Java and GUI - Where do ActionListeners belong according to MVC pattern?

Question

I'm currently writing a template Java application and somehow, I'm not sure about where the ActionListeners belong if I wanted to cleanly follow the MVC pattern.

The example is Swing based, but it's not about the framework but rather the basic concept of MVC in Java, using any framework to create GUI.

I started with an absolutely simple application containing a JFrame and a JButton (to dispose the frame hence close the application). The code trailing this post. Nothing really special, just to clearify what we're talking about. I didn't start with the Model yet as this question was bugging me too much.

There has already been more than one similar question(s), like these:
MVC pattern with many ActionListeners
Java swing - Where should the ActionListener go?

But non of them was really satisfying as I'd like to know two things:

• Is it reasonable to have all ActionListeners in a separate package?
  • I'd like to do so for the sake of readability of View and Controller, esp. if there's a lot of listeners
• How would I execute a Controller function from within an ActionListener, if the listener is not a sub class inside the Controller? (follow-up question)

I hope this is not too general or vague I'm asking here, but it makes me think for a while now. I always used sort of my own way, letting the ActionHandler know about the Controller, but this does not seem right, so I'd finally like to know how this is done properly.

Kind regards,
jaySon

---

Controller:

package controller;  import java.awt.event.ActionEvent; import java.awt.event.ActionListener;  import view.MainView;  public class MainController {     MainView mainView = new MainView();      public MainController()     {         this.initViewActionListeners();     }      private void initViewActionListeners()     {         mainView.initButtons(new CloseListener());     }      public class CloseListener implements ActionListener     {         @Override         public void actionPerformed(ActionEvent e)         {             mainView.dispose();         }     } } 

View:

package view;  import java.awt.Dimension; import java.awt.event.ActionListener;  import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JPanel;  public class MainView extends JFrame {     JButton button_close    = new JButton();     JPanel  panel_mainPanel = new JPanel();      private static final long   serialVersionUID    = 5791734712409634055L;      public MainView()     {         setDefaultCloseOperation(DISPOSE_ON_CLOSE);         this.setSize(500, 500);         this.add(panel_mainPanel);         setVisible(true);     }      public void initButtons(ActionListener actionListener)     {         this.button_close = new JButton("Close");         this.button_close.setSize(new Dimension(100, 20));         this.button_close.addActionListener(actionListener);         this.panel_mainPanel.add(button_close);     } } 

Answer 1

That's a very difficult question to answer with Swing, as Swing is not a pure MVC implementation, the view and controller are mixed.

Technically, a model and controller should be able to interact and the controller and view should be able to interact, but the view and model should never interact, which clearly isn't how Swing works, but that's another debate...

Another issue is, you really don't want to expose UI components to anybody, the controller shouldn't care how certain actions occur, only that they can.

This would suggest that the ActionListeners attached to your UI controls should be maintained by the view. The view should then alert the controller that some kind of action has occurred. For this, you could use another ActionListener, managed by the view, to which the controller subscribes to.

Better yet, I would have a dedicated view listener, which described the actions that this view might produce, for example...

public interface MainViewListener {     public void didPerformClose(MainView mainView); } 

The controller would then subscribe to the view via this listener and the view would call didPerformClose when (in this case) the close button is pressed.

Even in this example, I would be tempted to make a "main view" interface, which described the properties (setters and getters) and actions (listeners/callbacks) that any implementation is guaranteed to provide, then you don't care how these actions occur, only that when they do, you are expected to do something...

At each level you want to ask yourself, how easy would it be to change any element (change the model or the controller or the view) for another instance? If you find yourself having to decouple the code, then you have a problem. Communicate via interfaces and try and reduce the amount of coupling between the layers and the amount that each layer knows about the others to the point where they are simply maintaining contracts

Updated...

Let's take this for an example...

There are actually two views (discounting the actual dialog), there is the credentials view and the login view, yes they are different as you will see.

CredentialsView

The credentials view is responsible for collecting the details that are to be authenticated, the user name and password. It will provide information to the controller to let it know when those credentials have been changed, as the controller may want to take some action, like enabling the "login" button...

The view will also want to know when authentication is about to take place, as it will want to disable it's fields, so the user can't update the view while the authentication is taking place, equally, it will need to know when the authentication fails or succeeds, as it will need to take actions for those eventualities.

public interface CredentialsView {      public String getUserName();     public char[] getPassword();      public void willAuthenticate();     public void authenticationFailed();     public void authenticationSucceeded();      public void setCredentialsViewController(CredentialsViewController listener);  }  public interface CredentialsViewController {      public void credientialsDidChange(CredentialsView view);  } 

CredentialsPane

The CredentialsPane is the physical implementation of a CredentialsView, it implements the contract, but manages it's own internal state. How the contract is managed is irrelevent to the controller, it only cares about the contract been upheld...

public class CredentialsPane extends JPanel implements CredentialsView {      private CredentialsViewController controller;      private JTextField userNameField;     private JPasswordField passwordField;      public CredentialsPane(CredentialsViewController controller) {         setCredentialsViewController(controller);         setLayout(new GridBagLayout());         userNameField = new JTextField(20);         passwordField = new JPasswordField(20);          GridBagConstraints gbc = new GridBagConstraints();         gbc.gridx = 0;         gbc.gridy = 0;         gbc.insets = new Insets(2, 2, 2, 2);         gbc.anchor = GridBagConstraints.EAST;         add(new JLabel("Username: "), gbc);          gbc.gridy++;         add(new JLabel("Password: "), gbc);          gbc.gridx = 1;         gbc.gridy = 0;         gbc.anchor = GridBagConstraints.WEST;         gbc.fill = GridBagConstraints.HORIZONTAL;         add(userNameField, gbc);         gbc.gridy++;         add(passwordField, gbc);          DocumentListener listener = new DocumentListener() {             @Override             public void insertUpdate(DocumentEvent e) {                 getCredentialsViewController().credientialsDidChange(CredentialsPane.this);             }              @Override             public void removeUpdate(DocumentEvent e) {                 getCredentialsViewController().credientialsDidChange(CredentialsPane.this);             }              @Override             public void changedUpdate(DocumentEvent e) {                 getCredentialsViewController().credientialsDidChange(CredentialsPane.this);             }         };          userNameField.getDocument().addDocumentListener(listener);         passwordField.getDocument().addDocumentListener(listener);      }      @Override     public CredentialsViewController getCredentialsViewController() {         return controller;     }      @Override     public String getUserName() {         return userNameField.getText();     }      @Override     public char[] getPassword() {         return passwordField.getPassword();     }      @Override     public void willAuthenticate() {         userNameField.setEnabled(false);         passwordField.setEnabled(false);     }      @Override     public void authenticationFailed() {         userNameField.setEnabled(true);         passwordField.setEnabled(true);          userNameField.requestFocusInWindow();         userNameField.selectAll();          JOptionPane.showMessageDialog(this, "Authentication has failed", "Error", JOptionPane.ERROR_MESSAGE);     }      @Override     public void authenticationSucceeded() {         // Really don't care, but you might want to stop animation, for example...     }      public void setCredentialsViewController(CredentialsViewController controller){         this.controller = controller;     }  } 

LoginView

The LoginView is responsible for managing a CredentialsView, but also for notifying the LoginViewController when authentication should take place or if the process was cancelled by the user, via some means...

Equally, the LoginViewController will tell the view when authentication is about to take place and if the authentication failed or was successful.

public interface LoginView {      public CredentialsView getCredentialsView();      public void willAuthenticate();     public void authenticationFailed();     public void authenticationSucceeded();      public void dismissView();      public LoginViewController getLoginViewController();  }  public interface LoginViewController {      public void authenticationWasRequested(LoginView view);     public void loginWasCancelled(LoginView view);  } 

LoginPane

The LoginPane is kind of special, it is acting as the view for the LoginViewController, but it is also acting as the controller for the CredentialsView. This is important, as there is nothing saying that a view can't be a controller, but I would be careful about how you implement such things, as it might not always make sense to do it this way, but because the two views are working together to gather information and manage events, it made sense in this case.

Because the LoginPane will need to change it's own state based on the changes in the CredentialsView, it makes sense to allow the LoginPane to act as the controller in this case, otherwise, you'd need to supply more methods that controlled that state of the buttons, but this starts to bleed UI logic over to the controller...

public static class LoginPane extends JPanel implements LoginView, CredentialsViewController {      private LoginViewController controller;     private CredentialsPane credientialsView;      private JButton btnAuthenticate;     private JButton btnCancel;      private boolean wasAuthenticated;      public LoginPane(LoginViewController controller) {         setLoginViewController(controller);         setLayout(new BorderLayout());         setBorder(new EmptyBorder(8, 8, 8, 8));          btnAuthenticate = new JButton("Login");         btnCancel = new JButton("Cancel");          JPanel buttons = new JPanel();         buttons.add(btnAuthenticate);         buttons.add(btnCancel);          add(buttons, BorderLayout.SOUTH);          credientialsView = new CredentialsPane(this);         add(credientialsView);          btnAuthenticate.addActionListener(new ActionListener() {             @Override             public void actionPerformed(ActionEvent e) {                 getLoginViewController().authenticationWasRequested(LoginPane.this);             }         });         btnCancel.addActionListener(new ActionListener() {             @Override             public void actionPerformed(ActionEvent e) {                 getLoginViewController().loginWasCancelled(LoginPane.this);                 // I did think about calling dispose here,                 // but's not really the the job of the cancel button to decide what should happen here...             }         });          validateCreientials();      }      public static boolean showLoginDialog(LoginViewController controller) {          final LoginPane pane = new LoginPane(controller);          JDialog dialog = new JDialog();         dialog.setTitle("Login");         dialog.setModal(true);         dialog.add(pane);         dialog.pack();         dialog.setLocationRelativeTo(null);         dialog.setDefaultCloseOperation(JDialog.DO_NOTHING_ON_CLOSE);         dialog.addWindowListener(new WindowAdapter() {             @Override             public void windowClosing(WindowEvent e) {                 pane.getLoginViewController().loginWasCancelled(pane);             }         });         dialog.setVisible(true);          return pane.wasAuthenticated();      }      public boolean wasAuthenticated() {         return wasAuthenticated;     }      public void validateCreientials() {          CredentialsView view = getCredentialsView();         String userName = view.getUserName();         char[] password = view.getPassword();         if ((userName != null && userName.trim().length() > 0) && (password != null && password.length > 0)) {              btnAuthenticate.setEnabled(true);          } else {              btnAuthenticate.setEnabled(false);          }      }      @Override     public void dismissView() {         SwingUtilities.windowForComponent(this).dispose();     }      @Override     public CredentialsView getCredentialsView() {         return credientialsView;     }      @Override     public void willAuthenticate() {         getCredentialsView().willAuthenticate();         btnAuthenticate.setEnabled(false);     }      @Override     public void authenticationFailed() {         getCredentialsView().authenticationFailed();         validateCreientials();         wasAuthenticated = false;     }      @Override     public void authenticationSucceeded() {         getCredentialsView().authenticationSucceeded();         validateCreientials();         wasAuthenticated = true;     }      public LoginViewController getLoginViewController() {         return controller;     }      public void setLoginViewController(LoginViewController controller) {         this.controller = controller;     }      @Override     public void credientialsDidChange(CredentialsView view) {         validateCreientials();     }  } 

Working example

import java.awt.BorderLayout; import java.awt.EventQueue; import java.awt.GridBagConstraints; import java.awt.GridBagLayout; import java.awt.Insets; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.awt.event.WindowAdapter; import java.awt.event.WindowEvent; import java.util.Random; import java.util.logging.Level; import java.util.logging.Logger; import javax.swing.JButton; import javax.swing.JDialog; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JOptionPane; import javax.swing.JPanel; import javax.swing.JPasswordField; import javax.swing.JTextField; import javax.swing.SwingUtilities; import javax.swing.UIManager; import javax.swing.UnsupportedLookAndFeelException; import javax.swing.border.EmptyBorder; import javax.swing.event.DocumentEvent; import javax.swing.event.DocumentListener; import sun.net.www.protocol.http.HttpURLConnection;  public class Test {      protected static final Random AUTHENTICATION_ORACLE = new Random();      public static void main(String[] args) {         new Test();     }      public interface CredentialsView {         public String getUserName();         public char[] getPassword();         public void willAuthenticate();         public void authenticationFailed();         public void authenticationSucceeded();         public CredentialsViewController getCredentialsViewController();     }      public interface CredentialsViewController {         public void credientialsDidChange(CredentialsView view);     }      public interface LoginView {         public CredentialsView getCredentialsView();         public void willAuthenticate();         public void authenticationFailed();         public void authenticationSucceeded();         public void dismissView();         public LoginViewController getLoginViewController();     }      public interface LoginViewController {         public void authenticationWasRequested(LoginView view);         public void loginWasCancelled(LoginView view);     }      public Test() {         EventQueue.invokeLater(new Runnable() {             @Override             public void run() {                 try {                     UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());                 } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {                     ex.printStackTrace();                 }                  LoginViewController controller = new LoginViewController() {                      @Override                     public void authenticationWasRequested(LoginView view) {                         view.willAuthenticate();                         LoginAuthenticator authenticator = new LoginAuthenticator(view);                         authenticator.authenticate();                     }                      @Override                     public void loginWasCancelled(LoginView view) {                          view.dismissView();                      }                 };                  if (LoginPane.showLoginDialog(controller)) {                      System.out.println("You shell pass");                  } else {                      System.out.println("You shell not pass");                  }                  System.exit(0);              }         });     }      public class LoginAuthenticator {          private LoginView view;          public LoginAuthenticator(LoginView view) {             this.view = view;         }          public void authenticate() {              Thread t = new Thread(new Runnable() {                 @Override                 public void run() {                     try {                         Thread.sleep(2000);                     } catch (InterruptedException ex) {                         Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);                     }                     SwingUtilities.invokeLater(new Runnable() {                         @Override                         public void run() {                             if (AUTHENTICATION_ORACLE.nextBoolean()) {                                 view.authenticationSucceeded();                                 view.dismissView();                             } else {                                 view.authenticationFailed();                             }                         }                     });                 }             });             t.start();          }      }      public static class LoginPane extends JPanel implements LoginView, CredentialsViewController {          private LoginViewController controller;         private CredentialsPane credientialsView;          private JButton btnAuthenticate;         private JButton btnCancel;          private boolean wasAuthenticated;          public LoginPane(LoginViewController controller) {             setLoginViewController(controller);             setLayout(new BorderLayout());             setBorder(new EmptyBorder(8, 8, 8, 8));              btnAuthenticate = new JButton("Login");             btnCancel = new JButton("Cancel");              JPanel buttons = new JPanel();             buttons.add(btnAuthenticate);             buttons.add(btnCancel);              add(buttons, BorderLayout.SOUTH);              credientialsView = new CredentialsPane(this);             add(credientialsView);              btnAuthenticate.addActionListener(new ActionListener() {                 @Override                 public void actionPerformed(ActionEvent e) {                     getLoginViewController().authenticationWasRequested(LoginPane.this);                 }             });             btnCancel.addActionListener(new ActionListener() {                 @Override                 public void actionPerformed(ActionEvent e) {                     getLoginViewController().loginWasCancelled(LoginPane.this);                     // I did think about calling dispose here,                     // but's not really the the job of the cancel button to decide what should happen here...                 }             });              validateCreientials();          }          public static boolean showLoginDialog(LoginViewController controller) {              final LoginPane pane = new LoginPane(controller);              JDialog dialog = new JDialog();             dialog.setTitle("Login");             dialog.setModal(true);             dialog.add(pane);             dialog.pack();             dialog.setLocationRelativeTo(null);             dialog.setDefaultCloseOperation(JDialog.DO_NOTHING_ON_CLOSE);             dialog.addWindowListener(new WindowAdapter() {                 @Override                 public void windowClosing(WindowEvent e) {                     pane.getLoginViewController().loginWasCancelled(pane);                 }             });             dialog.setVisible(true);              return pane.wasAuthenticated();          }          public boolean wasAuthenticated() {             return wasAuthenticated;         }          public void validateCreientials() {              CredentialsView view = getCredentialsView();             String userName = view.getUserName();             char[] password = view.getPassword();             if ((userName != null && userName.trim().length() > 0) && (password != null && password.length > 0)) {                  btnAuthenticate.setEnabled(true);              } else {                  btnAuthenticate.setEnabled(false);              }          }          @Override         public void dismissView() {             SwingUtilities.windowForComponent(this).dispose();         }          @Override         public CredentialsView getCredentialsView() {             return credientialsView;         }          @Override         public void willAuthenticate() {             getCredentialsView().willAuthenticate();             btnAuthenticate.setEnabled(false);         }          @Override         public void authenticationFailed() {             getCredentialsView().authenticationFailed();             validateCreientials();             wasAuthenticated = false;         }          @Override         public void authenticationSucceeded() {             getCredentialsView().authenticationSucceeded();             validateCreientials();             wasAuthenticated = true;         }          public LoginViewController getLoginViewController() {             return controller;         }          public void setLoginViewController(LoginViewController controller) {             this.controller = controller;         }          @Override         public void credientialsDidChange(CredentialsView view) {             validateCreientials();         }      }      public static class CredentialsPane extends JPanel implements CredentialsView {          private CredentialsViewController controller;          private JTextField userNameField;         private JPasswordField passwordField;          public CredentialsPane(CredentialsViewController controller) {             setCredentialsViewController(controller);             setLayout(new GridBagLayout());             userNameField = new JTextField(20);             passwordField = new JPasswordField(20);              GridBagConstraints gbc = new GridBagConstraints();             gbc.gridx = 0;             gbc.gridy = 0;             gbc.insets = new Insets(2, 2, 2, 2);             gbc.anchor = GridBagConstraints.EAST;             add(new JLabel("Username: "), gbc);              gbc.gridy++;             add(new JLabel("Password: "), gbc);              gbc.gridx = 1;             gbc.gridy = 0;             gbc.anchor = GridBagConstraints.WEST;             gbc.fill = GridBagConstraints.HORIZONTAL;             add(userNameField, gbc);             gbc.gridy++;             add(passwordField, gbc);              DocumentListener listener = new DocumentListener() {                 @Override                 public void insertUpdate(DocumentEvent e) {                     getCredentialsViewController().credientialsDidChange(CredentialsPane.this);                 }                  @Override                 public void removeUpdate(DocumentEvent e) {                     getCredentialsViewController().credientialsDidChange(CredentialsPane.this);                 }                  @Override                 public void changedUpdate(DocumentEvent e) {                     getCredentialsViewController().credientialsDidChange(CredentialsPane.this);                 }             };              userNameField.getDocument().addDocumentListener(listener);             passwordField.getDocument().addDocumentListener(listener);          }          @Override         public CredentialsViewController getCredentialsViewController() {             return controller;         }          @Override         public String getUserName() {             return userNameField.getText();         }          @Override         public char[] getPassword() {             return passwordField.getPassword();         }          @Override         public void willAuthenticate() {             userNameField.setEnabled(false);             passwordField.setEnabled(false);         }          @Override         public void authenticationFailed() {             userNameField.setEnabled(true);             passwordField.setEnabled(true);              userNameField.requestFocusInWindow();             userNameField.selectAll();              JOptionPane.showMessageDialog(this, "Authentication has failed", "Error", JOptionPane.ERROR_MESSAGE);         }          @Override         public void authenticationSucceeded() {             // Really don't care, but you might want to stop animation, for example...         }          public void setCredentialsViewController(CredentialsViewController controller) {             this.controller = controller;         }      }  }