFile) {     
    [...] // some stuffs

    infoComponent = new InformationComponent(studentRepo);
    add(infoComponent);
    }

    //update when job process is over
    private void uploadFileSuccceed() {
       infoComponent.update(myUploadComponent.getFile());
    }

InformationComponent:

public class InformationComponent extends HorizontalLayout {

    StudentRepository studentRepo;

    Label nbLineInFile = new Label();

    VerticalLayout componentLeft = new VerticalLayout();;
    VerticalLayout componentRight = new VerticalLayout();;

    public InformationComponent(StudentRepository studentRepo) {
    [...] // some init and style stuff

    addLine("Nombre de lignes dans le fichier", nbLineInFile);
    }

    private void addLine(String label, Label value) {
    componentLeft.add(new Label(label));
    componentRight.add(value);
    }

    public void update(File file) {
    try {


        long nbLines = Files.lines(file.toPath(), Charset.defaultCharset()).count();

        System.out.println("UPDATED! " +nbLines); // value is display in console ok!
        UI.getCurrent().access(() -> nbLineInFile.setText(nbLines)); // UI is not updated!!
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
  }
}

When I call InformationComponent from MainView the Label is not update in the browser.

UI.getCurrent().access(() -> nbLineInFile.setText(nbLines))

also try wwith @Push(PushMode.MANUAL) and ui.push(); but doesn't work either...

Complete source code is here: https://github.com/Tyvain/ProcessUploadedFile-Vaadin_SpringBatch/tree/push-not-working

Answer 1

I suspect the problem here is that uploadFileSuccceed() is run from a background thread, in which case UI.getCurrent() will return null. This would cause a NullPointerException that either kills the background thread or alternatively the exception is caught and silently ignored by the caller. Another alternative is that uploadFileSuccceed() happens through a different browser window and thus also a different UI instance, which means that the changes would be pushed in the context of the wrong UI.

For exactly these reasons, UI.getCurrent().access(...) is generally an anti pattern, even though it's unfortunately quite widely used in old examples.

You can check whether this is the cause of your problem by logging the value of UI.getCurrent() in the beginning of the update method, and comparing that to the value of UI.getCurrent() e.g. in the constructor of InformationComponent.

To properly fix the problem, you should pass the correct UI instance through the entire chain of events originating from whatever triggers the background processing to start. You should also note that it might be tempting to use the getUI() method that is available in any Component subclass, but that method is not thread safe and should thus be avoided in background threads.

As a final notice, I would recommend using the Span or Text component instead of Label in cases like this. In Vaadin 10, the Label component has been changed to use the <label> HTML element, which means that it's mainly intended to be used as the label of an input component.

Answer 2

Based on information provided by Leif you should do something like the following example.

At runtime, when this HorizontalLayout subclass object is attached to a parent UI object, its onAttach method is called. At that point we can remember the UI by storing its reference is a member variable named ui. Actually, an Optional<UI> is returned rather than a UI object, so we need to test for null, though it should never be null at point of onAttach.

public class InformationComponent extends HorizontalLayout {

    UI ui;

    StudentRepository studentRepo;

    Label nbLineInFile = new Label();

    VerticalLayout componentLeft = new VerticalLayout();;
    VerticalLayout componentRight = new VerticalLayout();;

    public InformationComponent(StudentRepository studentRepo) {
        [...] // some init and style stuff

        addLine("Nombre de lignes dans le fichier", nbLineInFile);
    }

    private void addLine(String label, Label value) {
        componentLeft.add(new Label(label));
        componentRight.add(value);
    }

    public void update(File file) {
    try {
        long nbLines = Files.lines(file.toPath(), Charset.defaultCharset()).count();

        System.out.println("UPDATED! " +nbLines); // value is display in console ok!
        this.ui.access(() -> nbLineInFile.setText(nbLines)); // UI is not updated!!
    } catch (IOException e) {
        throw new RuntimeException(e);
    } catch (UIDetachedException e) {
        // Do here what is needed to do if UI is no longer attached, user has closed the browser
    }

    @Override  // Called when this component (this `HorizontalLayout`) is attached to a `UI` object.
    public void onAttach() {
        ui = this.getUI().orElseThrow( () -> new IllegalStateException("No UI found, which should be impossible at point of `onAttach` being called.") );

}