android - Why does Gradle build my module in Release mode when the app is in Debug

Question

I'm making a new Android project, with the standard 'app' module, as well as a library project (let's call it 'custom_lib'). In the app's build.gradle file, I link the module as such :

dependencies {
    compile project(':custom_lib')
}

When I trigger the build process (Menu Build > Make Project), I get the following output in the Gradle console

Executing tasks: [clean, :app:compileDebugSources, :custom_lib:compileDebugSources]

Configuration on demand is an incubating feature.
:app:clean
:custom_lib:clean
:app:preBuild
:app:preDebugBuild
:app:checkDebugManifest
:app:preReleaseBuild
:custom_lib:compileLint
:custom_lib:copyReleaseLint UP-TO-DATE
:custom_lib:mergeReleaseProguardFiles UP-TO-DATE
:custom_lib:preBuild
:custom_lib:preReleaseBuild
:custom_lib:checkReleaseManifest
:custom_lib:prepareReleaseDependencies
:custom_lib:compileReleaseAidl
:custom_lib:compileReleaseRenderscript
:custom_lib:generateReleaseBuildConfig
:custom_lib:generateReleaseAssets UP-TO-DATE
:custom_lib:mergeReleaseAssets
:custom_lib:generateReleaseResValues UP-TO-DATE
:custom_lib:generateReleaseResources
:custom_lib:packageReleaseResources
:custom_lib:processReleaseManifest
:custom_lib:processReleaseResources
:custom_lib:generateReleaseSources
:custom_lib:compileReleaseJava
:custom_lib:processReleaseJavaRes UP-TO-DATE
:custom_lib:packageReleaseJar
:custom_lib:compileReleaseNdk
:custom_lib:packageReleaseJniLibs UP-TO-DATE
:custom_lib:packageReleaseLocalJar UP-TO-DATE
:custom_lib:packageReleaseRenderscript UP-TO-DATE
:custom_lib:bundleRelease
:app:prepareComAndroidSupportAppcompatV72102Library
:app:prepareComAndroidSupportSupportV42102Library
:app:prepareTestDoubleBuildCustom_libUnspecifiedLibrary
:app:prepareDebugDependencies
:app:compileDebugAidl
:app:compileDebugRenderscript
:app:generateDebugBuildConfig
:app:generateDebugAssets UP-TO-DATE
:app:mergeDebugAssets
:app:generateDebugResValues UP-TO-DATE
:app:generateDebugResources
:app:mergeDebugResources
:app:processDebugManifest
:app:processDebugResources
:app:generateDebugSources
:app:compileDebugJava
:app:compileDebugNdk
:app:compileDebugSources
:custom_lib:preDebugBuild
:custom_lib:checkDebugManifest
:custom_lib:prepareDebugDependencies
:custom_lib:compileDebugAidl
:custom_lib:compileDebugRenderscript
:custom_lib:generateDebugBuildConfig
:custom_lib:generateDebugAssets UP-TO-DATE
:custom_lib:mergeDebugAssets
:custom_lib:generateDebugResValues UP-TO-DATE
:custom_lib:generateDebugResources
:custom_lib:packageDebugResources
:custom_lib:processDebugManifest
:custom_lib:processDebugResources
:custom_lib:generateDebugSources
:custom_lib:compileDebugJava
:custom_lib:compileDebugNdk
:custom_lib:compileDebugSources

BUILD SUCCESSFUL

Total time: 2.184 secs

What puzzles me is that the build mechanism triggers a Debug build (as the first line says), but almost immediately, Gradle uses the task :app:preReleaseBuild which make my custom_lib module to be built with the Release configuration.

And then, after the app is fully built, Gradle compiles my module with the Debug configuration.

So my questions are :

• Why is it doing this double build which seems incoherent ?
• How can I make sure that the library is built with debug configuration when I launch a Debug build process ?

EDIT :

app/build.gradle :

apply plugin: 'com.android.application'

android {
    compileSdkVersion 21
    buildToolsVersion "21.1.1"

    defaultConfig {
        applicationId "com.deezer.testdoublebuild"
        minSdkVersion 8
        targetSdkVersion 21
        versionCode 1
        versionName "1.0"
    }

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }
    buildTypes {
        debug{
            debuggable true
        }
        release {
            debuggable false
            minifyEnabled false
        }
    }
}

dependencies {
    compile project(':custom_lib')
}

custom_lib/build.gradle :

apply plugin: 'com.android.library'

android {
    compileSdkVersion 21
    buildToolsVersion "21.1.1"

    defaultConfig {
        applicationId "com.deezer.mylibrary"
        minSdkVersion 8
        targetSdkVersion 21
        versionCode 1
        versionName "1.0"
    }

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
}

Note : I'm using Android Studio 1.0 RC 1 / Gradle 2.2, and reproduced this issue by creating a new project from scratch, add an empty Android Library module, and "voila"

Answer 1

Put this in your app dependencies:

dependencies {
    debugCompile project(path: ':custom_lib', configuration: "debug")
    releaseCompile project(path: ':custom_lib', configuration: "release")
}

and in your library's build.gradle add:

android {

    defaultConfig {
        defaultPublishConfig 'release'
        publishNonDefault true
    }

}

Then the library will be built in the same mode as the app. Contrary to previous revisions of this answer, I've confirmed a flavor is not required on the library (this may be due to Gradle or Android plugin versions - I'm using Gradle 2.14 and Android plugin 2.1.0 and did not require it).

Edit: You may have trouble if you don't clean/rebuild after modifying the gradle files, as outlined in this answer here.