How Java 9 would help resolve NoClassDefFoundError through it's modularity ?

Java 9 would provide an error at compile time if there are different modules with the same package mapped to the same class loader and hence would provide a compile time check for this issue.

NoClassDefFoundError exists because of ambiguity at realtime because of multiple versions of same package / class exist. As Java 9 would provide a compile time check for it through clearly defined dependencies and exports and a check on duplicate packages being loaded by single class loader, it will fix the problem.