R8 says type is defined multiple times: in build\.transforms and in build\tmp\kotlin-classes

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06-10-2024

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R8 Error: "Type is defined multiple times" - A Guide to Troubleshooting

The error message "R8 says type is defined multiple times: in build.transforms and in build/tmp/kotlin-classes" can be frustrating, but it's actually a pretty common issue when dealing with Android projects and R8 optimization. This error typically arises when R8 encounters conflicting definitions of the same type during the code shrinking process.

Understanding the Problem

R8, the code shrinking tool in Android, tries to optimize your app by removing unused code and shrinking the resulting APK file size. It achieves this by analyzing your code and identifying types (classes, interfaces, etc.) that are no longer needed.

The error message indicates that R8 has found two different definitions of the same type, one in your project's build.transforms directory and another in the build/tmp/kotlin-classes directory. This often happens when:

• Multiple libraries are using the same type with different versions: This can occur if two libraries depend on the same external library, but use different versions of it. R8 might find conflicting implementations of the type between these libraries.
• Duplicate dependencies: You might have accidentally included the same library multiple times in your project, leading to conflicting definitions.
• Code generation differences: Sometimes, different tools (like Kotlin or Java compilation) might generate slightly different code for the same type, which can confuse R8 during the shrinking process.

The Scenario

Let's imagine you're building an Android app that uses both a networking library and a UI library, both of which depend on a third-party library called core-utils. The networking library uses version 1.0.0 of core-utils, while the UI library uses version 1.1.0. This creates the potential for conflicts, as the two versions might contain different implementations of classes within core-utils.

Here's a simplified example of the build.gradle file that might lead to this issue:

dependencies {
  implementation("com.example:networking:1.0.0")
  implementation("com.example:ui:1.0.0")
}

Troubleshooting Strategies

Here are some strategies to resolve this "type defined multiple times" error:

1. Check for duplicate dependencies: Carefully examine your build.gradle file to ensure you're not including the same library multiple times. Look for instances where the same dependency is declared in the dependencies block or in subprojects.
2. Upgrade dependencies: If you're using different versions of the same library, try upgrading all of them to the latest compatible version. This might resolve conflicts caused by version-specific code changes.
3. Use dependency exclusion: Exclude specific dependencies from conflicting libraries to avoid duplicate definitions. This is often necessary when two libraries depend on the same external library with different versions. You can do this using the exclude keyword within your dependencies. For example:

implementation("com.example:networking:1.0.0") {
  exclude group: 'com.example', module: 'core-utils'
}
implementation("com.example:ui:1.0.0")

4. Use the proguardFiles directive: Customize your proguard-rules.pro file to instruct R8 on how to handle conflicting types. This is a more advanced approach and requires a deeper understanding of ProGuard's syntax. You can define custom rules to keep specific classes or methods, or to instruct R8 to remove specific types.

5. Try cleaning and rebuilding: Sometimes, a simple clean and rebuild of your project can resolve the issue, especially if it's caused by temporary files or build cache inconsistencies.

Additional Insights

• Version conflicts are common: Always strive to use the latest compatible versions of libraries to minimize the chances of encountering conflicts.
• R8 optimization: While R8 aims to optimize your app, it can also be a source of issues. Carefully review your dependencies and configurations to understand how R8 might be affecting your code.
• Documentation: Refer to the official documentation for R8 and ProGuard for more advanced configuration options and troubleshooting techniques.

By understanding the cause of the error and applying the right strategies, you can effectively address the "type defined multiple times" issue and ensure your Android project builds and runs smoothly. Remember to always be mindful of dependency management and code optimization practices to avoid these types of errors in the future.