title: Android Studio proguardRelease FAILED due empty output jar library
body: <p>I have imported a project from Eclipse into Android Studio 1.0.1 that consists in an Android Library Project (RuletaAfortunadaCore) and an Android Project (RuletaAfortunada), also have some third parties library dependencies. During the import everything seemed fine, but now when I try to build it I get this error message from Gradle:</p>
  
  <pre><code>FAILURE: Build failed with an exception.
  
  * What went wrong:
  Execution failed for task ':ruletaAfortunadaCore:proguardRelease'.
  &gt; java.io.IOException: The output jar is empty. Did you specify the proper '-keep' options?
  </code></pre>
  
  <p>The whole output from the start of the building process:</p>
  
  <pre><code>Executing tasks: [clean, :ruletaAfortunada:compileDebugSources, :facebookSDK:compileDebugSources, :ruletaAfortunadaCore:compileDebugSources]
  
  Configuration on demand is an incubating feature.
  :facebookSDK:clean
  :ruletaAfortunada:clean UP-TO-DATE
  :ruletaAfortunadaCore:clean
  :facebookSDK:compileLint
  :facebookSDK:copyReleaseLint UP-TO-DATE
  :facebookSDK:mergeReleaseProguardFiles UP-TO-DATE
  :facebookSDK:preBuild
  :facebookSDK:preReleaseBuild
  :facebookSDK:checkReleaseManifest
  :facebookSDK:prepareReleaseDependencies
  :facebookSDK:compileReleaseAidl
  :facebookSDK:compileReleaseRenderscript
  :facebookSDK:generateReleaseBuildConfig
  :facebookSDK:generateReleaseAssets UP-TO-DATE
  :facebookSDK:mergeReleaseAssets
  :facebookSDK:generateReleaseResValues UP-TO-DATE
  :facebookSDK:generateReleaseResources
  :facebookSDK:packageReleaseResources
  :facebookSDK:processReleaseManifest
  :facebookSDK:processReleaseResources
  :facebookSDK:generateReleaseSources
  :facebookSDK:compileReleaseJava
  :facebookSDK:processReleaseJavaRes UP-TO-DATE
  :facebookSDK:packageReleaseJar
  :facebookSDK:compileReleaseNdk
  :facebookSDK:packageReleaseJniLibs UP-TO-DATE
  :facebookSDK:packageReleaseLocalJar
  :facebookSDK:packageReleaseRenderscript UP-TO-DATE
  :facebookSDK:bundleRelease
  :ruletaAfortunada:preBuild
  :ruletaAfortunada:preDebugBuild
  :ruletaAfortunada:checkDebugManifest
  :ruletaAfortunada:preReleaseBuild
  :ruletaAfortunadaCore:compileLint
  :ruletaAfortunadaCore:copyReleaseLint UP-TO-DATE
  :ruletaAfortunadaCore:preBuild
  :ruletaAfortunadaCore:preReleaseBuild
  :ruletaAfortunadaCore:checkReleaseManifest
  :ruletaAfortunadaCore:preDebugBuild
  :ruletaAfortunadaCore:preDebugTestBuild
  :ruletaAfortunadaCore:prepareComAndroidSupportSupportV42100Library
  :ruletaAfortunadaCore:prepareComGoogleAndroidGmsPlayServices6587Library
  :ruletaAfortunadaCore:prepareRuletaAfortunadaFacebookSDKUnspecifiedLibrary
  :ruletaAfortunadaCore:prepareReleaseDependencies
  :ruletaAfortunadaCore:compileReleaseAidl
  :ruletaAfortunadaCore:compileReleaseRenderscript
  :ruletaAfortunadaCore:generateReleaseBuildConfig
  :ruletaAfortunadaCore:generateReleaseAssets UP-TO-DATE
  :ruletaAfortunadaCore:mergeReleaseAssets
  :ruletaAfortunadaCore:generateReleaseResValues UP-TO-DATE
  :ruletaAfortunadaCore:generateReleaseResources
  :ruletaAfortunadaCore:mergeReleaseResources
  :ruletaAfortunadaCore:processReleaseManifest
  :ruletaAfortunadaCore:processReleaseResources
  :ruletaAfortunadaCore:generateReleaseSources
  :ruletaAfortunadaCore:compileReleaseJava
  :ruletaAfortunadaCore:extractReleaseAnnotations
  :ruletaAfortunadaCore:mergeReleaseProguardFiles UP-TO-DATE
  :ruletaAfortunadaCore:compileReleaseNdk
  :ruletaAfortunadaCore:packageReleaseJniLibs UP-TO-DATE
  :ruletaAfortunadaCore:packageReleaseRenderscript UP-TO-DATE
  :ruletaAfortunadaCore:packageReleaseResources
  :ruletaAfortunadaCore:proguardRelease FAILED
  
  FAILURE: Build failed with an exception.
  
  * What went wrong:
  Execution failed for task ':ruletaAfortunadaCore:proguardRelease'.
  &gt; java.io.IOException: The output jar is empty. Did you specify the proper '-keep' options?
  </code></pre>
  
  <p>The contents of the build.gradle file for such library project:</p>
  
  <pre><code>apply plugin: 'com.android.library'
  
  android {
      compileSdkVersion 13
      buildToolsVersion "21.1.2"
  
      defaultConfig {
          minSdkVersion 9
          targetSdkVersion 17
      }
  
      buildTypes {
          release {
              minifyEnabled true
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-project.txt'
          }
      }
  }
  
  dependencies {
      compile project(':facebookSDK')
      compile 'com.android.support:support-v4:18.0.0'
      compile 'com.google.code.gson:gson:2.2.4'
      compile 'com.google.android.gms:play-services:+'
      compile files('libs/chartboost.jar')
      compile files('libs/mint-4.0.7.jar')
  }
  </code></pre>
  
  <p><strong>UPDATE</strong>: I have realized that the Android Library Project named RuletaAfortunadaCore can be build as an standalone project or made as a module without problems at all. It only fails to build when building the whole RuletaAfortunada Android Project that uses it.</p>
  
  <p>The build.gradle file for such Android Project is the default one created by Android Studio, I think, so has nothing interesting inside. Anyway, here it is, just in case:</p>
  
  <pre><code>// Top-level build file where you can add configuration options common to all sub-projects/modules.
  buildscript {
      repositories {
          jcenter()
      }
      dependencies {
          classpath 'com.android.tools.build:gradle:1.0.0'
      }
  }
  
  allprojects {
      repositories {
          jcenter()
      }
  }
  </code></pre>
  
  <p>This is the settings.gradle:</p>
  
  <pre><code>include ':facebookSDK'
  include ':ruletaAfortunadaCore'
  include ':ruletaAfortunada'
  </code></pre>
  
  <p>And finally, just to have all of them, this is the facebookSDK module build.gradle:</p>
  
  <pre><code>apply plugin: 'com.android.library'
  
  android {
      compileSdkVersion 9
      buildToolsVersion "21.1.2"
  
      defaultConfig {
          minSdkVersion 9
          targetSdkVersion 9
      }
  
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
          }
      }
  }
  
  dependencies {
      compile 'com.android.support:support-v4:18.0.0'
      compile files('libs/bolts-android-1.1.2.jar')
  }
  </code></pre>