Jtextarea receives the append but does not display the text

Question

I’m having trouble using a Jtextarea. I need that every time something happens on my system, information is added in real time. However, he appears nothing.

I’ve tried using the setText() and the append(), but neither worked. However, if I do it here

JOptionpane.showMessageDialog(null, JtextArea.getText())

It returns to me everything that I had written before but that was not appearing in it. I’ve tried to use repaint() also and nothing.

Someone’s been through it?

---

Here’s a piece of code. When I click on a button, it has this call:

    if (!isStartServer()) {
        area.append("Iniciando servidor de aplicação...\n");
        Integer portaSocket = Integer.parseInt(tfPorta.getText());
        Integer limite = Integer.parseInt(tfLimite.getText());

        setStartServer(true);
        StartServidor s = new StartServidor(portaSocket, limite, this);
        new Thread(s).start();

        btStart.setText("Stop Server");
    }else{
        System.exit(0);
}

In this way, Startservidor became a Thread, which is usually where I write messages in the Jtextarea. When I didn’t have this "Startservidor" as a Thread, it didn’t update the Jtextarea when I called writeTextArea(String msg);

Code of the Startservidor:

  public void run() {

    setPortaSocket(getPortaSocket());
    setLimite(getLimite());

    try {

        setServerSocket(new ServerSocket(getPortaSocket()));
        writeTextArea("Ouvindo a porta " + getPortaSocket() + "...");

        while (servidor.isStartServer()) {
            setSocketCliente(getServerSocket().accept());

            setUser(new User());
            getUser().setCliente(getSocketCliente());

            scanner = new Scanner(getUser().getCliente().getInputStream());

            if (scanner.hasNext()) {
                getUser().setNome(scanner.nextLine());
            }

            writeTextArea(getUser().getNome() + " conectou-se.");

            if (!clientes.containsKey(getUser().getNome())) {
                clientes.put(getUser().getNome(), getUser());

                getUser().setIpComputador(getUser().getCliente().getInetAddress().getHostAddress());
                getUser().setNomeComputador(getUser().getCliente().getInetAddress().getHostName());

                ServidorReadMessage read = new ServidorReadMessage(getUser(), this);
                new Thread(read).start();
            }

        }

    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (getServerSocket() != null) {
                getServerSocket().close();
            }
            if (getSocketCliente() != null) {
                getSocketCliente().close();
            }
            if (scanner != null) {
                scanner.close();
            }
        } catch (IOException e) {
        }
    }
}

This way, it is writing correctly, however, I do not know if it is the best way, I do not know nor if it is correct to do so. That is why I would like your help. I have looked at the documentation and it says nothing about it.

Answer 1

When I click on a button, it has this call:

Any code executed in response to a button click runs on Event Dispatcher Thread. This thread is used for virtually everything that interfaces with the Swing library. So, if you have a code:

void actionPerformed(ActionEvent e) {
    // Código
}

As long as this code does not finish running the Swing will do nothing else (repaint screens, update components, etc).

In this way, Startservidor became a Thread, which is usually where I write messages in the Jtextarea. When I didn’t have this "Startservidor" as a Thread, it didn’t update the Jtextarea when I called writeTextArea(String msg);

In fact, before you moved your code to a separate thread, the loop that awaited new connections ran on Event Dispatcher Thread. That means that the actionPerformed never returned! He stood still, waiting for the socket connections, and with that Swing did nothing else. When you tried to use setText in Jtextarea, he updated his model (Document) but did not touch the vision. So, if you did:

area.getText();

It would give you updated content (because the template was updated), but if you looked at the text box on the screen you would only see the old value (because who updates the screen is the Event Dispatcher Thread, and he’s standing around waiting for his code to finish running).

That way, you are writing correctly, however, I do not know if it is the best way, I do not know nor if it is correct to do so.

Yes, that way is correct. But the best way would be to use a SwingWorker: a specific thread type for tasks started via Swing. Here a documentation (in English) describing its use and its benefits. But if you want to keep doing it the way it is, you don’t have the slightest problem.

Important detail: the vast majority of Swing library methods are not thread-safe. This means that - if you need to change some component from another thread (including the main thread), you must do it by one of the special methods of SwingUtilities. Example:

// Seu thread (ou main thread)

/* area.setText("texto"); // ERRADO */
SwingUtilities.invokeLater(new Runnable() {
    public void run() {
        area.setText("texto"); // Certo
    }
});

// Event Dispatcher Thread

area.setText("texto"); // Certo

Answer 2

Your method code writeTextArea(String msg) could be like this:

private void writeTextArea(String msg){
    area.setText(area.getText()+msg+"\n");
    area.repaint();
}

Basically it defines your Jtextarea text for any text that was already inside, adds the message and renders Jtextarea again. If it doesn’t work, I have no idea what it is, and the JVM itself should render the component again if it has been modified.