The definitive guide to Android library development

Video: https://www.droidcon.com/2021/11/10/the-definitive-guide-to-android-library-development/

Android library/SDK development unfortunately is harder than it should be. This is caused by inferior tooling (Why do you need fat AAR?), missing documentation (What causes transitive dependency conflicts?), and different architecture considerations (Should you modularize your SDK?).

Having switched to full-time Android SDK development two years ago, I'm summarizing my lessons learned into a talk I wish would have existed when I got started.

⚠️ This talk is aimed at anyone curious to level up their knowledge of the Android ecosystem, not just at beginning/seasoned SDK developers!

---------------------------

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 conflict by Google
- All about Opt-in annotationsby Márton Braun

Transcript

1. @MOLSJEROEN
   THE DEFINITIVE GUIDE
   TO ANDROID LIBRARY
   DEVELOPMENT
   

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2. @MOLSJEROEN
   

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

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

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5. GOAL
   

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6. @MOLSJEROEN
   ANDROID LIBRARIES
   APP
   

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7. @MOLSJEROEN
   ANDROID LIBRARIES
   APP
   COMPOSE OKHTTP …
   

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8. @MOLSJEROEN
   ANDROID LIBRARIES
   APP
   COMPOSE OKHTTP …
   MAVEN
   

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

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

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

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12. OUR FIRST LIBRARY
    

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13. ANDROID STUDIO TO THE
    RESCUE!
    

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14. @MOLSJEROEN
    

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15. TOOLING FOR ANDROID
    LIBRARIES IS LACKING
    

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16. @MOLSJEROEN
    ANDROID STUDIO PROJECT
    PROJECT APP
    

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17. @MOLSJEROEN
    

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18. @MOLSJEROEN
    ANDROID STUDIO PROJECT
    LIBRARY
    APP
    PROJECT
    

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19. @MOLSJEROEN
    

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20. @MOLSJEROEN
    

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21. @MOLSJEROEN
    

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22. @MOLSJEROEN
    

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

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24. @MOLSJEROEN
    BUILD
    $ ./gradlew :library:assembleRelease
    

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25. @MOLSJEROEN
    

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26. @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
    

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27. @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
    

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28. @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/
    

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29. TOOLING FOR ANDROID
    LIBRARIES IS LACKING
    

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30. TRANSITIVE DEPENDENCIES
    

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31. @MOLSJEROEN
    EXTERNAL DEPENDENCIES
    LIBRARY
    APP
    PROJECT
    MAVEN OKHTTP
    

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32. @MOLSJEROEN
    EXTERNAL DEPENDENCIES
    PROJECT
    MAVEN
    CUSTOMER APP
    LIBRARY
    

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33. @MOLSJEROEN
    EXTERNAL DEPENDENCIES
    PROJECT
    MAVEN OKHTTP
    CUSTOMER APP
    LIBRARY
    

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

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

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36. @MOLSJEROEN
    EXTERNAL DEPENDENCIES
    PROJECT
    MAVEN
    OKHTTP
    CUSTOMER APP
    LIBRARY
    

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37. @MOLSJEROEN
    POM - XML FILE
    
    com.jeroenmols
    library
    1.0.0
    
    
    com.squareup.okhttp3
    okhttp3
    4.9.0
    
    
    
    

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38. @MOLSJEROEN
    POM - XML FILE
    >
    
    com.jeroenmols
    library
    1.0.0
    >
    
    >
    
    com.squareup.okhttp3>
    
    okhttp3>
    
    4.9.0>
    
    >
    
    >
    
    
    

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39. @MOLSJEROEN
    POM - XML FILE
    >
    
    com.jeroenmols>
    
    library>
    
    1.0.0>
    
    
    
    com.squareup.okhttp3
    okhttp3
    4.9.0
    
    >
    
    
    

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40. @MOLSJEROEN
    

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41. ALWAYS TEST LIBRARIES
    THROUGH MAVEN
    

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42. MODULARIZATION
    

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43. @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
    

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44. @MOLSJEROEN
    LIBRARY MODULARIZATION
    LIBRARY
    APP
    PROJECT
    DATABASE UI-COMPONENTS
    

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45. @MOLSJEROEN
    

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46. @MOLSJEROEN
    

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47. @MOLSJEROEN
    

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

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49. PROBLEM #1
    

    SUBMODULES NOT INCLUDED IN AAR
    

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

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51. @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')
    ...
    }
    

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52. @MOLSJEROEN
    

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53. @MOLSJEROEN
    

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54. TOOLING FOR ANDROID
    LIBRARIES IS LACKING
    

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55. @MOLSJEROEN
    

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

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

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58. @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"
    }
    

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59. @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')
    }
    

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60. @MOLSJEROEN
    3. SINGLE MODULE SDK
    LIBRARY
    APP
    PROJECT
    

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61. PROBLEM #2
    

    PUBLIC INTERFACE SUBMODULES EXPOSED
    

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62. @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
    

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63. @MOLSJEROEN
    1
    

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

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

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

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

    

    

    package com.jeroenmols.internal.a
    

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68. @MOLSJEROEN
    3. SINGLE MODULE SDK
    LIBRARY
    APP
    PROJECT
    

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69. RECOMMENDATION
    
    
    LIBRARY MODULARIZATION
    

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70. @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
    

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71. TRANSITIVE DEPENDENCY
    CONFLICTS
    

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

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

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

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75. PROBLEM #1
    
    
    TRANSITIVE DEPENDENCY CONFLICTS
    

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

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

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

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79. @MOLSJEROEN
    

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

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

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

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

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

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85. @MOLSJEROEN
    OkHtttp v2
    
    
    OkHttp v3
    

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

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87. PROBLEM #2
    
    
    INCOMPATIBLE TRANSITIVE DEPENDENCIES
    

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

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

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90. @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)
    
    
    ...
    
    
    

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91. @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
    

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

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

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

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95. @MOLSJEROEN
    2. REMOVE FROM TRANSITIVE DEPENDENCY
    ..
    .
    
    
    pro.streem.pbandk>
    
    pbandk-runtime-jvm
    0.9.0
    
    
    com.google.protobuf
    protobuf-java
    
    
    
    

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96. @MOLSJEROEN
    2. REMOVE FROM TRANSITIVE DEPENDENCY
    ..
    .
    
    >
    
    pro.streem.pbandk>
    
    pbandk-runtime-jvm>
    
    0.9.0>
    
    
    
    com.google.protobuf
    protobuf-java
    
    >
    
    
    

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97. @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")
    } } }
    

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98. @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")
    } } }
    

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

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

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

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102. RECOMMENDATION
     
     
     TRANSITIVE DEPENDENCIES
     

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103. @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
     

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104. @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
     
     
     
     
     

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105. LAST SDK INTEGRATED WILL
     GET THE BLAME
     

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106. TIPS & TOOLS
     

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107. @MOLSJEROEN
     2. LIBRARY RESOURCES
     

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108. @MOLSJEROEN
     2. LIBRARY RESOURCES - HIDE
     
     
     
     
     
     

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

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

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

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

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113. @MOLSJEROEN
     3. COMPATIBILITY: SOURCE
     class CustomerApp
     {
     
     fun useLibrary() = Library().initialize(
     )
     
     }
     // Library v2
     class Library {
     fun initialize(useCaching: Boolean) = Database().initialize()
     }
     

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114. @MOLSJEROEN
     3. COMPATIBILITY: SOURCE
     class CustomerApp {
     fun useLibrary() = Library().initialize()
     }
     // Library v2
     class Library {
     fun initialize(useCaching: Boolean) = Database().initialize()
     }
     no value passed for parameter useCaching
     

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115. @MOLSJEROEN
     3. COMPATIBILITY: SOURCE
     class CustomerApp {
     fun useLibrary() = Library().initialize()
     }
     // Library v2
     class Library {
     fun initialize(useCaching: Boolean = true) = Database().initialize()
     }
     

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116. @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
     
     }
     

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117. @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
     
     }
     

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118. @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
     
     }
     

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

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

     LIBRARY V2
     LIBRARY V2
     

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121. @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/
     

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122. @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"]
     }
     

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

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

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125. @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
     

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126. @MOLSJEROEN
     4. MINIMIZE API SURFACE
     // build.gradle
     fi
     le of each module
     android {
     ...
     kotlinOptions {
     freeCompilerArgs += '-Xexplicit-api=strict'
     }
     }
     // librar
     y
     
     class Library
     {
     
     fun initialize() { ...
     }
     
     }
     

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

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128. @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
     

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129. @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
     

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

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

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132. @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
     

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133. @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
     )
     
     }
     

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

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

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136. WRAP UP
     

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

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138. @MOLSJEROEN
     HTTPS://JEROENMOLS.COM/BLOG
     

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

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140. @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
     

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141. MOLSJEROEN
     

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