Is my JAVA code thread safe, as the code is called by only one thread?

Question

I would like some informations about calling swing methods by one thread at once.

EDIT: I use Java 7.

I saw the following topic:

Thread Safety of JTextArea.append

I developped a mini swing application.

This is my main class that is a thread safe class. I call the method SwingUtilities.invokeLater for making it thread safe class.

MainClass :

package swingex;

import javax.swing.SwingUtilities;

public final class MainClass extends Thread {

    public static void main(String[] _args) {
        SwingUtilities.invokeLater(new MainClass());
    }

    @Override
    public void run() {
        new MainWindow();
    }
}

Here is a class inheriting from JFrame. I put a text area and a button in the content pane.

MainWindow:

package swingex;

import java.awt.Dimension;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;

public final class MainWindow extends JFrame {

    private JTextArea textArea = new JTextArea(32,32);

    //AddText inherits from Thread class
    private AddText thread = new AddText(textArea);

    public MainWindow() {
        JPanel panel_ = new JPanel();
        panel_.setLayout(new BoxLayout(panel_, BoxLayout.PAGE_AXIS));
        JScrollPane scr_ = new JScrollPane(textArea);
        scr_.setPreferredSize(new Dimension(128, 128));
        panel_.add(scr_);
        //The button is used for adding rows in the text area
        JButton button_ = new JButton("Add rows");
        //Adding the event
        button_.addActionListener(new AddTextEvent(this));
        panel_.add(button_);
        setContentPane(panel_);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        pack();
        setVisible(true);
    }

    //Called by actionPerformed method
    public void setText() {
        if (thread.isAlive()) {
            //prevent from setting the text area by multi threading
            return;
        }
        //For avoiding issues, the text area is affected by less than two threads.
        thread = new AddText(textArea);
        thread.start();
    }
}

Clicking the button makes sleeping the thread while 5 seconds then the thread adds 200 rows to the text area. AddText:

package swingex;

import java.awt.Rectangle;

import javax.swing.JTextArea;
import javax.swing.text.BadLocationException;

public final class AddText extends Thread {

    private JTextArea textArea;

    public AddText(JTextArea _textArea) {
        textArea = _textArea;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(5000);
        } catch (InterruptedException _0) {
            _0.printStackTrace();
        }
        //Only one thread can access the code
        for (int i = 0; i < 200; i++) {
            textArea.append("Text"+i+"\n");
        }
        int endPosition_ = textArea.getDocument().getLength();
        Rectangle bottom_;
        try {
            bottom_ = textArea.modelToView(endPosition_);
            textArea.scrollRectToVisible(bottom_);
        } catch (BadLocationException _0) {
            _0.printStackTrace();
        }
    }
}

This class implements ActionListener, it is used for clicking button.

AddTextEvent:

package swingex;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public final class AddTextEvent implements ActionListener {

    private MainWindow window;

    public AddTextEvent(MainWindow _window) {
        window = _window;
    }

    @Override
    public void actionPerformed(ActionEvent _e) {
        window.setText();
    }

}

Thank you in advance.

EDIT 2: my "new thread" class is the following one: (The other classes remain the same ones.)

package swingex;

import java.awt.Rectangle;

import javax.swing.JTextArea;
import javax.swing.SwingWorker;
import javax.swing.text.BadLocationException;

/**Now the class AddText inherits from SwingWorker*/
public final class AddText extends SwingWorker<Void, Void> {

    private JTextArea textArea;

    public AddText(JTextArea _textArea) {
        textArea = _textArea;
    }


    /**The equivalent in the class Thread*/
    public void start() {
        execute();
    }

    @Override
    public Void doInBackground() {
        try {
            Thread.sleep(5000);
        } catch (InterruptedException _0) {
            _0.printStackTrace();
        }
        for (int i = 0; i < 200; i++) {
            textArea.append("Text"+i+"\n");
        }
        int endPosition_ = textArea.getDocument().getLength();
        Rectangle bottom_;
        try {
            bottom_ = textArea.modelToView(endPosition_);
            textArea.scrollRectToVisible(bottom_);
        } catch (BadLocationException _0) {
            _0.printStackTrace();
        }
        return null;
    }

    /**The equivalent in the class Thread*/
    public boolean isAlive() {
        return !isDone() || !isCancelled();
    }
}

EDIT 3: it is a custom class inheriting from a "swing" component:

Are my custom methods "thread-safe" or not?

MyTextArea:

package swingex;

import javax.swing.JTextArea;

public class MyTextArea extends JTextArea {

    private String aField = "";

    public String getaField() {
        return aField;
    }

    public void setaField(String _aField) {
        aField = _aField;
    }
}

Now, I use the method "invokeLater" of SwingUtilities from the "doInBackground" method of SwingWorker

Answer 1

I think you should do actions in Swing's EDT. So everything after the sleep should be called with invokeLater.

This is not a problem of concurrent access. We are not supposed to modify UI in other threads.

What you could do is:

1. not create your own thread for this action
2. wrap actions in a Runnable and call it with invokeLater. Then the right order will be guaranteed in EDT.

For long time actions, take a look at swingworker.

And why there is an issue? There are at least 2 threads:

1. Swing EDT: where you create all your components and Swing modifies its UI.
2. Thread in background: where you do long treatment and try to append text. And here you access objects in the EDT thread.