main_class as mandatory attribute in kt_jvm_binary

[jxramos]

I'm stumbling across this restriction trying to write the most basic hello world application to verify our integration of the bazel rules was successful.

rules_kotlin/src/main/starlark/core/compile/rules.bzl

Line 182 in 45d4446

"main_class": attr.string(mandatory = True, doc = ""),

This is composed of the simple source given below...

// hello_world_app.kt
fun main() {
    println("Hello, World!")
}

and the BUILD file

kt_jvm_binary(
    name = "hello_world_app",
    srcs = ["hello_world_main.kt"],
)

That simple combination yields the mandatory attribute error

ERROR: /home/userX/orgY/repoQ/third_party/external/rules_kotlin/tests/BUILD:29:14: //third_party/external/rules_kotlin/tests:hello_world_app: missing value for mandatory attribute 'main_class' in 'kt_jvm_binary' rule

Is there something flawed or incomplete with the hello world example above that I'm missing, or perhaps this reveals some underlying Java dependency for these rules that isn't obvious from the surface of this all? What is the reason why a kotlin JVM binary would need a main class precisely? Sorry for the confusion, and lack of background I'm very new to the Kotlin language and and trying to see how much of the language is standalone and independent vs standing on the shoulders of Java.

[jxramos]

Looking up some related attempts at folks doing hello world in general

Running compilers and executing manually is a kind of advanced usage, it requires some additional knowledge.
https://discuss.kotlinlang.org/t/how-to-run-my-first-kotlin-hello-world/27296/2

But that individual with IntelliJ was seemingly able to run that hello world example so I think it is valid, but these bazel rules prevent this example from executing, or perhaps there's some generated code that Kotlin creates that exposes the Java class that I need to align first.

Seems like I can build with

kt_jvm_binary(
    name = "hello_world_app",
    srcs = ["hello_world_main.kt"],
    main_class = "",
)

Target //third_party/external/rules_kotlin/tests:hello_world_app up-to-date:
  bazel-out/xxx/bin/third_party/external/rules_kotlin/tests/hello_world_app.jar
  bazel-out/xxx/bin/third_party/external/rules_kotlin/tests/hello_world_app.jdeps
INFO: 8 processes: 5 internal, 1 linux-sandbox, 2 worker.
INFO: Total action wall time 0.50s
INFO: Execution critical path 0.64s (setup 0.14s, action wall time 0.47s)
INFO: Elapsed time 4.30s (preparation 3.61s, execution 0.69s)
INFO: Build completed successfully, 8 total actions

trying to run this target breaks the argument parsing however

 Arguments following the main class, source file, -jar <jarfile>,
 -m or --module <module>/<mainclass> are passed as the arguments to
 main class.

I'll see if the build output can be examined to show me this class path I need to hunt down, maybe some symbol can be the giveaway in the class file assuming it's not influenced at build time by the main_class bazel attribute value.

$ jar -tf bazel-out/xxx/bin/third_party/external/rules_kotlin/tests/hello_world_app.jar
META-INF/
META-INF/MANIFEST.MF
Hello_world_mainKt.class
META-INF/third_party_external_rules_kotlin_tests-hello_world_app.kotlin_module

[jxramos]

no inspiration found looking into the class file

$ jar xf  bazel-out/xxx/bin/third_party/external/rules_kotlin/tests/hello_world_app.jar Hello_world_mainKt.class
$ cat Hello_world_mainKt.class 
Hello, Worldjava/lang/System/lanoutLjava/io/PrintStream;



java/io/PrintStreamprintln(Ljava/lang/Object;)V

([Ljava/lang/String;)V

args[Ljava/lang/String;Lkotlin/Metadata;mvkxi0d1��
��

����0¨d27third_party_external_rules_kotlin_tests-hello_world_apphello_world_main.ktCodeLineNumberTableLocalVariableTable
SourceFileRuntimeVisibleAnnotations1(&
�_��)
                ("��*
                     +',4       III I!"[s#$[ss%s&

also if I give a dummy value for main_class="abc" then as expected the class could not be found.

Error: Could not find or load main class abc
Caused by: java.lang.ClassNotFoundException: abc

[restingbull]

Have a look at the integration tests

Since the underlying runner is still the jvm, it needs a classname to invoke from the classpath. And kt_jvm_binary takes multiple source files... I think you can see why it's mandatory. (There is a usecase for making it not mandatory, but that would confuse things even further for your investigation.)

I have not tried running packageless, bare kotlin source -- but the jvm follows a pretty standard naming convention for loading classes (unless you get funky with class loaders, or start trying to load anonymous classes): convert the package to path and remove .class.

All that said, try setting Hello_world_mainKt as the main_class and tell me how it goes.

(You might also find javap Hello_world_mainKt.class an enlightening exercise)

[jxramos]

ah yes that's a perfect example

rules_kotlin/src/test/data/jvm/basic/BUILD

Lines 19 to 23 in 45d4446

	kt_jvm_binary(
	name = "test_module_name_bin",
	srcs = glob(["helloworld/Main.kt"]),
	main_class = "helloworld.Main",
	)

this @file thingy seems to be the means by which the main_class gets brought into the picture, so that indeed may be the missing piece of the puzzle.

rules_kotlin/src/test/data/jvm/basic/helloworld/Main.kt

Line 16 in 45d4446

@file:JvmName("Main")

thank you so much, I just played around with things, your suggestion for main_class was spot on, the kt_jvm_binary ran, printed the line, and exited. I'm in a great place now to advance. @restingbull you saved my bacon! I owe you one.