# Gradle brainteasers 2/2: relocatable input files
This is a follow up to [this other post](https://blog.mbonnin.net/gradle-brainteasers-12-aggregating-artifacts) about having fun with the Gradle APIs.
In this post, I'm talking about how I spent a shameful amount of time understanding how Gradle handles input files.
# The problem
The Apollo Gradle Plugin is generating Kotlin code from GraphQL files.
If you have GraphQL files like this:
```plaintext
.
└── src
└── main
└── graphql
└── com
└── example
├── GetUser.graphql
├── GetProduct.graphql
└── GetReview.graphql
```
The Apollo Compiler generates Kotlin classes like this:
```plaintext
com.example.GetUser
com.example.GetProduct
com.example.GetReview
```
ℹ
This behaviour of using the path as package name is optional and probably questionable but provides a very good use case for us diving into file inputs APIs.
In a nutshell, the compiler looks at the relative path of the file and uses that as the package name.
# Build cache relocation
A naive implementation could use the "base" directory as input so that it can compute the package names:
```kotlin
@get:PathSensitive(PathSensitivity.RELATIVE)
@get:InputFiles
abstract val inputFiles: Set
// The base directory to compute the package name
// This isn't great
@get:Input
abstract var baseDir: String
@TaskAction
fun taskAction() {
inputFiles.forEach {
val packageName = it.relativeTo(File(baseDir))
.canonicalPath
.replace(File.separatorChar, '.')
// ...
}
}
```
That works but it also completely breaks [build cache relocation](https://docs.gradle.org/current/userguide/build_cache_concepts.html#relocatability). If we were to copy our repository to another directory on our machine (or in CI), then no cache results could ever be reused.
# Entering FileCollection
The solution is to use [`FileCollection`](https://docs.gradle.org/current/javadoc/org/gradle/api/file/FileCollection.html) and its mutable cousin [`ConfigurableFileCollection`](https://docs.gradle.org/current/javadoc/org/gradle/api/file/ConfigurableFileCollection.html).
`FileCollection` is a [lazy API](https://docs.gradle.org/current/userguide/lazy_configuration.html) that can resolve [`Configuration`](https://mbonnin.medium.com/actual-footage-of-different-kinds-of-gradle-configurations-9678bd681793)s but most importantly may contain "roots". In other words, a `FileCollection` contains \`File\`s but also their relative path to the root(s).
At a high level, **Gradle doesn't have**`java.io.File`**inputs**. It's always `java.io.File` + `"fileIdentifier"`.
`fileIdentifier` being a name, a relative or absolute path, or even empty if only the contents of the file matter and you're using [`PathSensitivity.NONE`](https://docs.gradle.org/current/javadoc/org/gradle/api/tasks/PathSensitivity.html#NONE).
So how do we get that identifier? Well, for `NAME_ONLY` we can use `file.name`, for `NONE`, we can use `""` and for `ABSOLUTE`, we can use `file.asbolutePath`. But what about `RELATIVE`?
The secret to getting the relative path from a `FileCollection` is using [`asFileTree`](https://docs.gradle.org/current/javadoc/org/gradle/api/file/FileCollection.html#getAsFileTree()) which returns the underlying `FileTree` if any (which is really a `FileForest` because `FileTree`s can contain multiple roots):
```kotlin
@get:PathSensitive(PathSensitivity.RELATIVE)
@get:InputFiles
abstract val inputFiles: ConfigurableFileCollection
@TaskAction
fun taskAction() {
inputFiles.asFileTree.visit {
// Yay, we have the File here alongside its relative path 🎉
val packageName = path.replace(File.separatorChar, '.')
// do codegen
}
}
```
Using `ConfigurableFileCollection`, we get all the information about our files, including their relative path.
It's a very convenient API that non only allows resolving dependencies (`Configurations` are `FileCollections`) and filtering (`FileTree.include()`) and do so in a [lazy](https://docs.gradle.org/current/userguide/lazy_configuration.html) way but most importantly, they make it possible to model input files without breaking cache relocation! SUCCESS!
![](https://cdn.hashnode.com/res/hashnode/image/upload/v1721037893936/f23ea5e9-1bd2-4b6e-8d08-13312133e60b.webp align="center")
Looking ahead, [Gratatouille](https://github.com/GradleUp/gratatouille) will not allow simple \`File\` inputs to model the fact that task inputs also have a name (even if empty). Hopefully that will make the task of writing Gradle tasks easier!
---
*Brainteasers pictures from*[*mtairymd*](https://www.instructables.com/member/mtairymd/)*on*[*Instructables*](https://www.instructables.com/Metal-Wire-Puzzle-Solutions/)