ADDITIONAL SYSTEM INFORMATION :
I'm using Java 17, but the issue is probably way older - I've tested with Java 11 and it is also there.

A DESCRIPTION OF THE PROBLEM :
UNC paths with regular url are not working when used in a jar manifest:

Manifest-Version: 1.0
Class-Path: file://server/path/
Main-Class: com.main.Start

But this irregular syntax does work:

Manifest-Version: 1.0
Class-Path: file:////server/path/
Main-Class: com.main.Start

Background: Maven surefire plugin creates a temporary jar for forked test execution and adds all the classpath entries to the manifest. At some point the implementation changed from File to Path. But the Java implementation of File.toURI() and Path.toUri() behaves differently for UNC paths (File produces file://// which was working in the jar manifest, but Path produces file:// which won't work). See also https://issues.apache.org/jira/browse/SUREFIRE-2211

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
* Create some class with a main method at an unc path like //server/path/com/main/Start.class.

* Create a jar with such a manifest

Manifest-Version: 1.0
Class-Path: file://server/path/
Main-Class: com.main.Start

* execute the jar via

java -jar test.jar

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Class gets executed
ACTUAL -
java.lang.ClassNotFoundException: de.main.Start

CUSTOMER SUBMITTED WORKAROUND :
stay with the old File.toURI() for absolute UNC paths