Handle multiple JavaFX application launches within a loop

Question

My code currently reads my Gmail inbox via IMAP (imaps) and javamail, and once it finds an email with zip/xap attachment, it displays a stage (window) asking whether to download the file, yes or no.

I want the stage to close once I make a selection, and then return to the place within the loop from which the call came. My problem arises because you cannot launch an application more than once, so I read here that I should write Platform.setImplicitExit(false); in the start method, and then use primartyStage.hide() (?) and then something like Platform.runLater(() -> primaryStage.show()); when I need to display the stage again later.

The problem occuring now is that the flow of command begins in Mail.java's doit() method which loops through my inbox, and launch(args) occurs within a for loop within the method. This means launch(args) then calls start to set the scene, and show the stage. Since there is a Controller.java and fxml associated, the Controller class has an event handler for the stage's buttons which "intercept" the flow once start has shown the stage. Therefore when I click Yes or No it hides the stage but then just hangs there. As if it can't return to the start method to continue the loop from where launch(args) occurred. How do I properly hide/show the stage whenever necessary, allowing the loop to continue whether yes or no was clicked.

Here is the code for Mail.java and Controller.java. Thanks a lot!

Mail.java

 [Other variables set here]
    public static int launchCount = 0;@FXML public Text subjectHolder;

    public static ReceiveMailImap obj = new ReceiveMailImap();

    public static void main(String[] args) throws IOException,        MessagingException {
        ReceiveMailImap.doit();
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        loader = new FXMLLoader(getClass().getResource("prompts.fxml"));
        root = loader.load();
        controller = loader.getController();
        controller.setPrimaryStage(primaryStage);
        scene = new Scene(root, 450, 250);
        controller.setPrimaryScene(scene);
        scene.getStylesheets().add("styleMain.css");
        Platform.setImplicitExit(false);
        primaryStage.setTitle("Download this file?");
        primaryStage.initStyle(StageStyle.UNDECORATED);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void doit() throws MessagingException, IOException {
        Folder inbox = null;
        Store store = null;

        try {
            Properties props = System.getProperties();
            Session session = Session.getDefaultInstance(props, null);
            store = session.getStore("imaps");
            store.connect("imap.gmail.com", "myAccount@gmail.com", "Password");
            inbox = store.getFolder("Inbox");
            inbox.open(Folder.READ_WRITE);
            Message[] messages = inbox.getMessages();
            FetchProfile fp = new FetchProfile();
            fp.add(FetchProfile.Item.ENVELOPE);
            fp.add(UIDFolder.FetchProfileItem.FLAGS);
            fp.add(UIDFolder.FetchProfileItem.CONTENT_INFO);
            fp.add("X-mailer");

            inbox.fetch(messages, fp);
            int doc = 0;
            int maxDocs = 400;
            for (int i = messages.length - 1; i >= 0; i--) {
                Message message = messages[i];
                if (doc < maxDocs) {
                    doc++;
                    message.getSubject();

                    if (!hasAttachments(message)) {
                        continue;
                    }
                    String from = "Sender Unknown";
                    if (message.getReplyTo().length >= 1) {
                        from = message.getReplyTo()[0].toString();
                    } else if (message.getFrom().length >= 1) {
                        from = message.getFrom()[0].toString();
                    }
                    subject = message.getSubject();
                    if (from.contains("myAccount@gmail.com")) {
                        saveAttachment(message.getContent());
                        message.setFlag(Flags.Flag.SEEN, true);
                    }
                }
            }
        } finally {
            if (inbox != null) {
                inbox.close(true);
            }
            if (store != null) {
                store.close();
            }
        }
    }

    public static boolean hasAttachments(Message msg) throws MessagingException, IOException {
        if (msg.isMimeType("multipart/mixed")) {
            Multipart mp = (Multipart) msg.getContent();
            if (mp.getCount() > 1) return true;
        }
        return false;
    }

    public static void saveAttachment(Object content)
    throws IOException, MessagingException {
        out = null; in = null;
        try {
            if (content instanceof Multipart) {
                Multipart multi = ((Multipart) content);
                parts = multi.getCount();
                for (int j = 0; j < parts; ++j) {
                    part = (MimeBodyPart) multi.getBodyPart(j);
                    if (part.getContent() instanceof Multipart) {
                        // part-within-a-part, do some recursion...
                        saveAttachment(part.getContent());
                    } else {
                        int allow = 0;
                        if (part.isMimeType("application/x-silverlight-app")) {
                            extension = "xap";
                            allow = 1;
                        } else {
                            extension = "zip";
                            allow = 1;
                        }
                        if (allow == 1) {
                            if (launchCount == 0) {
                                launch(args);
                                launchCount++;
                            } else {
                                Platform.runLater(() -> primaryStage.show());
                            }
                        } else {
                            continue;
                        }
                    }
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if ( in != null) { in .close();
            }
            if (out != null) {
                out.flush();
                out.close();
            }
        }
    }

    public static File createFolder(String subject) {
        JFileChooser fr = new JFileChooser();
        FileSystemView myDocs = fr.getFileSystemView();
        String myDocuments = myDocs.getDefaultDirectory().toString();
        dir = new File(myDocuments + "\\" + subject);
        savePathNoExtension = dir.toString();
        dir.mkdir();
        System.out.println("Just created: " + dir);
        return dir;
    }

}

Controller.java

public class Controller implements Initializable {

    @FXML
    private Text subjectHolder;
    public Button yesButton, noButton;
    public ReceiveMailImap subject;

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        subject= new ReceiveMailImap();
        subjectHolder.setText(subject.returnSubject());
    }
    public Stage primaryStage;
    public Scene scene;

    @FXML
    ComboBox<String> fieldCombo;

    public void setPrimaryStage(Stage stage) {
        this.primaryStage = stage;
    }

    public void setPrimaryScene(Scene scene) {
        this.scene = scene;
    }

    public String buttonPressed(ActionEvent e) throws IOException, MessagingException {
        Object source = e.getSource();
        if(source==yesButton){
            System.out.println("How to tell Mail.java that user clicked Yes?");
            return "POSITIVE";}
        else{subject.dlOrNot("no");
            System.out.println("How to tell Mail.java that user clicked No?");
            primaryStage.hide();
            return "NEGATIVE";}
    }

}

Answer 1

There are a lot of issues with the code you have posted, but let me just try to address the ones you ask about.

The reason the code hangs is that Application.launch(...)

does not return until the application has exited

In general, you've kind of misunderstood the entire lifecycle of a JavaFX application here. You should think of the start(...) method as the equivalent of the main(...) method in a "traditional" Java application. The only thing to be aware of is that start(...) is executed on the FX Application Thread, so if you need to execute any blocking code, you need to put it in a background thread.

The start(...) method is passed a Stage instance for convenience, as the most common thing to do is to create a scene graph and display it in a stage. You are under no obligation to use this stage though, you can ignore it and just create your own stages as and when you need.

I think you can basically structure your code as follows (though, to be honest, I have quite a lot of trouble understanding what you're doing):

public class Mail extends Application {

    @Override
    public void start(Stage ignored) throws Exception {

        Platform.setImplicitExit(false);

        Message[] messages = /* retrieve messages */ ;
        for (Message message : messages) {
            if ( /* need to display window */) {
                showMessage(message);
            }
        }
    }

    private void showMessage(Message message) {
        FXMLLoader loader = new FXMLLoader(getClass().getResource("prompts.fxml"));
        Parent root = loader.load();

        Controller controller = loader.getController();
        Scene scene = new Scene(root, 450, 250);
        stage.setScene(scene);
        stage.initStyle(StageStyle.UNDECORATED);
        stage.setTitle(...);

        // showAndWait will block execution until the window is hidden, so 
        // you can query which button was pressed afterwards:

        stage.showAndWait();

        if (controller.wasYesPressed()) {
            // ...
        }
    }

    // for IDEs that don't support directly launching a JavaFX Application:
    public static void main(String[] args) { 
        launch(args);
    }
}

Obviously your logic for decided whether to show a window is more complex, but this will give you the basic structure.

To check which button was pressed, use showAndWait as above and then in your controller do

public class Controller {

    @FXML
    private Button yesButton ;

    private boolean yesButtonPressed = false ;

    public boolean wasYesPressed() {
        return yesButtonPressed ;
    }

    // use different handlers for different buttons:
    @FXML
    private void yesButtonPressed() {
        yesButtonPressed = true ;
        closeWindow();
    }

    @FXML
    private void noButtonPressed() {
        yesButtonPressed = false ; // not really needed, but makes things clearer
        closeWindow();
    }

    private void closeWindow() {
        // can use any @FXML-injected node here:
        yesButton.getScene().getWindow().hide();
    }
}