仅在显示JFrame之后如何在Java中读取文件？

Question

I'm using Java Swing. Initially I want to read a file (which is quite big). So the frame gets displayed after the file is completely. Whereas I want the frame to first load (displayed) and then the file should be read.

class Passwd {

    JFrame jfrm; 
    // other elements

    Passwd() {
        start();

        // Display frame.
        jfrm.setVisible(true);
    }

    public void start() {

        // Create a new JFrame container.
        jfrm = new JFrame("Password Predictability & Strength Measure");

        // Specify FlowLayout for the layout manager.
        //jfrm.setLayout(new FlowLayout());
        jfrm.setLayout(null);

        // Give the frame an initial size.
        jfrm.setSize(450, 300);

        // align window to center of screen
        jfrm.setLocationRelativeTo(null);  
        // Terminate the program when the user closes the application.
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // some elements

        File file = new File("file.txt");
        try (BufferedReader br = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = br.readLine()) != null) {
                // operation
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } 

    }

    public static void main(String args[]) {

        // Create the frame on the event dispatching thread.
        SwingUtilities.invokeLater(new Runnable() {

            public void run() {

                new Passwd();                   

            }
        });
    }
}

How can I read the file after the frame is displayed?

Answer 1

The JFrame should display immediately, so that's not the problem. The problem is that you're reading in the file on the Swing event thread, and this blocks its ability to display the JFrame. The solution is to not do this, to instead read the file in a background thread, such as via a SwingWorker. This way the JFrame can display unimpeded, and the file reading will not interfere with Swing functioning.

So if the file reading will not change the state of Swing components, use a simple background thread:

new Thread(() -> {
    File file = new File("file.txt");
    try (BufferedReader br = new BufferedReader(new FileReader(file))) {
        String line;
        while ((line = br.readLine()) != null) {
            // operation
        }
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}).start();

If the reading in will change the state of the GUI as the reading occurs, again, use a SwingWorker.

Side issue: avoid using null layouts as they'll come back to bite you.