Hi. I’m using a third-party Kotlin library compil...

# announcements

[natpryce]

Hi. I’m using a third-party Kotlin library compiled for JVM 1.6. My application is targeting JVM 1.8. When I try to compile my code that calls an inline higher-order function in the library, I get the following compilation error:

Error:(17, 5) Kotlin: Cannot inline bytecode built with JVM target 1.8 into bytecode that is being built with JVM target 1.6. Please specify proper '-jvm-target' option

What’s the solution to this?

[Casey Brooks]

Make sure that Kotlin is also targeting jdk8, not just Java. For example, in gradle:

apply plugin: 'java'
apply plugin: "kotlin"

sourceCompatibility = '1.8'
targetCompatibility = '1.8'

compileKotlin {
    kotlinOptions {
        jvmTarget = '1.8'
    }
}

[natpryce]

I think that’s why I’m getting the error … My code is being compiled to 1.8 because of that kotlinOptions.jvmTarget declaration in Gradle, but the bytecode it’s being inlined into is 1.6 bytecode in the library my code depends on.

[christophsturm]

why is your code inlined into the library code? shouldnt it be the other way around?

[natpryce]

The library defines an inline higher order function

[natpryce]

I pass it a lambda

[natpryce]

The compiler would normally inline the library function and body of the lambda parameter into the body of the enclosing function

[natpryce]

But the enclosing function (and therefore the lambda) is compiled for JDK1.8 and the library function is compiled for JDK1.6

[christophsturm]

you could copy that one source file to your own project.

[natpryce]

The library is mostly higher-order functions.

[natpryce]

Thanks to defining a DSL

[natpryce]

so that would mean copying almost all functions from the library into my own code

[natpryce]

Plus many of the library functions call private features of the library, and so cannot be copied into my own compilation units

[Casey Brooks]

Which library is this?

[natpryce]

In this case it’s minutest.

[natpryce]

But in the general case… is there a way round this bytecode version mismatch when using libraries?

[Casey Brooks]

I think I’ve figured it out. Setting the Kotlin version in compileKotlin {} doesn’t set the same value for the test sourceset. Try adding

compileTestKotlin {
    kotlinOptions {
        jvmTarget = "1.8"
    }
}

to my snippet above

[natpryce]

Thanks! You’re a star

[gildor]

Better to use withType<KotlinCompile> to configure all available Kotlin tasks