Error:Execution failed for task ':app:packageRelease'. > Unable to compute hash of /../AndroidStudioProjects/../classes.jar

Question

I'm trying to do a 'release' build on an Android app and I keep on getting this error of:

Unable to compute hash of /../AndroidStudioProjects/../classes.jar

And then when I look into that directory for 'classes.jar' the file isn't there. Do I have to create this file myself with a gradle task?

There's something going on with proguard here but it's not giving much useful information other than 'Unable to compute hash...."

Here's my gradle.build file:

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"
    defaultConfig {
        applicationId "com.example"
        minSdkVersion 11
        targetSdkVersion 22
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        debug {
            buildConfigField "String", "SERVER_URL", '"http://10.0.2.2:3000"'
        }
        release {
            buildConfigField "String", "SERVER_URL", '"https://example.com"'
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
        debugRelease {
            buildConfigField "String", "SERVER_URL", '"https://example.com"'
            debuggable true
            jniDebuggable false
            renderscriptDebuggable false
            minifyEnabled false
            zipAlignEnabled true
        }
    }
    productFlavors {
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:22.1.1'
    compile 'com.android.support:recyclerview-v7:22.1.1'
    compile 'com.android.support:cardview-v7:22.1.1'
    compile 'com.google.android.gms:play-services-gcm:7.5.0'
}

If someone could help me debug this issue that'd be great.

Answer 1

All the current answers to this question are just giving the Proguard rules that worked for them, every fix will be different. First off, confirm it's a Proguard problem by checking that the classes-proguard directory is somewhere in the error message, something like this: Unable to compute hash of /Users/Documents/projectX/app/build/intermediates/classes-proguard/release/classes.jar

This means it's caused by an earlier Proguard error so you need to scroll up in the Messages window or Gradle Console window and check what warnings or errors you're getting. Just as an example, in my current project, Square's Picasso library is causing the error: Warning: com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.OkHttpClient. I just added -dontwarn com.squareup.okhttp.** to ignore the warnings, and the app still worked as normal.

A lot of the Proguard errors will be warnings about some class so just adding -dontwarn for whatever class is causing it in your project often works.

I know the StackOverflow way is just Google the error message, copy and paste the top answer and hope for the best but here you've to understand it a bit and figure out the proguard rules for you!

Answer 2

I figured out the problem:

Open up the proguard-rules.pro for your project and add this to the bottom:

-dontwarn java.nio.file.Files
-dontwarn java.nio.file.Path
-dontwarn java.nio.file.OpenOption
-dontwarn org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement

Basically how I solved it was this I tried to run my app in 'release' mode and got a bunch of errors similar to this guy here: https://github.com/square/okio/issues/144

I pretty much followed what he said and fixed it.

Hope this can help others with generating their APK's!