Android Studio - Proguard Error Shows Unable to compute hash

In Android development, the standard development kit includes a tool to shrink, optimise and obfuscate the APK. This tool is known as Proguard.

Android Studio will provide the options to use Proguard if it is enabled. However, some classes or files do NOT like to be optimised and obfuscated. In particular Javascript files and reference to classes that do not exist in the path. Here is an example of how the error message may appear when building the signed APK on Linux & MS Windows.

Linux

Error:Execution failed for task ':app:packageMinirelease'.

> Unable to compute hash of /usr/development/BakersPercentage/app/build/intermediates/classes-proguard/minirelease/classes.jar

MS Windows

Error from proguard during build

Since the problem is a warning on GMS packages, I resolved on Android Studio 1.4 as follows;

Step 1: Verify project works

Open the project and make sure it can build without errors. 

Step 2: Edit build.gradle (Module: app)

Copy the existing release build type and paste as a new build release named minirelease (or any name you fancy).

buildTypes {

        release {

            minifyEnabled false

            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'

            signingConfig signingConfigs.config

        }

        minirelease {

            minifyEnabled true

            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'

            signingConfig signingConfigs.config

        }

    }

Step 3: Edit proguard-rules.pro

Add the following lines

 -keep class com.google.android.gms.** { *; }

 -dontwarn com.google.android.gms.**

Step 4: Build Signed APK

In Android Studio menu, choose Build-> Clean Project

Build-> Generate Signed APK...

Enter Key details and click Next.

In Build Type: choose minirelease.

Click Finish.

Done.