garbage collector for JFrame's object

问题:

import javax.swing.*;

public class Main
{
    public Main()
    {
        JFrame jf = new JFrame("Demo");
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setSize(100, 100);
        jf.setVisible(true);
    }
    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            @Override
            public void run()
            {
                new Main();
            }
        });
        Runtime.getRuntime().gc();

    }

}

I call Runtime.getRuntime().gc(); for explicit garbage collector invoking. But window doesn't dissapear from screen, why doesn't garbage collector reclaim JFrame's object?

回答1:

When a JFrame is created, it registers itself in some internal Swing data structures which allow it to receive events like mouse clicks. This means there is a reference to your object lurking somewhere until you tell Swing to get rid of the window using dispose().

回答2:

Given the invokeLater() call, the call to GC will probably occur 1st¹.

BTW - calling Runtime.gc() is generally pointless, the JRE won't GC till it needs to.

1. E.G.

Output

GC called
Frame visible

Code

package test;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class VisibleFrameGC {

    VisibleFrameGC() {
        JFrame jf = new JFrame("Demo");
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setSize(100, 100);
        jf.setVisible(true);
        System.out.println("Frame visible");
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new VisibleFrameGC();
            }
        });
        Runtime.getRuntime().gc();
        System.out.println("GC called");
    }
}

回答3:

The frame is visible and the reference to the object is reachable by at least one of the GUI threads (the Event Dispatch Thread). That is why it isn't garbage collected.

---

If you want it to disapear, use frame.dispose().