Fix: Duplicate Classes Commons-io 2.19.0 Vs 2.4

Hey everyone! Running into "Duplicate class" errors in your Android projects can be a real headache, especially when they stem from library dependencies. Today, we're diving deep into a common scenario: dealing with duplicate classes arising from different versions of Apache Commons IO (specifically, versions 2.19.0 and 2.4). This article is here to provide a clear, step-by-step guide to resolve this issue cleanly, ensuring your builds run smoothly and your app functions as expected.

Understanding the Problem

So, you're building your Android app, and suddenly, you're hit with a barrage of "Duplicate class" errors. Looking closer, you see something like this:

Duplicate class org.apache.commons.io.FileUtils found in modules:
commons-io:commons-io:2.19.0 (transitive) and commons-io-2.4.jar

This means the same class (org.apache.commons.io.FileUtils in this example) is present in two different places:

• commons-io:commons-io:2.19.0: This is likely being pulled in as a transitive dependency. A transitive dependency is a library that your direct dependencies rely on. So, one of the libraries you're directly including in your project needs Commons IO 2.19.0.
• commons-io-2.4.jar: This indicates that you've manually included the Commons IO 2.4 JAR file in your project's libs/ directory.

The conflict arises because the Android build process doesn't know which version of the class to use, leading to the build failure. It's like having two people with the same name in a room – things can get confusing!

Why does this happen, guys? Often, it's due to older libraries in your project that depend on older versions of Commons IO. While you might have a newer library pulling in 2.19.0, the older JAR file is still lingering in your libs/ folder.

To effectively tackle this, it's crucial to understand how Gradle, Android's build system, handles dependencies. Gradle attempts to resolve dependencies by selecting a single version of a library. However, when it encounters multiple versions of the same library, especially when one is a direct dependency (like the JAR in libs/) and the other is a transitive dependency, conflicts can arise. The goal is to tell Gradle exactly which version to use and avoid the duplication.

Step-by-Step Solutions to Eliminate Duplicate Classes

Okay, let's get down to the nitty-gritty. Here are several approaches you can take to resolve these duplicate class errors. We'll start with the simplest and most recommended solutions, then move on to more advanced techniques if needed.

1. Removing the Explicit JAR

This is the most straightforward and often the best solution. If you're including commons-io-2.4.jar directly in your libs/ folder, the first step is to remove it. Why? Because you're already getting a newer version (2.19.0) transitively. Having the older version just creates conflict.

1. Locate the JAR: Find the commons-io-2.4.jar file in your project's libs/ directory.
2. Delete it: Simply delete the file. If it's under version control (like Git), make sure to remove it properly from the repository.
3. Clean and Rebuild: In Android Studio, go to Build > Clean Project, and then Build > Rebuild Project. This forces Gradle to re-evaluate your dependencies without the older JAR.

This approach works because you're relying on Gradle's dependency resolution to provide the necessary Commons IO library. By removing the explicit JAR, you're letting Gradle choose the version that's already being brought in transitively. This keeps your project cleaner and avoids potential compatibility issues that might arise from using an older version.

2. Excluding the Transitive Dependency

Sometimes, you might need the older version of Commons IO for a specific reason, or removing the JAR isn't feasible (perhaps it's required by a legacy module). In this case, you can tell Gradle to exclude the transitive dependency (2.19.0) and explicitly use the 2.4 JAR.

How do we do this, you ask? You'll need to modify your app's build.gradle file. Let's say you have a dependency that's pulling in Commons IO 2.19.0 transitively. You can exclude it like this:

dependencies {
    implementation('your.group:your-library:1.0') {
        exclude group: 'commons-io', module: 'commons-io'
    }
    implementation files('libs/commons-io-2.4.jar')
}

Let's break this down:

• implementation('your.group:your-library:1.0'): This is the dependency that's bringing in Commons IO 2.19.0.
• exclude group: 'commons-io', module: 'commons-io': This is the key part! It tells Gradle to exclude any transitive dependency with the group ID commons-io and module ID commons-io. This effectively blocks the 2.19.0 version.
• implementation files('libs/commons-io-2.4.jar'): This ensures that your project explicitly includes the 2.4 JAR from your libs/ directory.

Important Considerations:

• Identify the culprit: You need to figure out which dependency is pulling in the unwanted version of Commons IO. Gradle's dependency insight can help with this (we'll cover it later).
• Compatibility: Be absolutely sure that using the older version (2.4) won't break anything in your project. Newer versions often contain bug fixes and security updates, so sticking with an older version should be a deliberate choice.

3. Force a Specific Version Using force

Another powerful technique is to force Gradle to use a specific version of a dependency. This can be useful when you have multiple dependencies pulling in different versions of Commons IO, and you want to ensure consistency.

Here's how you can force a version in your build.gradle file:

dependencies {
    implementation files('libs/commons-io-2.4.jar')
    implementation('another.library:that-uses-commons-io:1.0') {
        force true
    }
    configurations.all {
        resolutionStrategy {
            force 'commons-io:commons-io:2.4'
        }
    }
}

Let's dissect this:

• implementation files('libs/commons-io-2.4.jar'): We're explicitly including the 2.4 JAR.
• implementation('another.library:that-uses-commons-io:1.0') { force: true }: This forces another.library:that-uses-commons-io:1.0 to use the forced version of commons-io
• configurations.all { ... }: This block applies to all dependency configurations in your project.
• resolutionStrategy { force 'commons-io:commons-io:2.4' }: This is the magic! It tells Gradle to force the use of Commons IO version 2.4 across the entire project. Any dependency that transitively includes a different version will be overridden.

Caveats:

• Global Impact: Forcing a version has a global effect. It affects every dependency in your project. Make sure this is what you want, as it could potentially lead to unexpected behavior if other libraries are not compatible with the forced version.
• Careful Consideration: Use force sparingly. It's a powerful tool, but it can mask underlying problems. It's generally better to address the root cause of version conflicts rather than simply forcing a version.

4. Dependency Insight: Your Detective Tool

Before you start excluding or forcing dependencies, it's crucial to understand why you're getting the different versions of Commons IO in the first place. Gradle's dependency insight feature is your best friend here. It allows you to trace where a specific dependency is coming from.

How to use Dependency Insight:

1. Open the Gradle Tool Window: In Android Studio, go to View > Tool Windows > Gradle. If you do not see the Gradle Tool Window, make sure you are in the Android project view.
2. Run the Dependency Insight Task: In the Gradle Tool Window, navigate to Tasks > android > dependencies. Double-click on dependencies to run the task.
3. Search for the Dependency: In the Gradle Console (at the bottom of Android Studio), you'll see a massive output of all your project's dependencies. Search for commons-io. You can usually do this with Ctrl+F (Windows/Linux) or Cmd+F (Mac).

The output will show you something like this:

+--- commons-io:commons-io:2.19.0
|    
+--- your.group:your-library:1.0
|    

This tells you that commons-io:commons-io:2.19.0 is being pulled in by your.group:your-library:1.0. Now you know exactly which dependency to target if you need to exclude the transitive dependency.

Best Practices for Dependency Management

Preventing duplicate class errors is far better than fixing them after they occur. Here are some best practices for managing dependencies in your Android projects:

• Use a Dependency Management System (Gradle): Gradle is a powerful tool for managing dependencies. Use it to its full potential.
• Centralized Dependency Versions: Define dependency versions in a central location (like the ext block in your root build.gradle or in gradle.properties) and reuse them throughout your project. This makes it easier to update dependencies consistently.
• Regularly Review Dependencies: Keep an eye on your project's dependencies. Regularly check for updates and address any potential conflicts proactively.
• Be Mindful of Transitive Dependencies: Understand the dependencies your dependencies are bringing in. Use dependency insight to investigate.
• Avoid Explicit JARs: Whenever possible, rely on Gradle's dependency management rather than including JAR files directly in your libs/ directory.

Conclusion: Conquering Duplicate Classes

Duplicate class errors can be frustrating, but they're definitely solvable! By understanding the problem, using Gradle's features effectively, and following best practices, you can keep your Android projects clean and buildable.

Remember, the key steps are:

1. Identify the conflict: Recognize the duplicate classes and the libraries they're coming from.
2. Understand the source: Use dependency insight to trace where the dependencies are being pulled in.
3. Choose the right solution: Remove the explicit JAR, exclude the transitive dependency, or force a specific version, depending on your needs.
4. Maintain best practices: Keep your dependencies organized and up-to-date.

So, go forth and conquer those duplicate class errors, guys! Happy coding!