Gradle Build Logic JDK Version Conflict Resolution

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Hey folks! Let's dive into a common Gradle issue: JDK version conflicts when your build logic lives in a separate repository and gets pulled into another project. We'll tackle how to fix the "Cannot inline bytecode" error you might hit when dealing with different JDK versions in your build scripts. Specifically, we'll explore the situation where your build logic, built with JDK 8, is used by a project that leverages JDK 24. Let's get down to business and figure this out!

The Problem: JDK Version Mismatch

So, you've got this awesome build-logic project (let's call it 'A') that you've diligently built using JDK 8. You've thoughtfully published it to Maven Central, thinking, "This is gonna be great!" Then, another project (project 'B') comes along, and it wants to use your build logic. Project B, however, is a bit more modern and is using JDK 24. When project B tries to use your build logic from project A, Gradle throws a fit, usually with an error message that looks something like this: "Cannot inline bytecode built with JVM target 24 into bytecode that is being built with JVM target 1.8. Specify proper '-jvm-target' option." That's a mouthful, right? Basically, Gradle is saying, "Hey, the bytecode from your build logic (JDK 8) isn't compatible with the bytecode project B is trying to create (JDK 24)."

To make matters worse, imagine a situation where your build logic in project A itself uses tool classes compiled with a higher JDK version, say, JDK 24. And you are trying to pull it to an older project B. This situation creates a perfect storm for version conflicts! This means that when project B imports it, the problem becomes even more complex. You'll likely encounter build failures and compatibility issues. The crux of the matter is that the compiler in project B attempts to use the higher JDK version's bytecode within a context expecting an older version, creating a conflict. Let's break down the issue into actionable steps.

Current Behavior

Let's paint a clearer picture using the example repos provided. You've got addzero-lib-jvm (Repo A) which is your build-logic, and then dotfiles-cli-graalvm (Repo B) which imports it. In Repo B's build.gradle.kts, you include the build logic with an id like this: id("site.addzero.buildlogic.jvm.kotlin-convention") version "+". This version "+" is a wildcard that grabs the latest version published. Because of this, you might end up with the newer version. As a result, when you build project B, Gradle will complain because the bytecode targets from build logic don't align with the JVM target of project B.

Expected Behavior

The ultimate goal? When you explicitly specify the JDK version, the precompiled script should play nice and use the JDK version set by the consumer (project B). This means project B should be able to dictate the JDK version, and the build logic should adapt without throwing those nasty incompatibility errors.

Understanding the Root Cause: JVM Target and Kotlin Compiler

The heart of the problem lies with the JVM target set by the Kotlin compiler (if you're using Kotlin). The JVM target dictates the version of Java bytecode the compiler generates. When project A and project B have different JVM targets, the conflict arises. Project A's build logic, compiled for JDK 8, generates bytecode that's only compatible with JDK 8. Project B, with its JDK 24, creates bytecode for the newer version.

When Gradle tries to merge these during the build, it hits a roadblock because the older JVM target (8) can't handle the newer bytecode (24). This is similar to trying to put a square peg into a round hole; it just doesn't fit! Gradle, being the smart build tool it is, prevents this incompatibility and throws an error to flag the issue. To fix it, you need to ensure the JVM targets align, either by making them compatible or by managing the dependency versions carefully.

Understanding the specifics of JVM targets and how the Kotlin compiler handles them is very useful in resolving this issue. The Kotlin compiler allows you to specify the jvmTarget using the kotlinOptions block in your build.gradle.kts file. For instance, in your build logic, you might set the jvmTarget during compilation to match the JVM target of the projects consuming it, thus ensuring compatibility. It's also important to configure the right Kotlin and Gradle plugin versions to work well with different JDK versions and JVM targets. This will help you manage dependencies and keep build processes smooth. When you understand how these settings work together, you will easily manage different versions and compatibility.

Solutions: Aligning JVM Targets and Dependency Management

Now for the good stuff: How to fix this? Here are a couple of approaches to get your projects playing nicely:

1. Specifying the jvmTarget in Build Logic

The most direct way is to configure your Kotlin compiler in your build logic project (A) to use the target JVM version that is compatible with the consumer project (B). This is done in your build.gradle.kts file:

kotlin {
    jvmToolchain(8)
}

This makes sure that the bytecode generated is compatible with older JVMs. Also, make sure the kotlin-stdlib dependency is compatible with the JVM target.

2. Matching the JDK Version

  • Modify Build Logic (Repo A): If possible, update your build logic in Repo A to target the same or a compatible JDK version as Repo B. This might involve upgrading the JDK used to build the logic.

  • Configure the Consumer (Repo B): In project B's build.gradle.kts, specify the desired JDK version or use a compatible version. You might also need to ensure that the Kotlin compiler and other related plugins are set up to use that version.

3. Dependency Management and Versioning

  • Careful Versioning: Avoid using + in the version declaration of your build logic. Pin to specific versions. When pulling the build logic into project B, define a specific version in your build.gradle.kts file: `id(