FULL PRODUCT VERSION :
JRE 1.6.0_03-b05

ADDITIONAL OS VERSION INFORMATION :
Linux 2.6.21-1.3194.fc7 #1 SMP Wed May 23 22:35:01 EDT 2007 i686 i686 i386 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
Graphics.getFontMetrics() throws NullPointerException.

Happens on FontMetrics.charsWidth  also.

The problem seems to be in sun.font.FontManager.addToPool(). The following assertion fails.

// is it possible for this to be the same font?
assert fontFileCache[lastPoolIndex] != font;

This can happen when the sun.font.TrueTypeFont.readBlock() was executed, and during execution if the threads were interrupted.  Several threads executing readBlock() needs to be interrupted, for this problem to show up.  In such a scenario, FontManager cache might be containing multiple references to the same TrueType font.  When FontManager.addToPool attempts to close a font file which is same as the one getting added to pool, we get this exception.

One solution would be in the TrueTypeFont.open(), assign the fontfile after adding it to pool.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached testcase and keep selecting entries in  the combo box which contains the list of fonts.  Selecting a entry in the Combox  enables getting the fontmetrics of that font.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Should not throw NPE
ACTUAL -
throws NPE

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "Thread-1" java.lang.NullPointerException
at sun.font.TrueTypeFont.readBlock(Unknown Source)
        at sun.font.TrueTypeFont.readBlock(Unknown Source)
        at sun.font.TrueTypeFont.createScaler(Native Method)
        at sun.font.TrueTypeFont.getScaler(Unknown Source)
        at sun.font.FileFontStrike.<init>(Unknown Source)
        at sun.font.FileFont.createStrike(Unknown Source)
        at sun.font.Font2D.getStrike(Unknown Source)
        at sun.font.Font2D.getStrike(Unknown Source)
        at sun.font.CompositeStrike.getStrikeForSlot(Unknown Source)
        at sun.font.CompositeStrike.getFontMetrics(Unknown Source)
        at sun.font.FontDesignMetrics.initMatrixAndMetrics(Unknown Source)
        at sun.font.FontDesignMetrics.<init>(Unknown Source)
        at sun.font.FontDesignMetrics.getMetrics(Unknown Source)
        at sun.java2d.SunGraphics2D.getFontMetrics(Unknown Source)
        at lab.FontNPETest$2.run(FontNPETest.java:57)
        at java.lang.Thread.run(Unknown Source)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package lab;

import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.GraphicsEnvironment;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JComboBox;
import javax.swing.JFrame;

public class FontNPETest {

  private static Font gfont = null;
  
  public static void main(String[] args) {
    final JFrame fr = new JFrame("FontTest");
    final JComboBox box = createCombo();
    fr.getContentPane().add(box);
    fr.pack();
    fr.setVisible(true);
    fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    startFontManipulation(fr);
  }
  
  private static JComboBox createCombo() {
    final JComboBox box = new JComboBox(
        GraphicsEnvironment.getLocalGraphicsEnvironment().getAllFonts());
    box.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        Font f = (Font)box.getSelectedItem();
        f = f.deriveFont(12f);
        setFont(f);
      }
    });
    return box;
  }
  
  private static synchronized Font getFont() {
    return gfont;
  }
  
  private static synchronized void setFont(Font f) {
    gfont = f;
  }
  
  private static void startFontManipulation(final JFrame fr) {
    Thread t1 = new Thread(new Runnable() {
      public void run() {
          while(true) {
              Font f = getFont();
              if(f != null) {
                Image img = fr.createImage(100, 100);
                Graphics g = img.getGraphics();
                FontMetrics fm = g.getFontMetrics(gfont);
                final Thread thisT = Thread.currentThread();
                Thread t = new Thread(new Runnable() {
                    public void run() {
                        thisT.interrupt();
                    }
                });
                t.start();
                try {
                   fm.charsWidth("ABCDE".toCharArray(), 0, 5);
                } catch(NullPointerException e) {
                  e.printStackTrace();
                }
                g.dispose();
              }
          }
      }
    });
    t1.start();
  }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Don't interrupt the thread.