swing JNI how to free memory

[John_Woo]

Hi,

I had a class Screen extends JWindow, and this class had a few
attributes as

ImageIcon icon = new ImageIcon("big jpg");

JLabel lable.setIcon(icon);

then in main class, after creating Screen s, did the following:

System.gc();

Runtime r = Runtime.getruntime();

long before = r.freeMemory();

s.icon = null;
s.lable = null;
s.dispose();
s = null;
System.gc();

long after = r.freeMemory();

from (after-before), I saw free memory increased a lots, but from some
trace tool, I saw that the reference still there, like

Paths from "GC Roots" to "Object ToolkitImage #004fdf56"
sun.awt.image.ToolkitImage
image of javax.swing.ImageIcon
defaultIcon of javax.swing.JLabel
[0] of java.awt.Component[3]
component of javax.swing.JPanel
contentPane of javax.swing.JRootPane
rootPane of Screen [JNI Global]

consulting the tool vendor, it said the object is referenced by native
called, can't really free up by setting null

I'm wondering, how to clean up such object or free swing component?