This is probably a Windows specific issue.

Suppose java is located here

C:\Program Files\Java\jre1.8.0_91

getRTJarPath() returns this String:

/C:/Program%20Files/Java/jre1.8.0_91/lib/rt.jar

getBootClassPath() uses the String to create a URL from
a File (line 659). But

1. new File("/C:/Program%20Files/Java/jre1.8.0_91/lib/rt.jar")
   is not a valid File.
2. new File("/C:/Program%20Files/Java/jre1.8.0_91/lib/rt.jar").toURI()
   produces
   file:/C:/Program%2520Files/Java/jre1.8.0_91/lib/rt.jar
   which is a corrupt URI. toURI() wants to encode the '%' character
   without realizing that the space character has already been
   encoded. To be expected because File does not recognize encoded
   characters.

toURL() propagates the error.

searchJarForClasses() tries to URL.openStream() and it fails.

Try this:

cp = this.namespace.getClassManager().getClassPath();
cp.getClassSource("java.lang.Object");

It returns null.

Possible Solutions

(1.) Do

urlString.replaceAll("%20", " ");

before returning from getRTJarPath(). This handles encoded space characters in
the path. There could still be other encoded characters though.

(2.) Create the URL directly from getRTJarPath().

public static BshClassPath getBootClassPath() 
  throws ClassPathException  {
     ...
  URL url = new URL("file:" + getRTJarPath());
     ...
}