Want javax.swing hook that tells me WHICH component in the hierarchy is executing an action

Question

How with minimum code uglification can I write a debugging hook in a Swing program that would tell me which component in the hierarchy is actually handling each KeyStroke or mouse click and performing the action mapped to it in the component's action map? We are writing a complicated GUI and it would be very useful to know this information.

Answer 1

Put in a custom event dispatcher: http://tips4java.wordpress.com/2009/09/06/global-event-dispatching/

Also look up AWTEvent.getID(), MouseEvent, KeyEvent in the API docs.

This is how the software I work on monitors mouse and keyboard to see if the user is busy working, and locks the window if they're not.

Answer 2

Using AspectJ to weave a logging aspect into your code base can be done. Below is an example of advice on a joinpoint within execution of any objects with the method actionPerformed(ActionEvent) you have in your code base. Similar constructions can be used to advise other Listeners.

Below is the aspect to advise button presses and other components having ActionListeners. It simply outputs the class name of the source of the action and the signature of the actionPerformed method.

import java.awt.event.ActionEvent;

import org.aspectj.lang.Signature;

public aspect Logger {
  before(ActionEvent e) : execution(* *.actionPerformed(ActionEvent)) && args(e) {
    Signature sig = thisJoinPoint.getSignature();
    System.out.println(e.getSource().getClass() + " lead to " + sig);
  }
}

A test class which produces two buttons of different classes (in file StackTraceExample.java):

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

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.SwingUtilities;

class MyButton extends JButton {
  public MyButton(String string) {
    super(string);
  }
}

class MyOtherButton extends JButton {
  public MyOtherButton(String string) {
    super(string);
  }
}

class ButtonStackDisplay implements ActionListener {
  private final JTextArea stackTraceText;

  ButtonStackDisplay(JTextArea textArea) {
    this.stackTraceText = textArea;
  }

  public void actionPerformed(ActionEvent e) {
    String endl = System.getProperty("line.separator");
    StringBuilder b = new StringBuilder();

    // you can see the source of the event 
    b.append(e.getSource()).append(endl).append(endl);

    // the stack trace shows that events don't propagate through the components
    // originating them, but instead processed in a different thread
    for (StackTraceElement se : new Throwable().getStackTrace()) {
      b.append(se.toString());
      b.append(endl);
    }
    stackTraceText.setText(b.toString());
  }
}

public class StackTraceExample {
  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        f.setLayout(new BorderLayout());
        JPanel top = new JPanel();
        JButton button = new MyButton("Stack Trace");
        top.setLayout(new GridLayout(2, 1));
        top.add(button);
        JButton otherButton = new MyOtherButton("Stack Trace");
        top.add(otherButton);
        f.getContentPane().add(top, BorderLayout.NORTH);
        JTextArea stackTraceText = new JTextArea();
        f.add(stackTraceText, BorderLayout.CENTER);
        ButtonStackDisplay bsd = new ButtonStackDisplay(stackTraceText);
        button.addActionListener(bsd);
        otherButton.addActionListener(bsd);
        f.setSize(400, 400);
        f.setLocationRelativeTo(null);
        f.setVisible(true);
        f.toFront();
      }
    });
  }
}