In JDK 6, for ease of use the Font class supports all
TextAttributes as font attributes. Previously there was a hardcoded subset.
This makes it easy to (say) specify a foreground colour for text in that
font without using AttributedString.
Attributes which might require such behaviours are treated as 'layout'
attributes as they need to invoke code which can properly render using
all these attributes.

When measuring text which requires layout, the implementation internally
constructs a TextLayout. But suppose the application passes an empty
string, or zero-length character array to be measured?
TextLayout (as described in bug 4138921) throws an exception on being
constructed with a zero-length string. There was a plan to fix this
in JDK 6 but it did not happen. As a consequence measuring a zero
with string of text in a font which has layout attributes throws
that exception.

The following simple program demonstrates this :
import java.util.*;
import java.awt.*;
import java.awt.font.*;
import java.awt.image.*;

public class zws {

    public static void main(String args[]) {
        BufferedImage bi=new BufferedImage(1,1,BufferedImage.TYPE_INT_RGB);
        Graphics g = bi.getGraphics();

        Font f = new Font("Dialog", Font.PLAIN, 12);
        Map map = new HashMap();
        map.put(TextAttribute.BACKGROUND, Color.BLUE);
        f = f.deriveFont(map);
        g.setFont(f);
        FontMetrics fm = g.getFontMetrics();
        fm.stringWidth("");
   }
}
/java/re/jdk/1.6.0/latest/binaries/solaris-sparc/bin/java zws
Exception in thread "main" java.lang.IllegalArgumentException: Zero length string passed to TextLayout constructor.
        at java.awt.font.TextLayout.<init>(TextLayout.java:364)
        at sun.font.FontDesignMetrics.stringWidth(FontDesignMetrics.java:463)
        at zws.main(zws.java:18)