Using Gradle, how can I ensure that a file exists at a certain location?

Question

I am starting to use Gradle for an Android application. I would like the build to fail if the developer fails to create a file at a specific location such as ./src/res/values/specialfile.xml

A bit of searching led me to believe that a .doFirst would work

android.doFirst { 
    assert file("./src/res/values/specialfile.txt").exists() 
} 

However, Gradle says "unsupported Gradle DSL method found: 'doFirst()'!"

What can I do to assert a file's existence?

Answer 1

doFirst only exists on tasks object. android is not a task.

If would want this test to always be done even if the developer doesn't try to build (for example when running the tasks task), you should simply put in your build.gradle

assert file("./src/res/values/specialfile.txt").exists() 

However this is really not recommended as this would be executed even for non build tasks, or even when the model is built for IDE integration.

There is a task called preBuild that is executed before anything in the android build, so you can hook your test to it, either through another task or through doFirst:

preBuild.doFirst { 
    assert file("./src/res/values/specialfile.txt").exists() 
} 

Answer 2

One may want to check in a module build.gradle ...

• below the plugins configuration block
• above the android configuration block

eg. in order to determine which Gradle plugins to apply:

plugins {
    id "com.android.application"
}

def json_google = "src/google-services.json";
if (project.file(json_google).exists()) {
    println "found: ${project.file(json_google)}"
    apply plugin: "com.google.gms.google-services"
    apply plugin: "com.google.firebase.crashlytics"
    ...
} else {
    def message = "missing: ${project.file(json_google)}"
    // throw new GradleException(message)
    println message
}

android { ... }

rootProject.file() and project.file() might be more convenient than file() (it depends). Using assert seems a bit harsh - but then one has to take care that it will not crash elsewhere.

My projects usually have a few optional config files, which the environment may or may not provide, because not every product flavor may require the same set of plugins & config files.