GSON/Gradle: java.lang.NoClassDefFoundError in 3-module Android Studio Project

Question

I know there are a few dozen people ( like this one ) who've posted with the same stacktrace, but after working through each of them I'm still missing a solution. I have a school project I'm working on which we're building in Android Studio (Gradle is the build tool) that depends on Gson. It is structured with three modules:

• app (the android app generated by Android Studio)
• server (the server which the app will be directing its calls to)
• shared (classes that the server and app both depend on)

Both app and server have the following line in their build.gradle dependency:

implementation project(':shared')

So both depend on shared . shared , in turn, has gson-2.6.2.jar in shared/libs (though I've tried removing it and using the remote repository instead too), and all three have this as a dependency:

implementation fileTree(dir: 'libs', include: ['*.jar'])

There's a class in shared called Serializer which uses Gson. But when I'm running the server module within Android Studio and it gets to a call to Serializer, I get this stack trace:

Exception in thread "Thread-2" java.lang.NoClassDefFoundError: com/google/gson/Gson
    at my.t2r.comm.Serializer.<init>(Serializer.java:25)
...
Caused by: java.lang.ClassNotFoundException: com.google.gson.Gson
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 13 more

Solutions on Stack Overflow focus on making sure the dependencies are in the build.gradle files and cleaning and rebuilding the project. I've tried including the dependency on Gson in all three modules individually, tried using a remote Gson, and cleaned and rebuilt countless times. I'm always able to use Gson while coding, but it refuses to work at runtime. I'm at a loss, and any help would be appreciated!

app/build.gradle:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "my.t2r"
        minSdkVersion 24
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    buildToolsVersion '26.0.2'
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
    implementation project(':shared')
    compile 'com.android.support:recyclerview-v7:26.1.0'
    compile 'com.bignerdranch.android:expandablerecyclerview:1.0.3'
}

server/build.gradle:

apply plugin: 'java-library'

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation project(':shared')
    testImplementation 'junit:junit:4.12'
    implementation 'com.google.code.gson:gson:2.8.2' // I've also tried api instead
}

sourceCompatibility = "1.7"
targetCompatibility = "1.7"

shared/build.gradle:

apply plugin: 'java-library'

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    testImplementation 'junit:junit:4.12'
    implementation 'com.google.code.gson:gson:2.8.2' // I've also tried api instead
}

sourceCompatibility = "1.7"
targetCompatibility = "1.7"

EDIT: Updated to indicate that I'm using the directive to use remote instead of libs/

Answer 1

在您的app\\build.gradle添加此依赖app\\build.gradle

implementation 'com.google.code.gson:gson:2.8.2'

Answer 2

I found an answer!

I'm assuming there was something wrong with the IDE. When I loaded up the project in IDEA instead of Android Studio, the Gson dependency was scoped as Provided instead of Compile. Unfortunately, in Android Studio there's no way to see those settings. After hours of googling different, I finally turned up this answer in an SO chat room:

artifacts.add("default", file('libs/gson-2.6.2.jar'))

Adding this line to shared/build.gradle (not inside of any brackets, just free) solved my problem! I did not need anything changed in server/build.gradle .

Answer 3

I had the same problem, nothing like cleans or cache invaidates or Gradle changes helped. Finally it turned out that it was exclusive to one emulator instance (API 19). Both other emulators and physical device work fine.