FULL PRODUCT VERSION :
java version "1.5.0_01"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_01-b08)
Java HotSpot(TM) Client VM (build 1.5.0_01-b08, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
I'm trying to run simple javadoc from the command line on a test heirarchy that contains only a single Main class.  I've tried several configurations, but it always fails.  It also fails when I try it as an ant task.

One command line I've tried is:

javadoc com.kitfox.violet


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a package structure and create a Main file at com.kitfox.violet.Main.  It does not need to contain much more than the main(String[] argv) method

Type the following line at the command prompt:

javadoc com.kitfox.violet

Source Code:

package com.kitfox.violet;

public class Main {

public static void main(String args[]) {

}
}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Javadoc should be produced
ACTUAL -
Received an error message

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Loading source files for package com.kitfox.violet...
Constructing Javadoc information...
Standard Doclet version 1.5.0_01
Building tree for all the packages and classes...
Generating com/kitfox/violet/\Main.html...
java.lang.IllegalArgumentException
        at sun.net.www.ParseUtil.decode(ParseUtil.java:183)
        at sun.misc.URLClassPath$FileLoader.<init>(URLClassPath.java:863)
        at sun.misc.URLClassPath$3.run(URLClassPath.java:319)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.misc.URLClassPath.getLoader(URLClassPath.java:313)
        at sun.misc.URLClassPath.getLoader(URLClassPath.java:290)
        at sun.misc.URLClassPath.findResource(URLClassPath.java:141)
        at java.net.URLClassLoader$2.run(URLClassLoader.java:362)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findResource(URLClassLoader.java:359)
        at java.lang.ClassLoader.getResource(ClassLoader.java:977)
        at java.lang.ClassLoader.getResourceAsStream(ClassLoader.java:1159)
        at javax.xml.parsers.SecuritySupport$4.run(SecuritySupport.java:72)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.xml.parsers.SecuritySupport.getResourceAsStream(SecuritySupport
.java:65)
        at javax.xml.parsers.FactoryFinder.findJarServiceProvider(FactoryFinder.
java:213)
        at javax.xml.parsers.FactoryFinder.find(FactoryFinder.java:185)
        at javax.xml.parsers.SAXParserFactory.newInstance(SAXParserFactory.java:
107)
        at com.sun.tools.doclets.internal.toolkit.builders.LayoutParser.parseXML
(LayoutParser.java:72)
        at com.sun.tools.doclets.internal.toolkit.builders.ClassBuilder.build(Cl
assBuilder.java:108)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.generateClassFiles(Html
Doclet.java:155)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.generateClassFi
les(AbstractDoclet.java:164)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.startGeneration
(AbstractDoclet.java:106)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.start(AbstractD
oclet.java:64)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.start(HtmlDoclet.java:4
2)
        at com.sun.tools.doclets.standard.Standard.start(Standard.java:23)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:585)
        at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:269)
        at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:143)
        at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:340)
        at com.sun.tools.javadoc.Start.begin(Start.java:128)
        at com.sun.tools.javadoc.Main.execute(Main.java:41)
        at com.sun.tools.javadoc.Main.main(Main.java:31)
com.sun.tools.doclets.internal.toolkit.util.DocletAbortException
        at com.sun.tools.doclets.internal.toolkit.builders.LayoutParser.parseXML
(LayoutParser.java:79)
        at com.sun.tools.doclets.internal.toolkit.builders.ClassBuilder.build(Cl
assBuilder.java:108)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.generateClassFiles(Html
Doclet.java:155)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.generateClassFi
les(AbstractDoclet.java:164)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.startGeneration
(AbstractDoclet.java:106)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.start(AbstractD
oclet.java:64)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.start(HtmlDoclet.java:4
2)
        at com.sun.tools.doclets.standard.Standard.start(Standard.java:23)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:585)
        at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:269)
        at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:143)
        at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:340)
        at com.sun.tools.javadoc.Start.begin(Start.java:128)
        at com.sun.tools.javadoc.Main.execute(Main.java:41)
        at com.sun.tools.javadoc.Main.main(Main.java:31)
com.sun.tools.doclets.internal.toolkit.util.DocletAbortException
        at com.sun.tools.doclets.formats.html.HtmlDoclet.generateClassFiles(Html
Doclet.java:159)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.generateClassFi
les(AbstractDoclet.java:164)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.startGeneration
(AbstractDoclet.java:106)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.start(AbstractD
oclet.java:64)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.start(HtmlDoclet.java:4
2)
        at com.sun.tools.doclets.standard.Standard.start(Standard.java:23)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:585)
        at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:269)
        at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:143)
        at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:340)
        at com.sun.tools.javadoc.Start.begin(Start.java:128)
        at com.sun.tools.javadoc.Main.execute(Main.java:41)
        at com.sun.tools.javadoc.Main.main(Main.java:31)

REPRODUCIBILITY :
This bug can be reproduced always.

Release Regression From : 1.4.2_06
The above release value was the last known release where this 
bug was known to work. Since then there has been a regression.
###@###.### 2005-1-21 00:13:53 GMT
As noted by Doug, this isn't a bug. If the classpath is unset, everything works fine.
An RFE could be filed to have javadoc give a more helpful warning regarding this