Specification for method :
http://jre.us.oracle.com/java/re/jdk/9/promoted/latest/docs/api/java/lang/module/ModuleFinder.html#of-java.nio.file.Path...-

says:

"If the JAR file has a Main-Class attribute in its main manifest then its value is the main class."

which could give an impression that the value is taken without any verification. However standard builder API is used so if for example empty value is discovered the following happens:

======================
java.lang.module.FindException: Unable to derive module descriptor for: ..... emptymainclass.jar
at java.lang.module.ModulePath.readJar(java.base@9-ea/ModulePath.java:513)
at java.lang.module.ModulePath.readModule(java.base@9-ea/ModulePath.java:274)
at java.lang.module.ModulePath.scan(java.base@9-ea/ModulePath.java:193)
at java.lang.module.ModulePath.scanNextEntry(java.base@9-ea/ModulePath.java:145)
at java.lang.module.ModulePath.find(java.base@9-ea/ModulePath.java:109)
...
Caused by: java.lang.IllegalArgumentException: Empty main class name
at jdk.internal.module.Checks.requireJavaIdentifier(java.base@9-ea/Checks.java:70)
at java.lang.module.ModuleDescriptor$Builder.mainClass(java.base@9-ea/ModuleDescriptor.java:1519)
at java.lang.module.ModulePath.deriveModuleDescriptor(java.base@9-ea/ModulePath.java:475)
at java.lang.module.ModulePath.readJar(java.base@9-ea/ModulePath.java:509)
======================

Constraints set by builder API (+FindException) deserve to be mentioned in the spec for MF.of() or assertion ""If the JAR file has a Main-Class attribute in its main manifest then its value is the main class." needs to be updated