Develop Reference

Swing Timer is not garbage collected

I came across problem of memory leak in Swing Application due to Swing Timer.
I have used Timer to display slideshow of images in Page1.
When I profiled the application, I noticed that when navigating to Page2, the Timer object, the Page1 object and any object within Page1 object were not Garbage Collected.
I came to know that stopping the Timer allows it to be garbage collected.
I was assuming that if any object is not being referenced, it is ready for garbage collection. But this assumption failed in this case.
The code below summarizes my application and does not have memory leak. To see memory leak, comment the line where I have called stopTimer method of Timer.
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.Timer;
public class TimerMemoryLeak {
public static void main(String[] args) {
TimerMemoryLeak timer = new TimerMemoryLeak();
timer.buildUI();
}
public void buildUI() {
showPanel1();
frame.setSize(600, 400);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public void showPanel1() {
Page1 page1 = new Page1();
if (currentPanel != null) {
pane.remove(((Page2) currentPanel).getPanel());
}
pane.add(page1.getPanel());
currentPanel = page1;
page1.startTimer();
page1.setNextAction(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
showPanel2();
}
});
pane.revalidate();
pane.repaint();
}
public void showPanel2() {
Page2 page2 = new Page2();
if (currentPanel != null) {
((Page1) currentPanel).stopTimer(); // Comment this for memory leak
pane.remove(((Page1) currentPanel).getPanel());
}
pane.add(page2.getPanel());
currentPanel = page2;
page2.setPreviousAction(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
showPanel1();
}
});
pane.revalidate();
pane.repaint();
}
private JFrame frame = new JFrame();
private Container pane = frame.getContentPane();
private Object currentPanel;
}
class Page1 {
public Page1() {
panel.add(title, BorderLayout.NORTH);
panel.add(textTimer);
panel.add(btnNext, BorderLayout.SOUTH);
}
public void setNextAction(ActionListener listener) {
btnNext.addActionListener(listener);
}
public JPanel getPanel() {
return panel;
}
public void startTimer() {
timer.setInitialDelay(0);
timer.start();
}
public void stopTimer() {
timer.stop();
}
private JPanel panel = new JPanel(new BorderLayout());
private JLabel title = new JLabel("Panel 1");
private JButton btnNext = new JButton("Next");
private JLabel textTimer = new JLabel();
private int timerInterval = 1000;
private ActionListener timerAction = new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
textTimer.setText(Math.random() + "");
}
};
private Timer timer = new Timer(timerInterval, timerAction);
}
class Page2 {
public Page2() {
panel.add(title, BorderLayout.NORTH);
panel.add(btnPrev, BorderLayout.SOUTH);
}
public void setPreviousAction(ActionListener listener) {
btnPrev.addActionListener(listener);
}
public JPanel getPanel() {
return panel;
}
private JPanel panel = new JPanel(new BorderLayout());
private JLabel title = new JLabel("Panel 2");
private JButton btnPrev = new JButton("Previous");
}
What could be the possible reason for this?

I profiled your example in an artificially small heap, as shown here. The profile showed the expected result: periodic garbage collection returns the used heap space to baseline, as shown here. Selecting page two for the last half of the profile resulted in smaller amplitude collections. Sampling memory showed that the Timer instance present on page one was collected promptly on page two; no instances proliferated. Some additional suggestions:
Construct and manipulate Swing GUI objects only on the event dispatch thread.
Consider using CardLayout to switch pages dynamically.
Comment [out] the line where I have called [the] stopTimer() method.
The same result prevails. Note that instances of Swing Timer "share the same, pre-existing timer thread." As the Timer runs, instances of the inner class DoPostEvent, appearing in the profiler with the name javax.swing.Timer$1, will accumulate transiently. They too will be collected, albeit eventually in a later phase of garbage collection. As suggested here, you can click the Perform GC button to effect a more aggressive collection. Click the Deltas button in the Sampler tab to see other instances that accumulate transiently in the course of executing the timer's ActionListener; click Perform GC again to see the effect.
Console:
$ jvisualvm &
$ java TimerMemoryLeak.java
$ java -Xms32m -Xmx32m TimerMemoryLeak
Code, as tested:
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.Timer;
public class TimerMemoryLeak {
public static void main(String[] args) {
TimerMemoryLeak timer = new TimerMemoryLeak();
timer.buildUI();
}
public void buildUI() {
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
showPanel1();
frame.setSize(600, 400);
frame.setVisible(true);
}
public void showPanel1() {
Page1 page1 = new Page1();
if (currentPanel != null) {
pane.remove(((Page2) currentPanel).getPanel());
}
pane.add(page1.getPanel());
currentPanel = page1;
page1.startTimer();
page1.setNextAction(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
showPanel2();
}
});
pane.revalidate();
pane.repaint();
}
public void showPanel2() {
Page2 page2 = new Page2();
if (currentPanel != null) {
((Page1) currentPanel).stopTimer();
pane.remove(((Page1) currentPanel).getPanel());
}
pane.add(page2.getPanel());
currentPanel = page2;
page2.setPreviousAction(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
showPanel1();
}
});
pane.revalidate();
pane.repaint();
}
private JFrame frame = new JFrame();
private Container pane = frame.getContentPane();
private Object currentPanel;
private static class Page1 {
public Page1() {
panel.add(title, BorderLayout.NORTH);
panel.add(textTimer);
panel.add(btnNext, BorderLayout.SOUTH);
}
public void setNextAction(ActionListener listener) {
btnNext.addActionListener(listener);
}
public JPanel getPanel() {
return panel;
}
public void startTimer() {
timer.setInitialDelay(0);
timer.start();
}
public void stopTimer() {
timer.stop();
}
private JPanel panel = new JPanel(new BorderLayout());
private JLabel title = new JLabel("Panel 1");
private JButton btnNext = new JButton("Next");
private JLabel textTimer = new JLabel();
private int timerInterval = 1000;
private ActionListener timerAction = new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
textTimer.setText(Math.random() + "");
}
};
private Timer timer = new Timer(timerInterval, timerAction);
}
private static class Page2 {
public Page2() {
panel.add(title, BorderLayout.NORTH);
panel.add(btnPrev, BorderLayout.SOUTH);
}
public void setPreviousAction(ActionListener listener) {
btnPrev.addActionListener(listener);
}
public JPanel getPanel() {
return panel;
}
private JPanel panel = new JPanel(new BorderLayout());
private JLabel title = new JLabel("Panel 2");
private JButton btnPrev = new JButton("Previous");
}
}

JButton setText() ERROR

I am trying to set a new text into a button when you press on it. However it does not seem to work, I am doing something wrong, and I do not know what...
EDIT -----I attach the code for easier comprehension of what I mean
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class GrowAndShrinkSquareGUItest {
JFrame frame;
SquareDrawPanel bigGreen;
SquareDrawPanel smallGreen;
JButton button;
growAndShrinkListener listener;
public class SquareDrawPanel extends JPanel {
int width;
int height;
SquareDrawPanel(int w, int h) {
width = w;
height = h;
}
public void paintComponent(Graphics g) {
g.setColor(Color.green);
g.fillRect(frame.getWidth() / 2 - (width / 2), frame.getHeight()
/ 2 - (height / 2) - 15, width, height);
}
}
public class growAndShrinkListener implements ActionListener {
// JButton button;
growAndShrinkListener(JButton button) {
button = new JButton("Click me to grow the Square");
frame.add(button, BorderLayout.NORTH);
button.addActionListener(this);
}
#Override
public void actionPerformed(ActionEvent e) {
button.setText("Unselect all");
}
}
public static void main(String[] args) {
GrowAndShrinkSquareGUItest test = new GrowAndShrinkSquareGUItest();
test.go();
}
private void createPanels() {
bigGreen = new SquareDrawPanel(400, 400);
smallGreen = new SquareDrawPanel(100, 100);
}
private void drawPanel(JPanel panel) {
frame.add(panel);
panel.setVisible(true);
frame.add(panel, BorderLayout.CENTER);
}
private void createListenerButton() {
listener = new growAndShrinkListener(button);
}
private void loop(){}
public void go() {
frame = new JFrame();
frame.setSize(500, 500);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
createPanels();
drawPanel(smallGreen);
createListenerButton();
frame.setVisible(true);
}
}

It is because you use the button variable that is not defined in the scope of actionPerformed method. Java variables have scope and they are available inside curly braces where they're defined. The actionPerformed method is out of the curly braces of growAndShrinkListener method. The fixed code:
public class growAndShrinkListener implements ActionListener {
growAndShrinkListener(JButton button) {
button = new JButton("Click me to grow the Square");
frame.add(button, BorderLayout.NORTH);
button.addActionListener(this);
}
#Override
public void actionPerformed(ActionEvent e) {
Object source = e.getSource();
if(source instanceof JButton) {
button = (JButton) source;
button.setText("Click to shrink square");
}
}
}
Alternatively, you can use a private variable:
public class growAndShrinkListener implements ActionListener {
private JButton button;
growAndShrinkListener() {
button = new JButton("Click me to grow the Square");
frame.add(button, BorderLayout.NORTH);
button.addActionListener(this);
}
#Override
public void actionPerformed(ActionEvent e) {
button.setText("Click to shrink square");
}
}
Note: You should not overwrite the content of the argument in the constructor. The argument that you got is not a "pointer" to the callee's variable, but just a simple reference to that. If you overwrite, then the content will be lost for you and calle will not know that.

change like this..
public class YourSuperClass{
private JButton button;
public YourSuperClass(){
// your logics
button = new JButton();
button.addActionListener(new GrowAndShrinkListener(button));
frame.add(button, BorderLayout.NORTH);
}
class GrowAndShrinkListener implements ActionListener {
GrowAndShrinkListener(JButton button) {
}
#Override
public void actionPerformed(ActionEvent e) {
if(e.getSource() == button){
button.setText("Click to shrink square");
}
}
}
}

Try This :
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
class frame extends JFrame
{
JButton b;
public frame()
{
setLayout(new BorderLayout());
b=new JButton("Press");
add(b,BorderLayout.SOUTH);
b.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e)
{
b.setText("Clicked");
}
});
}
public static void main (String[] args) {
frame f=new frame();
f.setExtendedState(MAXIMIZED_BOTH);
f.setVisible(true);
}
}

Closing and reopeing of frame happens with increasing frequency each a button is pressed a single time

I am using a self made toolbar to navigate through my application and the toolbar is present on all pages. Each time a new page is displayed I am closing the current frame and opening a new one, using the following code:
java.awt.Window win[] = java.awt.Window.getWindows();
for(int i=0;i<win.length;i++){
win[i].dispose();
}
I am doing it this way as the ActionListeners are declared in the toolbar class, whilst the frames for each page are declared at runtime and are not static.
This all works fine except for one particular case-the "cancel" button, where the first time the frame is accessed it will close once. The second time it will close and re open 2 times, the third 3 and so on. I have tracked this using the "counter" in the code.
I have minimised the code to recreate the same behaviour, as below:
Toolbar Class
public class Toolbar {
static JButton buttonCancel = new JButton("Cancel");
static int counter;
public static JPanel Toolbar(String panelname){
FlowLayout layout = new FlowLayout();
JPanel Toolbar = new JPanel(new BorderLayout());
Toolbar.setLayout(layout);
GridLayout GLayout = new GridLayout(2,1);
GLayout.setVgap(0);
JPanel container2 = new JPanel();
if(panelname.matches("Customers")){
container2.setLayout(GLayout);
JButton buttonAddCust = new JButton("Add Cust");
container2.add(buttonAddCust, BorderLayout.PAGE_START);
buttonAddCust.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
java.awt.Window win[] = java.awt.Window.getWindows();
for(int i=0;i<win.length;i++){
win[i].dispose();
}
Customers.AddCustomersGui();
}
});
}
JPanel container21 = new JPanel();
if(panelname.matches("Add Customers")){
container21.setLayout(GLayout);
container21.add(buttonCancel, BorderLayout.PAGE_START);
buttonCancel.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
counter ++;
java.awt.Window win[] = java.awt.Window.getWindows();
for(int i=0;i<win.length;i++){
win[i].dispose();
}
System.out.println("Coutner " + counter);
Customers.CustomersGui();
}
});
}
Toolbar.add(container2);
Toolbar.add(container21);
return Toolbar;
}
}
GUI class
public class Customers extends Toolbar{
public static void CustomersGui(){
final JFrame frame = new JFrame("Customers");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel customers = new JPanel();
customers.add(Toolbar.Toolbar(frame.getTitle()));
frame.setContentPane(customers);
frame.setSize(1200,500);
frame.setVisible(true);
}
public static void AddCustomersGui(){
final JFrame frame1 = new JFrame("Add Customers");
frame1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel Addcustomers = new JPanel();
Addcustomers.add(Toolbar.Toolbar(frame1.getTitle()));
frame1.setContentPane(Addcustomers);
frame1.setSize(1200,500);
frame1.setVisible(true);
}
}
main class
public static void main(String[] args) {
Customers.CustomersGui();
}

You are adding a new ActionListener to the buttonCancel, with each iteration of your code and this is the reason for your program's behavior.
Also, as per my comment, you state,
Each time a new page is displayed I am closing the current frame and opening a new one.
A better design is probably not to swap windows which can be annoying, but rather to swap JPanel views using a CardLayout. Please read The Use of Multiple JFrames, Good/Bad Practice?.
For example, add this line of code to your program:
if (panelname.matches("Add Customers")) {
container21.setLayout(GLayout);
container21.add(buttonCancel, BorderLayout.PAGE_START);
buttonCancel.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
counter++;
java.awt.Window win[] = java.awt.Window.getWindows();
for (int i = 0; i < win.length; i++) {
win[i].dispose();
}
System.out.println("Coutner " + counter);
Customers.CustomersGui();
}
});
// ***** add this here **********
System.out.println("buttonCancel ActionListener count: "
+ buttonCancel.getListeners(ActionListener.class).length);
}
and you'll see that the ActionListeners get added multiple times to this button.
An example of swapping views:
import java.awt.*;
import java.awt.event.ActionEvent;
import javax.swing.*;
#SuppressWarnings("serial")
public class SwapPanels extends JPanel {
public static final String CUSTOMER = "customer";
public static final String ADD_CUSTOMER = "Add Customer";
protected static final int PREF_W = 800;
protected static final int PREF_H = 600;
public static final String CANCEL = "Cancel";
private CardLayout cardLayout = new CardLayout();
public SwapPanels() {
setLayout(cardLayout);
add(createCustomerPanel(CUSTOMER), CUSTOMER);
add(createAddCustomerPanel(ADD_CUSTOMER), ADD_CUSTOMER);
}
public void showCard(String key) {
cardLayout.show(this, key);
}
public JPanel createAddCustomerPanel(String name) {
JPanel addCustPanel = new JPanel() {
#Override
public Dimension getPreferredSize() {
if (isPreferredSizeSet()) {
return super.getPreferredSize();
}
return new Dimension(PREF_W, PREF_H);
}
};
addCustPanel.setName(name);
addCustPanel.setBorder(BorderFactory.createTitledBorder(name));
addCustPanel.add(new JButton(new AbstractAction(CANCEL) {
{
int mnemonic = (int)getValue(NAME).toString().charAt(0);
putValue(MNEMONIC_KEY, mnemonic);
}
#Override
public void actionPerformed(ActionEvent e) {
if (CANCEL.equals(e.getActionCommand())) {
SwapPanels.this.showCard(CUSTOMER);
}
}
}));
return addCustPanel;
}
private JPanel createCustomerPanel(String name) {
JPanel custPanel = new JPanel() {
#Override
public Dimension getPreferredSize() {
if (isPreferredSizeSet()) {
return super.getPreferredSize();
}
return new Dimension(PREF_W, PREF_H);
}
};
custPanel.setName(name);
custPanel.setBorder(BorderFactory.createTitledBorder(name));
custPanel.add(new JButton(new AbstractAction(ADD_CUSTOMER) {
{
int mnemonic = (int)getValue(NAME).toString().charAt(0);
putValue(MNEMONIC_KEY, mnemonic);
}
#Override
public void actionPerformed(ActionEvent e) {
if (ADD_CUSTOMER.equals(e.getActionCommand())) {
SwapPanels.this.showCard(ADD_CUSTOMER);
}
}
}));
return custPanel;
}
private static void createAndShowGui() {
JFrame frame = new JFrame("SwapPanels");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(new SwapPanels());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}

JFrame Trigger Events on Close

I know the title isn't very explanatory, I was unsure of how to phrase the question. What I have is a GUI that I want to trigger an event when the window in closed (including when you force quit the window/application). Thank you in advance for any help!!! Here's my code:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.IOException;
import java.io.Serializable;
public class NotAVirus extends JFrame {
private JTextField statusField = new JTextField(20);
private JButton yesButton = new JButton("Open");
private JButton noButton = new JButton("Close");
private static NotAVirus app = new ImLost();
public static void main() {
app.setVisible(true);
app.setLocationRelativeTo(null);
}
/**
* Create the GUI and show it. For thread safety,
* this method should be invoked from the
* event-dispatching thread.
*/
public ImLost() {
super("ImLost");
statusField.setText("There's No Escape");
statusField.setHorizontalAlignment(JLabel.CENTER);
statusField.setEditable(false);
add(statusField, BorderLayout.CENTER);
JPanel p = new JPanel();
p.setLayout(new GridLayout(1, 2));
p.add(yesButton);
p.add(noButton);
add(p, BorderLayout.SOUTH);
yesButton.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e) {
app.setVisible(false);
for(int i = 0; i <= 10000; i ++)
{
JFrame frame = new JFrame("ImLost");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JLabel emptyLabel = new JLabel("");
emptyLabel.setPreferredSize(new Dimension(160, 1));
frame.getContentPane().add(emptyLabel, BorderLayout.CENTER);
//Display the window.
frame.setLocation((int)(Math.random() * ((1280) + 1)),(int)(Math.random() * ((800) + 1)));
frame.pack();
frame.setVisible(true);
}
}
});
noButton.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e) {
app.setVisible(false);
for(int i = 0; i <= 10000; i ++)
{
JFrame frame = new JFrame("ImLost");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JLabel emptyLabel = new JLabel("");
emptyLabel.setPreferredSize(new Dimension(160, 1));
frame.getContentPane().add(emptyLabel, BorderLayout.CENTER);
//Display the window.
frame.setLocation((int)(Math.random() * ((1280) + 1)),(int)(Math.random() * ((800) + 1)));
frame.pack();
frame.setVisible(true);
}
}
});
p.setPreferredSize(new Dimension(200, 35));
pack();
}

You can add a WindowListener to the JFrame:
frame.addWindowListener(new WindowListener() {
#Override
public void windowOpened(WindowEvent e) {
}
#Override
public void windowClosing(WindowEvent e) {
//window is being closed
}
#Override
public void windowClosed(WindowEvent e) {
//window is closed
}
#Override
public void windowIconified(WindowEvent e) {
}
#Override
public void windowDeiconified(WindowEvent e) {
}
#Override
public void windowActivated(WindowEvent e) {
}
#Override
public void windowDeactivated(WindowEvent e) {
}
});
I want to trigger an event when the window in closed (including when you force quit the window/application)
If you mean "when someone kills the process" i think it wouldn't be possible, as the process gets killed and so stops its execution immediately. If you mean "when the application freezes and the user forces it to close" I think it wouldn't be possible too, usually if you force the quit of an application, it means that it's frozen and it's not responding anymore, so executing other code of it wouldn't be possible.

How can i pass the data from one JPanel to other...?

I have a JFrame which contains 3 JPanels. I want to pass the JTextField value of one panel to other. Each panel is shown using JTabbedPane. I am getting null when i access the value of other text field. How can i access?

You don't show any code, and so it's impossible to know why you're getting "null" values. Two possible solutions if you want all three JPanels to hold JTextFields with the same content:
Put the shared JTextField outside of the JPanels held by the JTabbedPane and instead in a JPanel that holds the JTabbedPane, so that the field is always visible no matter what tab is displayed, or
Use several JTextFields but have them share the same Document or "model".
e.g.,
import java.awt.Dimension;
import javax.swing.*;
import javax.swing.text.PlainDocument;
public class SharedField extends JTabbedPane {
private static final int TAB_COUNT = 5;
private static final int MY_WIDTH = 600;
private static final int MY_HEIGHT = 300;
PlainDocument doc = new PlainDocument();
public SharedField() {
for (int i = 0; i < TAB_COUNT; i++) {
JTextField tField = new JTextField(10);
tField.setDocument(doc);
JPanel panel = new JPanel();
panel.add(tField);
add("Panel " + i, panel);
// to demonstrate some of the JTextFields acting like
// a label
if (i % 2 == 1) { // if i is odd
tField.setEditable(false);
tField.setBorder(null);
}
}
}
#Override
public Dimension getPreferredSize() {
return new Dimension(MY_WIDTH, MY_HEIGHT);
}
private static void createAndShowUI() {
JFrame frame = new JFrame("SharedField");
frame.getContentPane().add(new SharedField());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public static void main(String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
createAndShowUI();
}
});
}
}
Edit 1
I see that you've cross-posted this on java-forums.org/ where you show some of your code:
pacage Demotool;
Class:MainFrame
This is the actionPerformed code of first panel
both str and scrTxt is (public static)
public void actionPerformed(ActionEvent e)
{
String act=e.getActionCommand();
if(act.equals("ADD"))
{
str=scrnTxt.getText();
System.out.println("Hi :"+str);
Demotool.DemoTool.jtp.setSelectedIndex(1);
}
}
using the belove code i tried to access the data but I am getting null String:
System.out.println("Hello:"+Demotool.MainFrame.str);
Problems:
Don't use static variables or methods unless you have a good reason to do so. Here you don't.
You're may be trying to access the MainFrame.str variable before anything has been put into it, making it null, or you are creating a new MainFrame object in your second class, one that isn't displayed, and thus one whose str variable is empty or null -- hard to say.
Either way, this design is not good. You're better off showing us a small demo program that shows your problem with code that compiles and runs, an sscce, so we can play with and modify your code and better be able to show you a decent solution.
One such decent solution is to add a DocumentListener to the JTextField so that changes to the text held by the JTextField are "pushed" into the observers that are listening for changes (your other classes).
For example, using DocumentListeners:
import java.awt.Dimension;
import javax.swing.*;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.BadLocationException;
public class SharedField2 extends JTabbedPane {
private static final int LABEL_PANEL_COUNT = 4;
private static final int MY_WIDTH = 600;
private static final int MY_HEIGHT = 300;
public SharedField2() {
TextFieldPanel tfPanel = new TextFieldPanel();
LabelPanel[] labelPanels = new LabelPanel[LABEL_PANEL_COUNT];
add("TextFieldPanel", tfPanel);
for (int i = 0; i < labelPanels.length; i++) {
labelPanels[i] = new LabelPanel();
// add each label panel's listener to the text field
tfPanel.addDocumentListenerToField(labelPanels[i].getDocumentListener());
add("Label Panel " + i, labelPanels[i]);
}
}
#Override
public Dimension getPreferredSize() {
return new Dimension(MY_WIDTH, MY_HEIGHT);
}
private static void createAndShowUI() {
JFrame frame = new JFrame("SharedField2");
frame.getContentPane().add(new SharedField2());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public static void main(String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
createAndShowUI();
}
});
}
}
class TextFieldPanel extends JPanel {
private JTextField tField = new JTextField(10);
public TextFieldPanel() {
add(tField);
}
public void addDocumentListenerToField(DocumentListener listener) {
tField.getDocument().addDocumentListener(listener);
}
}
class LabelPanel extends JPanel {
private DocumentListener myListener;
private JLabel label = new JLabel();
public LabelPanel() {
add(label);
myListener = new DocumentListener() {
#Override
public void changedUpdate(DocumentEvent e) {
updateLabel(e);
}
#Override
public void insertUpdate(DocumentEvent e) {
updateLabel(e);
}
#Override
public void removeUpdate(DocumentEvent e) {
updateLabel(e);
}
private void updateLabel(DocumentEvent e) {
try {
label.setText(e.getDocument().getText(0,
e.getDocument().getLength()));
} catch (BadLocationException e1) {
e1.printStackTrace();
}
}
};
}
public DocumentListener getDocumentListener() {
return myListener;
}
}

One simple solution will be making JTextField global so all panel can access it.
Make sure all your panel can access JTextField that is textField is globally accessible.
Following code demonstrate this:
JTextField textField = new JTextField(25);
JLabel labelForPanel2 = new JLabel(),labelForPanel3 = new JLabel();
private void panelDemo() {
JFrame frame = new JFrame();
frame.setSize(400, 400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel panel1 = new JPanel();
JPanel panel2 = new JPanel();
JPanel panel3 = new JPanel();
JTabbedPane tabbedPane = new JTabbedPane();
tabbedPane.addTab("Tab 1", panel1);
tabbedPane.addTab("Tab 2", panel2);
tabbedPane.addTab("Tab 3", panel3);
panel1.add(textField);
panel2.add(labelForPanel2);
panel3.add(labelForPanel3);
textField.addKeyListener(new KeyAdapter() {
#Override
public void keyReleased(KeyEvent e) {
labelForPanel2.setText(textField.getText());
labelForPanel3.setText(textField.getText());
}
});
frame.add(tabbedPane);
frame.setVisible(true);
}

I don't know what exactly are you going to achieve, but maybe try data binding?
Take a look at BetterBeansBinding library.