Fixing classNotFoundExceptions in Gradle Multi-Module Projects: An Easy Guide

Описание: Struggling with `classNotFoundExceptions` in a Gradle multi-module project? Discover how to compile sub-modules successfully and create a monolithic JAR without dependencies.
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Fixing classNotFoundExceptions in Gradle Multi-Module Projects: An Easy Guide

In a complex Gradle multi-module Java project, it’s not uncommon to encounter compilation issues, particularly classNotFoundExceptions. This can be frustrating, especially when individual modules compile without issues, but the project fails when building from the root. If you've found yourself in this situation, don’t worry! We’re here to guide you through solving this problem seamlessly.

Understanding the Problem

Let’s begin by clarifying the core issue at hand: when you attempt to run ./gradlew compileJava from the root of your multi-module project, it fails due to classes not being found within specified dependencies. Conversely, running ./gradlew core-module:compileJava works perfectly fine. This is an indicator that while the sub-modules are set up correctly, the root project’s configuration is leading to compilation failures.

Common Causes

Here are a couple of reasons this might happen:

Local JAR References: The project may be referring to local JAR files that are not being recognized in the root project context.

Source Set Configuration: The sourceSets in the root build.gradle file might be incorrectly configured, leading to failures when attempting to reference classes across sub-modules.

Step-by-Step Solution

To remedy the issue and successfully compile the entire project while creating a monolithic JAR without dependency jars, follow these steps:

1. Adjust Your jar Task

Include the following configuration in your root build.gradle to create a single JAR file from all sub-modules while ensuring all necessary classes are included:

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2. Verify Your Source Sets Configuration

Ensure that your source sets section in the root build.gradle is correctly referencing the directories of the sub-modules. The configuration should not just point to the source directories; it should properly establish dependencies between them to ensure visibility of classes. For example:

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3. Build Your Project

Once you've adjusted your jar task and confirmed that your source sets are properly configured, you can proceed to build the project. Use the command below from the root of your multi-module setup:

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This should compile the entire project and produce a monolithic JAR file that incorporates all classes across sub-modules.

Conclusion

By carefully configuring the jar task and verifying your sourceSets, you can effectively resolve compilation issues in a Gradle multi-module project. Remember that the configurations and dependencies in multi-module setups can become complex, so it’s important to manage them diligently. Following the steps outlined above should lead to a successful build process without encountering classNotFoundExceptions.

If you find this guide helpful or have any questions, feel free to reach out or leave your comments below! Happy coding!