The definitive guide to Android library development

@MOLSJEROEN THE DEFINITIVE GUIDE TO ANDROID LIBRARY DEVELOPMENT

@MOLSJEROEN

@MOLSJEROEN GOAL OUR FIRST LIBRARY TRANSITIVE DEPENDENCIES MODULARIZATION DEPENDENCY CONFLICTS
TIPS & TOOLS

@MOLSJEROEN TERMINOLOGY • Software Development Kit (SDK) • Android library

@MOLSJEROEN ANDROID LIBRARIES APP

@MOLSJEROEN ANDROID LIBRARIES APP COMPOSE OKHTTP …

@MOLSJEROEN ANDROID LIBRARIES APP COMPOSE OKHTTP … MAVEN

@MOLSJEROEN ANDROID LIBRARIES COMPOSE OKHTTP … MAVEN APP MODULE 1
MODULE 2

@MOLSJEROEN ANDROID LIBRARIES COMPOSE OKHTTP LIBRARY MAVEN APP MODULE 1
MODULE 2

@MOLSJEROEN ANDROID LIBRARIES dependencies { implementation "com.jeroenmols:library:1.0.0" }

OUR FIRST LIBRARY

ANDROID STUDIO TO THE RESCUE!

@MOLSJEROEN

TOOLING FOR ANDROID LIBRARIES IS LACKING

@MOLSJEROEN ANDROID STUDIO PROJECT PROJECT APP

@MOLSJEROEN

@MOLSJEROEN ANDROID STUDIO PROJECT LIBRARY APP PROJECT

@MOLSJEROEN

@MOLSJEROEN RUN & TEST $./gradlew :app:installDebu g $ ./gradlew :library:testDebugUnitTes
t $ ./gradlew :library:connectedAndroidTest

@MOLSJEROEN BUILD $ ./gradlew :library:assembleRelease

@MOLSJEROEN

@MOLSJEROEN APK •Compiled code •Processed resources •Android manifest •Compiled transitive
dependencies •Can be run on Android device •Signed JAR •Compiled code •Cannot be run on Android device •Unsigned

@MOLSJEROEN APK •Compiled code •Processed resources •Android manifest •Compiled transitive
dependencies •Can be run on Android device •Signed AAR •Compiled code •Processed resources •Android manifest •Cannot be run on Android device •Unsigned

@MOLSJEROEN DISTRIBUTE - STEPS 1.Build a release (see before) 2.Generate
and publish PGP key 3.Sign build 4.Upload to Maven central => Tutorial by Márton Braun:   https://getstream.io/blog/publishing-libraries-to-mavencentral-2021/

TRANSITIVE DEPENDENCIES

@MOLSJEROEN EXTERNAL DEPENDENCIES LIBRARY APP PROJECT MAVEN OKHTTP

@MOLSJEROEN EXTERNAL DEPENDENCIES PROJECT MAVEN CUSTOMER APP LIBRARY

@MOLSJEROEN EXTERNAL DEPENDENCIES PROJECT MAVEN OKHTTP CUSTOMER APP LIBRARY

@MOLSJEROEN EXTERNAL DEPENDENCIES dependencies { implementation "com.jeroenmols:library:1.0.0" implementation "com.squareup.okhttp3:okhttp:4.9.2" }

@MOLSJEROEN TRANSITIVE DEPENDENCIES dependencies { implementation "com.jeroenmols:library:1.0.0" }

@MOLSJEROEN EXTERNAL DEPENDENCIES PROJECT MAVEN OKHTTP CUSTOMER APP LIBRARY

@MOLSJEROEN POM - XML FILE <project xmlns="http://maven.apache.org/POM/4.0.0" > <groupId>com.jeroenmols</groupId> <artifactId>library</artifactId>
<version>1.0.0</version> <dependencies> <dependency> <groupId>com.squareup.okhttp3</groupId> <artifactId>okhttp3</artifactId> <version>4.9.0</version> </dependency> </dependencies> </project>

@MOLSJEROEN POM - XML FILE <project xmlns="http://maven.apache.org/POM/4.0.0" > <groupId>com.jeroenmols</groupId> <artifactId>library</artifactId>
<version>1.0.0</version> <dependencies > <dependency > <groupId>com.squareup.okhttp3</groupId > <artifactId>okhttp3</artifactId > <version>4.9.0</version > </dependency > </dependencies > </project>

@MOLSJEROEN POM - XML FILE <project xmlns="http://maven.apache.org/POM/4.0.0" > <groupId>com.jeroenmols</groupId >
<artifactId>library</artifactId > <version>1.0.0</version > <dependencies> <dependency> <groupId>com.squareup.okhttp3</groupId> <artifactId>okhttp3</artifactId> <version>4.9.0</version> </dependency> </dependencies > </project>

@MOLSJEROEN

ALWAYS TEST LIBRARIES THROUGH MAVEN

MODULARIZATION

@MOLSJEROEN APP MODULARISATION APP FEATURE 2 FEATURE 3 FEATURE 4
FEATURE 5 FEATURE 6 CORE 4 CORE 4 CORE 2 CORE 3 FEATURE 1 CORE 1

@MOLSJEROEN LIBRARY MODULARIZATION LIBRARY APP PROJECT DATABASE UI-COMPONENTS

@MOLSJEROEN

@MOLSJEROEN TEST SETUP // app Library().initialize() // library class Library
{ fun initialize() = Database().initialize() } // database class Database { fun initialize() = System.out.println(“database ready") }

PROBLEM #1   SUBMODULES NOT INCLUDED IN AAR

@MOLSJEROEN LIBRARY SUBMODULE TEST ./gradlew :library:assembleRelease

@MOLSJEROEN LIBRARY SUBMODULE TEST ./gradlew :library:assembleReleas e // App build.gradl
e dependencies { // implementation project(':library') implementation fi les('../library/build/outputs/aar/library-release.aar') ... }

@MOLSJEROEN

@MOLSJEROEN SUBMODULES NOT INCLUDED IN AAR 1. Release submodules to
Maven 2. Fat AAR 3. Single module SDK

@MOLSJEROEN 1. RELEASE SUBMODULES TO MAVEN COMPOSE LIBRARY MAVEN APP
UI-COMPONENTS DATABASE

@MOLSJEROEN 1. RELEASE SUBMODULES TO MAVEN // Library build.gradl e
dependencies { implementation "com.jeroenmols:database:1.0.0" implementation "com.jeroenmols:ui-components:1.0.0" }

@MOLSJEROEN 2. FAT AAR apply plugin: 'com.kezong.fat-aar' ... dependencies {
embed project(path: ':modules:database', con fi guration:'default') embed project(path: ':modules:ui-components', con fi guration:'default') }

@MOLSJEROEN 3. SINGLE MODULE SDK LIBRARY APP PROJECT

PROBLEM #2   PUBLIC INTERFACE SUBMODULES EXPOSED

@MOLSJEROEN KOTLIN VISIBILITY MODIFIERS • private: visible inside this class
only • protected: same as private + visible in subclasses too • internal: visible to all classes inside this module • public: visible to all classes

@MOLSJEROEN 1

@MOLSJEROEN PUBLIC INTERFACE SUBMODULES EXPOSED 1. Internal package 2. Obfuscate
non-public classes 3. Single module SDK

@MOLSJEROEN 1. INTERNAL PACKAGE package com.jeroenmols.internal.database

@MOLSJEROEN 2. OBFUSCATE NON-PUBLIC CLASSES package a.a.a

@MOLSJEROEN 2. OBFUSCATE NON-PUBLIC CLASSES // proguard-rules.pr o repackageclasses com.jeroenmols.internal 
    package com.jeroenmols.internal.a

@MOLSJEROEN 3. SINGLE MODULE SDK LIBRARY APP PROJECT

RECOMMENDATION LIBRARY MODULARIZATION

@MOLSJEROEN MODULARIZATION RECOMMENDATION • Modularizing SDKs is tedious • Single
module for small and mid sized SDKs • Spin off reusable maven artefacts for larger SDKs • Reduce public API surface • Tooling is lacking

TRANSITIVE DEPENDENCY CONFLICTS

@MOLSJEROEN TRANSITIVE DEPENDENCY CONFLICTS PROJECT MAVEN TRANSITIVE DEPENDENCY 1 CUSTOMER
APP ANOTHER LIBRARY

@MOLSJEROEN TRANSITIVE DEPENDENCY CONFLICTS PROJECT MAVEN TRANSITIVE DEPENDENCY 1 CUSTOMER
APP ANOTHER LIBRARY LIBRARY TRANSITIVE DEPENDENCY 2 TRANSITIVE DEPENDENCY 3

@MOLSJEROEN TRANSITIVE DEPENDENCY CONFLICTS 1. con fl icting transitive dependency
versions 2. incompatible transitive dependencies

PROBLEM #1 TRANSITIVE DEPENDENCY CONFLICTS

@MOLSJEROEN CONFLICTING DEPENDENCY VERSIONS dependencies { implementation "com.jeroenmols:library:1.0.0" implementation "com.example:anotherlibrary:2.0.0"
}

@MOLSJEROEN TRANSITIVE DEPENDENCY CONFLICTS PROJECT MAVEN OKHTTP V3 CUSTOMER APP
ANOTHER LIBRARY LIBRARY OKHTTP V4

@MOLSJEROEN

@MOLSJEROEN CONFLICT RESOLUTION 1.Force dependency resolution in CustomerApp 2.Loosen dependency
requirements in library 3.Remove transitive dependency from library

@MOLSJEROEN 1. FORCE DEPENDENCY IN CUSTOMERAPP implementation('com.jeroenmols:library:1.0.0') { exclude group:
'com.squareup.okhttp3', module: 'okhttp' }

@MOLSJEROEN 1. FORCE DEPENDENCY IN CUSTOMERAPP con fi gurations.all {
resolutionStrategy { force 'com.squareup.okhttp3:okhttp:3.12.0' } }

@MOLSJEROEN 1. FORCE DEPENDENCY IN CUSTOMERAPP • No library update
required • Force untested combinations of dependencies • Burden for SDK customers

@MOLSJEROEN CASE STUDY: BREAKING API CHANGES PROJECT MAVEN OKHTTP V2
CUSTOMER APP ANOTHER LIBRARY LIBRARY OKHTTP V3

@MOLSJEROEN OkHtttp v2 OkHttp v3

@MOLSJEROEN CASE STUDY: BREAKING API CHANGES package com.square.okhttp 3 implementation
‘com.squareup.okhttp3:okhttp3:4.9.0'

PROBLEM #2 INCOMPATIBLE TRANSITIVE DEPENDENCIES

@MOLSJEROEN INCOMPATIBLE TRANSITIVE DEPENDENCIES PROJECT MAVEN PROTOBUF-JAVA CUSTOMER APP ANOTHER
LIBRARY LIBRARY PROTOBUF-JAVALITE

@MOLSJEROEN INCOMPATIBLE TRANSITIVE DEPENDENCIES $ ./gradlew clean assembleDebug > Task
:app:checkDebugDuplicateClasses FAILED FAILURE: Build failed with an exception. * What went wrong: Execution failed for task ':myproject:checkReleaseDuplicateClasses'. > 1 exception was raised by workers: java.lang.RuntimeException: Duplicate class com.google.protobuf.AbstractMessageLite found in modules protobuf-java-3.11.1.jar (com.google.protobuf:protobuf-java:3.11.1) and protobuf-javalite-3.11.0.jar (com.google.protobuf:protobuf-javalite:3.11.0) Duplicate class com.google.protobuf.AbstractMessageLite$Builder found in modules protobuf-java-3.11.1.jar (com.google.protobuf:protobuf-java:3.11.1) and protobuf-javalite-3.11.0.jar (com.google.protobuf:protobuf-javalite:3.11.0) Duplicate class com.google.protobuf.AbstractMessageLite$Builder$LimitedInputStream found in modules protobuf-java-3.11.1.jar (com.google.protobuf:protobuf-java:3.11.1) and protobuf-javalite-3.11.0.jar (com.google.protobuf:protobuf-javalite:3.11.0) ...

@MOLSJEROEN INCOMPATIBLE TRANSITIVE DEPENDENCIES +--- com.google. fi rebase: fi rebase-perf:19.0.7
| +--- com.google. fi rebase: fi rebase-con fi g:19.0.4 | | +--- com.google. fi rebase: fi rebase-abt:19.0.0 | | | \--- com.google.protobuf:protobuf-lite:3.0.1

@MOLSJEROEN INCOMPATIBILITY RESOLUTION 1.Substitute dependency in CustomerApp 2.Remove dependency from
transitive dependency of library 3.Remove transitive dependency from library

@MOLSJEROEN 2. REMOVE FROM TRANSITIVE DEPENDENCY PROJECT MAVEN PROTOBUF-JAVALITE CUSTOMER
APP ANOTHER LIBRARY LIBRARY PBANDK PROTOBUF-JAVA

@MOLSJEROEN 2. REMOVE FROM TRANSITIVE DEPENDENCY .. . <dependency> <groupId>pro.streem.pbandk</groupId
> <artifactId>pbandk-runtime-jvm</artifactId> <version>0.9.0</version> <exclusions> <exclusion> <groupId>com.google.protobuf</groupId> <artifactId>protobuf-java</artifactId> </exclusion> </exclusions> </dependency>

@MOLSJEROEN 2. REMOVE FROM TRANSITIVE DEPENDENCY .. . <dependency >
<groupId>pro.streem.pbandk</groupId > <artifactId>pbandk-runtime-jvm</artifactId > <version>0.9.0</version > <exclusions> <exclusion> <groupId>com.google.protobuf</groupId> <artifactId>protobuf-java</artifactId> </exclusion> </exclusions > </dependency>

@MOLSJEROEN 2. REMOVE FROM TRANSITIVE DEPENDENCY pom.withXml { def dependencies
= asNode().appendNode('dependencies') con fi gurations.getByName(“releaseCompileClasspath").getResolvedCon fi guration( ) .getFirstLevelModuleDependencies().each { ... def dependency = dependencies.appendNode(‘dependency’ ) if (it.moduleName.contains("pbandk")) { def exclusions = dependency.appendNode(‘exclusions' ) def protobufExclusion = exclusions.appendNode('exclusion') protobufExclusion.appendNode('groupId', "com.google.protobuf") protobufExclusion.appendNode('artifactId', "protobuf-java") } } }

@MOLSJEROEN 2. REMOVE FROM TRANSITIVE DEPENDENCY pom.withXml { def dependencies
= asNode().appendNode('dependencies' ) con fi gurations.getByName(“releaseCompileClasspath").getResolvedCon fi guration( ) .getFirstLevelModuleDependencies().each { .. . def dependency = dependencies.appendNode(‘dependency’ ) if (it.moduleName.contains("pbandk")) { def exclusions = dependency.appendNode(‘exclusions' ) def protobufExclusion = exclusions.appendNode('exclusion') protobufExclusion.appendNode('groupId', "com.google.protobuf") protobufExclusion.appendNode('artifactId', "protobuf-java") } } }

APP ANOTHER LIBRARY LIBRARY PBANDK

@MOLSJEROEN 2. REMOVE FROM TRANSITIVE DEPENDENCY PROJECT MAVEN CUSTOMER APP
LIBRARY PBANDK

APP ANOTHER LIBRARY LIBRARY PBANDK PROTOBUF- JAVALITE

RECOMMENDATION TRANSITIVE DEPENDENCIES

@MOLSJEROEN TRANSITIVE DEPENDENCY RECOMMENDATION 1. Minimize transitive dependencies 2. Use
stable versions 3. Use versions without known vulnerabilities 4. Don’t use latest dependency versions 5. Prefer exact versions over ranges 6. Only use libraries that handle breaking changes => Put exceptions in release notes

@MOLSJEROEN INVESTIGATE DEPENDENCY CONFLICTS ./gradlew :library:dependencie s $ ./gradlew --console
plain :app:dependencies --con fi guration releaseRuntimeClasspath > Task :app:dependencies ------------------------------------------------------------ Project :app ------------------------------------------------------------ releaseRuntimeClasspath - Runtime classpath of compilation 'release' (target (androidJvm)). +--- org.jetbrains.kotlin:kotlin-stdlib:1.3.72 | +--- org.jetbrains.kotlin:kotlin-stdlib-common:1.3.72 | \--- org.jetbrains:annotations:13.0 +--- androidx.core:core-ktx:1.3.2 | +--- org.jetbrains.kotlin:kotlin-stdlib:1.3.71 -> 1.3.72 (*) | +--- androidx.annotation:annotation:1.1.0 | \--- androidx.core:core:1.3.2 | \--- ... +--- androidx.appcompat:appcompat:1.2.0 | +--- androidx.annotation:annotation:1.1.0

LAST SDK INTEGRATED WILL GET THE BLAME

TIPS & TOOLS

@MOLSJEROEN 2. LIBRARY RESOURCES

@MOLSJEROEN 2. LIBRARY RESOURCES - HIDE  <?xml
version="1.0" encoding="utf-8"?> <resources> <public name="library_name" type="string"/> </resources>

@MOLSJEROEN 2. LIBRARY RESOURCES - UNIQUE NAME // library/build.gradle android
{ ... resourcePre fi x 'mylib_' }

@MOLSJEROEN 3. COMPATIBILITY: SOURCE PROJECT MAVEN CUSTOMER APP LIBRARY V1

@MOLSJEROEN 3. COMPATIBILITY: SOURCE PROJECT MAVEN CUSTOMER APP LIBRARY V2

@MOLSJEROEN 3. COMPATIBILITY: SOURCE class CustomerApp { fun useLibrary() =
Library().initialize( ) } // Library v1 class Library { fun initialize() = Database().initialize() }

Library().initialize( ) } // Library v2 class Library { fun initialize(useCaching: Boolean) = Database().initialize() }

Library().initialize() } // Library v2 class Library { fun initialize(useCaching: Boolean) = Database().initialize() } no value passed for parameter useCaching

Library().initialize() } // Library v2 class Library { fun initialize(useCaching: Boolean = true) = Database().initialize() }

@MOLSJEROEN 3. COMPATIBILITY: BINARY // Bytecode library v1 public fi
nal class com/jeroenmols/library/Library { public fi nal static initialize() V } // Bytecode library v2 public fi nal class com/jeroenmols/library/Library { public fi nal static initialize(Z) V public static synthetic initialize$default(ZILjava/lang/Object;) V }

@MOLSJEROEN 3. COMPATIBILITY: BINARY // Bytecode library v 1 public
fi nal class com/jeroenmols/library/Library { public fi nal static initialize() V } // Bytecode library v 2 public fi nal class com/jeroenmols/library/Library { public fi nal static initialize(Z) V public static synthetic initialize$default(ZILjava/lang/Object;) V }

@MOLSJEROEN 3. COMPATIBILITY: BINARY OTHER LIBRARY MAVEN CUSTOMER APP LIBRARY
V1 LIBRARY V2

@MOLSJEROEN 3. COMPATIBILITY: BINARY OTHER LIBRARY MAVEN APP < FORCED
>   LIBRARY V2 LIBRARY V2

@MOLSJEROEN 3. COMPATIBILITY: BINARY Maintaining Compatibility in Kotlin libraries by
Márton Braun   https://zsmb.co/maintaining-compatibility-in-kotlin-libraries/ Public API challenges in Kotlin by Jake Wharton https://jakewharton.com/public-api-challenges-in-kotlin/

@MOLSJEROEN 3. COMPATIBILITY: GRADLE PLUGIN // top level build.gradle plugins
{ id 'org.jetbrains.kotlinx.binary-compatibility-validator' version '0.8.0-RC' } apiValidation { ignoredPackages += ["com.jeroenmols.library.internal", “com.jeroenmols.library.databinding"] ignoredClasses += ["com.jeroenmols.library.BuildCon fi g"] ignoredProjects += ["app"] }

@MOLSJEROEN 3. COMPATIBILITY: GRADLE PLUGIN // Snapshot current API into
api subfolder $ ./gradlew apiDum p // Verify current API against snapshot $ ./gradlew apiCheck

@MOLSJEROEN 4. MINIMIZING API SURFACE • Easy to learn •
Hard to make mistakes • Reduce maintenance • Avoid deprecation • Easier to change

@MOLSJEROEN 4. MINIMIZE API SURFACE // build.gradle fi le of
each module android { ... kotlinOptions { freeCompilerArgs += '-Xexplicit-api=strict' } } // library class Library { fun initialize() { ... } } Visibility must be specified in explicit API mode

each module android { ... kotlinOptions { freeCompilerArgs += '-Xexplicit-api=strict' } } // librar y class Library { fun initialize() { ... } }

each modul e android { .. . kotlinOptions { freeCompilerArgs += '-Xexplicit-api=strict ' } } // library class Library { fun initialize() { ... } } Visibility must be specified in explicit API mode

@MOLSJEROEN 5. EXPERIMENTAL APIS @RequiresOptIn(level = RequiresOptIn.Level.ERROR, message = "This
is an experimental API.") @Retention(AnnotationRetention.BINARY) @Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION) public annotation class ExperimentalMyLibraryApi

@MOLSJEROEN 5. EXPERIMENTAL APIS @RequiresOptIn(level = RequiresOptIn.Level.ERROR, message = "This
is an experimental API.") @Retention(AnnotationRetention.BINARY ) @Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION ) public annotation class ExperimentalMyLibraryApi

@MOLSJEROEN 5. EXPERIMENTAL APIS // library @ExperimentalMyLibraryApi fun doSomething() //
ap p @OptIn(ExperimentalMyLibraryApi::class ) override fun myFunction() { doSomething( ) }

@MOLSJEROEN 5. EXPERIMENTAL APIS // librar y @ExperimentalMyLibraryAp i fun
doSomething( ) // app @OptIn(ExperimentalMyLibraryApi::class) override fun myFunction() { doSomething( ) }

@MOLSJEROEN 5. EXPERIMENTAL APIS @kotlin.RequiresOptIn( level = kotlin.RequiresOptIn.Level.ERROR, ) public
annotation class ExperimentalMyLibraryAp i @androidx.annotation.RequiresOptIn( level = androidx.annotation.RequiresOptIn.Level.ERROR, ) public annotation class ExperimentalMyLibraryApi

@MOLSJEROEN 6. DEPRECATION - REPLACE class Library { @Deprecated(message =
“Please use the version with caching options”, replaceWith = ReplaceWith(“initialize(true)")) fun initialize( ) fun initialize(useCaching: Boolean ) }

@MOLSJEROEN 6. DEPRECATION - HIDE class Library { @Deprecated(message =
“Please use the version with caching options”, level = DeprecationLevel.HIDDEN) fun initialize( ) fun initialize(useCaching: Boolean ) }

@MOLSJEROEN 7. PROGUARD android { defaultCon fi g { ...
consumerProguardFiles 'consumer-rules.pro' } }

WRAP UP

@MOLSJEROEN DRIVE INNOVATION TOOLS ARE LACKING MINIMIZE API SURFACE LIMIT
MODULARISATION REDUCE DEPENDENCIES

@MOLSJEROEN HTTPS://JEROENMOLS.COM/BLOG

@MOLSJEROEN IMAGE CREDITS Welcome image by James A. Molnar https://unsplash.com/photos/R8t7MjLXy2s
Font awesome https://fontawesome.com/

@MOLSJEROEN FURTHER READS Maintaining compatibility in Kotlin libraries by Márton
Braun The hidden Kotlin gem: Deprecations with ReplaceWith by Marc Reichelt Publishing libraries to Maven Central in 2021 by Marton Braun Java interoperability policy for major version updates by Jake Wharton Public API challenges in Kotlin by Jake Wharton How to publish and distribute your Android library by Marco Gomiero What is a diamond dependency con fl ict by Google All about Opt-in annotations by Márton Braun

MOLSJEROEN

The definitive guide to Android library develop...

The definitive guide to Android library development

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