FULL PRODUCT VERSION :
java version "1.6.0.05"
Java(TM) SE Runtime Environment (build 1.6.0.05-jinteg_14_oct_2009_01_44-b00)
Java HotSpot(TM) 64-Bit Server VM (build 14.2-b01-jre1.6.0.05-rc5, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
HP-UX drsaca05 B.11.31 U ia64 1459067606 unlimited-user license

A DESCRIPTION OF THE PROBLEM :
This problem arises because the strikeCacheMap in Font2D is populated by calling StrikeCache.getStrikeRef(FontStrike strike, boolean weak) which can return a SoftReference, WeakReference, or a StrikeCache.DisposableStrike.  Font2D assumes everything in strikeCacheMap implements StrikeCache.DisposableStrike and will occasionally throw a ClassCastException as a result.

I do not know why I'm running into the case where a FontStrike does not have a disposer, but this issue seems resolved after specifying java.awt.headless=true.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the provided test program on HP-UX.  After about 5 minutes, the program should throw the ClassCastException.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No error.
ACTUAL -
java.lang.ClassCastException: java.lang.ref.SoftReference cannot be cast to
sun.font.StrikeCache$DisposableStrike
    at sun.font.Font2D.getStrike(Font2D.java:308)
    at sun.font.Font2D.getStrike(Font2D.java:262)
    at sun.font.CompositeStrike.getStrikeForSlot(CompositeStrike.java:59)
    at sun.font.CompositeStrike.getFontMetrics(CompositeStrike.java:75)
    at
sun.font.FontDesignMetrics.initMatrixAndMetrics(FontDesignMetrics.java:345)
    at sun.font.FontDesignMetrics.<init>(FontDesignMetrics.java:336)
    at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:284)
    at sun.java2d.SunGraphics2D.getFontMetrics(SunGraphics2D.java:764)

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.ClassCastException: java.lang.ref.SoftReference cannot be cast to
sun.font.StrikeCache$DisposableStrike
    at sun.font.Font2D.getStrike(Font2D.java:308)
    at sun.font.Font2D.getStrike(Font2D.java:262)
    at sun.font.CompositeStrike.getStrikeForSlot(CompositeStrike.java:59)
    at sun.font.CompositeStrike.getFontMetrics(CompositeStrike.java:75)
    at
sun.font.FontDesignMetrics.initMatrixAndMetrics(FontDesignMetrics.java:345)
    at sun.font.FontDesignMetrics.<init>(FontDesignMetrics.java:336)
    at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:284)
    at sun.java2d.SunGraphics2D.getFontMetrics(SunGraphics2D.java:764)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package fonttest;

import java.awt.Color;
import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GraphicsEnvironment;
import java.awt.Rectangle;
import java.awt.image.BufferedImage;

public class Main {

    private static String[] fontNames;
    private static final int SIZE = 100;

    private static void _resetFont(Graphics g) {
        Font font = g.getFont();
        int numfonts = fontNames.length;
        for (int j = 0; j < numfonts; ++j) {
            String name = fontNames[j];
            for (int i = 1; i < 100; ++i) {
                Font f = new Font(name, font.getStyle(), i);
                g.setFont(f);
                FontMetrics fm = g.getFontMetrics();
            }
        }
        System.gc();
        Thread.yield();

        g.setFont(font);
    }

    public static void paint(Graphics g) {
        Font font = new Font("Arial", Font.PLAIN, 9);
        Rectangle r = new Rectangle(0, 0, SIZE, SIZE);
        g.drawLine(0, 0, r.width - 1, r.height - 1);
        g.setColor(new Color(255, 127, 0));
        g.drawLine(0, r.height - 1, r.width - 1, 0);
        g.setFont(font);
        _resetFont(g);
        g.drawString("Test", 32, 8);
    }

    public static void main(String[] args) {
        System.setProperty("java.awt.headless", "false");

       GraphicsEnvironment ge =
        GraphicsEnvironment.getLocalGraphicsEnvironment();
        System.out.println("Headless mode: " + ge.isHeadless());

        fontNames = ge.getAvailableFontFamilyNames();

        BufferedImage image = new BufferedImage(SIZE, SIZE, BufferedImage.TYPE_4BYTE_ABGR);
        Graphics2D gr = image.createGraphics();
        while (true) {
            try {
                paint(gr);
            } catch (Exception e) {
                e.printStackTrace();
                System.exit(0);
            }
        }
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
set java.awt.headless=true