How do I run Kotlin code using OkHttp from the command line?

I tried running Kotlin code on macOS 13 from the command line, first I installed it with

brew install kotlin

then I made a directory and downloaded the latest versions of some jar files that seemed relevant:

mkdir -p /tmp/kotlin 
cd /tmp/kotlin 
curl https://repo1.maven.org/maven2/com/squareup/okhttp3/okhttp/4.11.0/okhttp-4.11.0.jar > okhttp-4.11.0.jar 
curl https://repo1.maven.org/maven2/com/squareup/okio/okio/3.3.0/okio-3.3.0.jar > okio-3.3.0.jar 
curl https://repo1.maven.org/maven2/org/jetbrains/kotlin/kotlin-stdlib/1.8.21/kotlin-stdlib-1.8.21.jar > kotlin-stdlib-1.8.21.jar

(ChatGPT told me to install kotlin-stdlib too, I'm not sure that makes sense...)

then I created a sample file called OkHttpExample.kt with these contents:

import java.io.IOException;
import okhttp3.OkHttpClient
import okhttp3.Request

fun main() {
    val client = OkHttpClient()

    val request = Request.Builder()
        .url("https://example.com")
        .build()

    client.newCall(request).execute().use { response ->
        if (!response.isSuccessful) throw IOException("Unexpected code $response")

        println(response.body!!.string())
    }
}

and tried to compile and run it

kotlinc -classpath okhttp-4.11.0.jar:kotlin-stdlib-1.8.21.jar:okio-3.3.0.jar OkHttpExample.kt -include-runtime -d OkHttpExample.jar
java -classpath okhttp-4.11.0.jar:kotlin-stdlib-1.8.21.jar:okio-3.3.0.jar:OkHttpExample.jar OkHttpExampleKt

but the java command fails with this error:

Exception in thread "main" java.lang.NoClassDefFoundError: okio/Buffer
	at okhttp3.ResponseBody$Companion.create(ResponseBody.kt:248)
	at okhttp3.ResponseBody$Companion.create$default(ResponseBody.kt:247)
	at okhttp3.internal.Util.<clinit>(Util.kt:65)
	at okhttp3.OkHttpClient.<clinit>(OkHttpClient.kt:1073)
	at OkHttpExampleKt.main(OkHttpExample.kt:6)
	at OkHttpExampleKt.main(OkHttpExample.kt)
Caused by: java.lang.ClassNotFoundException: okio.Buffer
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
	... 6 more

Fixing your specific problem

NoClassDefFoundError: okio/Buffer means that some code tries to use the okio.Buffer class but the class is not in any jar that you put on the classpath.

This may happen because you just manually cherry-picked some jars to download, and you missed transitive dependencies. For instance, it's likely you're missing the okio-jvm dependency, which is a dependency of okio itself.

One solution is to find all transitive dependencies yourself (for instance by looking at .pom files in Maven Central) and download more and more jars by hand, but it's not very practical.

Recommended approaches

Instead, I highly suggest 2 options here:

1. create a "standard" Kotlin project and use a build tool like Gradle to manage dependencies and compilation. It requires a bit of setup, but it will be more familiar to most people, and you may easily find examples online.

2. if you're just playing around, you can try to use Kotlin Scripting with the .main.kts script definition, which may simplify your workflow here, but note that it's experimental and not very well known.

I'll elaborate on option 2 here because it's harder to find information about it.

To use the main-kts definition, make sure your file ends with .main.kts. Then put your code directly at the top level without wrapping it in a main() function.

Instead of manually downloading jars, add dependencies to it using @file:DependsOn at the top of the file, with the maven coordinates of the dependencies (you don't need to put transitive dependencies, just the ones you want to use directly):

script.main.kts (the extension matters!)

@file:DependsOn("com.squareup.okhttp3:okhttp:4.11.0")

import java.io.IOException
import okhttp3.OkHttpClient
import okhttp3.Request

val client = OkHttpClient()
val request = Request.Builder().url("https://example.com").build()
client.newCall(request).execute().use { response ->
    if (!response.isSuccessful) throw IOException("Unexpected code $response")

    println(response.body!!.string())
}

Then you can compile and run it in one simple command:

kotlin script.main.kts

Here is a more "official" example about main.kts usage:
https://github.com/Kotlin/kotlin-script-examples/blob/master/jvm/main-kts/MainKts.md