Kotlin compiler unresolved reference on Java classes

Unresolved reference: JavaClass1  
Unresolved reference: JavaClass2  
Unresolved reference: JavaClass3  
Unresolved reference: JavaClass4  
Unresolved reference: JavaClass5  
Unresolved reference: JavaClass6  
Unresolved reference: JavaClass7  
...

Some Java classes cannot be loaded by Kotlin compiler.

TL;DR;

If you compile your Java + Kotlin mixed code then get such errors. You probably need to check your Java source file's path.

General JVM Class Loader

In Java, it requires you to put class source file in the corresponding directory of it's package statement.

In Kotlin, you can put class source file anywhere regardless of the package they're in,

The difference of compilers feature.

Mixed compiling

Kotlin compiler would be invoked before Java compiler, and Kotlin have a little bit restricted Java Class Loader.

It's ok for Java compiler to load class A from package com.dummy.path.model in such directories graph:

- com
  - dummy.path
    - model
      - A.java

Kotlin compiler would not resolve class A in above directories graph, it's restricted to be one directory per path. See below:

```
- com
  - dummy
    - path
      - model
        - A.java

I agree with Kotlin compiler's implementation.

Since this issue is hard to identify, this article may make someone's life easier.

Ref:

The Class Loader Subsystem (The Java Virtual Machine by Bill Venners)