Fixing NoSuchMethodError: Mill And Zinc Compatibility Issue

Dec 21, 2025 by Alex Johnson 60 views

Understanding the `NoSuchMethodError` with Mill and Zinc 2.0.0-M10

If you're working with Scala build tools, you might occasionally run into cryptic errors that can halt your development process. One such error, the java.lang.NoSuchMethodError: 'xsbti.api.AnalyzedClass xsbti.api.AnalyzedClass.of(...)', often appears when there's a version mismatch between your build tool and its underlying compiler dependencies. Specifically, this error frequently surfaces when you upgrade to Zinc 2.0.0-M10 (or any subsequent milestone version) while still using Mill, a popular build tool for Scala. This incompatibility can be frustrating, especially when it affects even the simplest of projects.

The Core of the Problem: Version Mismatches

The NoSuchMethodError is a runtime error that occurs when the Java Virtual Machine (JVM) tries to call a method on an object, but that method no longer exists in the class definition it's expecting. In the context of Scala compilation, this almost always points to a version conflict between different components of the build process. The xsbti package, which is part of Zinc (Scala's incremental compiler), defines APIs that build tools like Mill rely on. When you introduce a newer version of Zinc that has changed its internal APIs without updating the build tool to match, you're bound to hit this kind of error.

Why Zinc Matters

Zinc is a crucial component in Scala development. It's responsible for compiling your Scala code efficiently, especially when dealing with incremental builds. This means it only recompiles the parts of your project that have changed since the last build, saving significant time. Zinc is developed by the Scala Center and is often used by build tools like sbt and, in this case, Mill.

The Mill and Zinc Relationship

Mill is designed to be a fast, modular, and user-friendly build tool for Scala. It leverages Zinc for its compilation tasks. When Mill is configured to use a specific version of Zinc, it expects that version to adhere to a certain API contract. If you manually force Mill to use a newer, perhaps unstable, version of Zinc (like Zinc 2.0.0-M10 in this scenario), and that new version of Zinc has altered its internal structure, including the AnalyzedClass.of method signature, Mill won't be able to find the expected method. This leads directly to the NoSuchMethodError.

Reproducing the Issue: A Step-by-Step Guide

The issue you've encountered is reproducible, which is great for debugging. The steps provided clearly outline how to trigger the error:

Clone the Mill Repository: Start by getting the latest code for Mill. git checkout https://github.com/com-lihaoyi/mill ensures you have the most up-to-date version.
Navigate to the Mill Directory: cd mill moves you into the project's root directory.
Modify Dependencies: The critical step is to alter the Zinc version used by Mill's own build. This is achieved by patching the mill-build/src/millbuild/Deps.scala file. The provided patch replaces the default Zinc dependency with org.scala-sbt::zinc:2.0.0-M10 and also explicitly includes zinc-core from the same version. This explicit inclusion is important as it ensures both parts of the Zinc library are aligned.
```
// Original snippet might look like:
// val zinc = mvn"org.scala-sbt::zinc:1.x.y"

// Modified snippet:
val zinc = mvn"org.scala-sbt::zinc:2.0.0-M10"
val zincCore = mvn"org.scala-sbt::zinc-core:${zinc.version}"
```
Attempt Compilation: After applying the patch, you can try to build a simple Mill project. The command mill -i dist.run example/scalalib/basic/3-simple -i -d --meta-level 1 compile attempts to compile a basic example project within Mill's own structure. The -i flag enables interactive mode, -d enables debug logging, and --meta-level 1 might influence how Mill handles meta-information, though the core issue lies in the compiler's API.

Because the problem occurs at the compiler setup level, the actual source code being compiled is less relevant. The error happens when Mill tries to initialize and use the Zinc compiler with the modified dependencies.

The Error Message Explained

The stack trace reveals the exact point of failure:

java.lang.Exception: fatal exception occurred: java.lang.NoSuchMethodError: 'xsbti.api.AnalyzedClass xsbti.api.AnalyzedClass.of(long, java.lang.String, xsbti.api.Lazy, int, xsbti.api.NameHash[], boolean, int, java.lang.String, long, long)'
    at sbt.internal.inc.AnalysisCallback.analyzeClass(Incremental.scala:1040)
    ...

This indicates that sbt.internal.inc.AnalysisCallback.analyzeClass is attempting to call a method named of on an AnalyzedClass object. However, the signature of this of method in the version of xsbti.api.AnalyzedClass that the compiler is using does not match the signature that AnalysisCallback expects. The long list of arguments (long, java.lang.String, xsbti.api.Lazy, int, xsbti.api.NameHash[], boolean, int, java.lang.String, long, long)) highlights the specific API that's missing or has changed.

Expected Behavior vs. Actual Outcome

The expectation is straightforward: the compiler should successfully process the code, identify dependencies, and produce compiled artifacts without throwing runtime exceptions. In a stable environment with compatible library versions, this is exactly what happens. The compilation should complete, and you should be able to run your project or further build steps.

The actual outcome, as demonstrated, is a complete failure. The build process halts abruptly with the NoSuchMethodError, making it impossible to proceed. This means that either Mill needs to be updated to be compatible with the newer Zinc API, or a version of Zinc that is compatible with the current Mill version must be used.

Notes and Potential Solutions

This specific issue highlights the challenges of managing dependencies in complex build systems. Upgrading a core component like the compiler can have ripple effects across the entire toolchain. Here are some potential considerations and solutions:

Version Compatibility is Key: The most direct solution is to ensure that the versions of Mill and Zinc are compatible. If you need to use Zinc 2.0.0-M10 or later, you might need to check if there's a version of Mill that explicitly supports it or has been updated to handle its API changes. Conversely, if you're using a specific version of Mill, ensure you're using a compatible version of Zinc (often the one bundled or recommended by Mill's own dependency management).
Check Mill's Release Notes: Mill's developers regularly update the tool to incorporate new features and maintain compatibility with upstream libraries like Zinc. Check the release notes for recent versions of Mill to see if this particular Zinc incompatibility has been addressed.
Report Issues: If you encounter such a problem, reporting it to the Mill or Zinc project (or both) with a clear reproducer, like the one you've provided, is crucial. This helps the maintainers identify and fix the problem.
Consider Stable Releases: Milestone releases (like 2.0.0-M10) are often for testing and might have breaking API changes. For stable builds, it's generally recommended to use official stable releases of both Mill and Zinc unless you have a specific reason to use a milestone.
Dependency Management: Tools like Mill and sbt have sophisticated dependency management systems. Manually overriding dependencies, as done in the patch, can sometimes lead to unexpected conflicts. If possible, try to let the build tool manage these dependencies unless absolutely necessary.

This NoSuchMethodError is a clear indicator of an API mismatch, and resolving it involves aligning the versions of the tools that depend on each other. For most users, sticking to the default or recommended dependency versions provided by Mill will prevent this issue.

For further information on Scala compilation and build tools, you might find the Scala documentation and the sbt documentation to be valuable resources.