(Unknown Source) in Exception stack trace

Note that if you are using Ant build and if the debug attribute set to false in javac command this could happen.

ex : if you need proper location in trace set debug = true in Ant build,

    <javac verbose="false" srcdir="${src}" destdir="${classdir}" debug="true" includes="**/*.java">
        <classpath refid="compile.classpath" />
    </javac>

This is normally related to missing debug information. You are probably using JRE (not JDK), which does not include debug information for rt.jar classes. Try using full JDK, you'll get proper locations in the stack trace:

Exception in thread "main" java.lang.NullPointerException
    at java.lang.String.<init>(String.java:177)
    at java.lang.String.valueOf(String.java:2840)
    at StringValueOfNull.main(StringValueOfNull.java:3)

I had the same problem, I'm using spring and apache ant for continuous integration.

The error I had was in the build.xml file.

The gender change log with more precise content was:

build.xml with the error:

    <javac srcdir="${src.home}" destdir="${work.home}/WEB-INF/classes">
        <classpath refid="compile.classpath" />
    </javac>

build.xml without error:

    <javac srcdir="${src.home}" destdir="${work.home}/WEB-INF/classes"  debug="true">
        <classpath refid="compile.classpath" />
    </javac>

Within the structure I lacked the courage debug = "true"

I ran the code in Eclipse and I got the following output,

public class Aloof {
    public static void main(String[] args) {
        String.valueOf(null);
    }
}

Exception in thread "main" java.lang.NullPointerException
    at java.lang.String.<init>(String.java:177)
    at java.lang.String.valueOf(String.java:2840)
    at mysql.Aloof.main(Aloof.java:19)

If you include the full source (from JDK), you can actually debug to the line 177 in String.java