---%<---
import javax.swing.JComponent;
import javax.swing.JLabel;
import javax.swing.UIDefaults;
import javax.swing.UIManager;
import javax.swing.plaf.ComponentUI;
import javax.swing.plaf.basic.BasicLabelUI;
public class TestUIDefaultsErrorHandling {
    public static void main(String[] args) {
        UIDefaults defaults = UIManager.getDefaults();
        defaults.put("BrokenUI", BrokenUI.class.getName());
        System.err.println(defaults.getUI(new JLabel() {
            public @Override String getUIClassID() {
                return "BrokenUI";
            }
        }));
    }
    public static class BrokenUI extends BasicLabelUI {
        public static ComponentUI createUI(JComponent target) {
            return new BrokenUI();
        }
        private BrokenUI() {
            throw new RuntimeException("oops, complex problem with diagnostics");
        }
    }
}
---%<---

produces

---%<---
java version "1.6.0_11"
Java(TM) SE Runtime Environment (build 1.6.0_11-b03)
Java HotSpot(TM) Server VM (build 11.0-b16, mixed mode)

UIDefaults.getUI() failed: createUI() failed for TestUIDefaultsErrorHandling$1[,0,0,0x0,invalid,alignmentX=0.0,alignmentY=0.0,border=,flags=0,maximumSize=,minimumSize=,preferredSize=,defaultIcon=,disabledIcon=,horizontalAlignment=LEADING,horizontalTextPosition=TRAILING,iconTextGap=4,labelFor=,text=,verticalAlignment=CENTER,verticalTextPosition=CENTER] java.lang.reflect.InvocationTargetException
java.lang.Error
        at javax.swing.UIDefaults.getUIError(UIDefaults.java:711)
        at javax.swing.MultiUIDefaults.getUIError(MultiUIDefaults.java:133)
        at javax.swing.UIDefaults.getUI(UIDefaults.java:757)
        at javax.swing.UIManager.getUI(UIManager.java:1016)
        at javax.swing.JLabel.updateUI(JLabel.java:256)
        at javax.swing.JLabel.<init>(JLabel.java:145)
        at javax.swing.JLabel.<init>(JLabel.java:216)
        at TestUIDefaultsErrorHandling$1.<init>(TestUIDefaultsErrorHandling.java:11)
        at TestUIDefaultsErrorHandling.main(TestUIDefaultsErrorHandling.java:11)
UIDefaults.getUI() failed: createUI() failed for TestUIDefaultsErrorHandling$1[,0,0,0x0,invalid,alignmentX=0.0,alignmentY=0.0,border=,flags=0,maximumSize=,minimumSize=,preferredSize=,defaultIcon=,disabledIcon=,horizontalAlignment=LEADING,horizontalTextPosition=TRAILING,iconTextGap=4,labelFor=,text=,verticalAlignment=CENTER,verticalTextPosition=CENTER] java.lang.reflect.InvocationTargetException
java.lang.Error
        at javax.swing.UIDefaults.getUIError(UIDefaults.java:711)
        at javax.swing.MultiUIDefaults.getUIError(MultiUIDefaults.java:133)
        at javax.swing.UIDefaults.getUI(UIDefaults.java:757)
        at TestUIDefaultsErrorHandling.main(TestUIDefaultsErrorHandling.java:11)
null
---%<---

Nowhere is the original exception's message or stack trace even mentioned; there is only a meaningless stack trace from Swing code.

In practice this means that if any runtime exception is thrown constructing a ComponentUI, if you want to see the details you either need to run in the debugger and set a breakpoint in UIDefaults, or patch UIDefaults and add to -Xbootclasspath/p. Neither is a good option for diagnosing a problem that arises unreproducibly in the field.

Encountered when trying to diagnose a quite subtle problem in JavaHelp. The original exception - when I found it after a couple hours of work - pointed me quickly to the root problem. The original application log output was useless.