Learning about @JvmMultifileClass

What is @JvmMultifileClass?

The JvmMultifileClass annotation is described as follows.

Instructs the Kotlin compiler to generate a multifile class with top-level functions and properties declared in this file as one of its parts. Name of the corresponding multifile class is provided by the JvmName annotation.

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/-jvm-multifile-class/
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/-jvm-name/

Let's decompile

Create a Utils.kt and define the foo method. The foo method just prints foo.

// Utils.kt fun foo() { println("foo") } 

When decompiled, it looks like this.

// Utils.decompiled.java public final class UtilsKt { public static final void foo() { String var0 = "foo"; boolean var1 = false; System.out.println(var0); } } 

Let's add @JvmName and @JvmMultifileClass.

// Utils.kt @file:JvmName("NewUtils") @file:JvmMultifileClass fun foo() { println("foo") } 

Let's decompile. I specified NewUtils in the @JvmName annotation, the NewUtils class was generated. In addition, a facade class (NewUtils__UtilsKt) has been generated.

// Utils.decompiled.java public final class NewUtils { public static final void foo() { NewUtils__UtilsKt.foo(); } } final class NewUtils__UtilsKt { public static final void foo() { String var0 = "foo"; boolean var1 = false; System.out.println(var0); } } 

If you specify @file: JvmName("NewUtils") fot multiple files, the following error will occur.

Duplicate JVM class name 'NewUtils' generated from: package-fragment, package-fragment

This can be solved by adding @JvmMultifileClass. @JvmMultifileClass annotation can be used to generate and organize facade classes.