Exploiting Kotlin Metadata + Annotation Processing

When compiling Kotlin code for the JVM, the resulting bytecode doesn't understand the advanced features of the language – like the difference between List and MutableList, or List<String> and List<String?>.

A year ago a prototype was developed to use these "metadata" during annotation processing; today, a few popular libraries (even from Google!) employ it under the hood to take their Kotlin integration to the next level.

Join us to explore what's possible, and have fun digging into some practical examples.

[ sequel to last year's presentation at Kotlin Night Berlin: "Unleash the secret power of Kotlin Metadata" ]

Transcript

1. View Slide

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3. List
   List
   MutableList
   MyTypeAlias
   !"
   !"
   !"
   !"
   List
   List
   List
   List
   

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4. class
   object
   data class
   sealed class
   !"
   !"
   !"
   !"
   class
   class
   class
   abstract class
   

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5. data class Sample(val counter: Int)
   

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9. public final class Sample {
   private final int counter;
   public final int getCounter() {
   return this.counter;
   }
   public Sample(int counter) {
   this.counter = counter;
   }
   public final int component1() {
   return this.counter;
   }
   @NotNull
   public final Sample copy(int counter) {
   return new Sample(counter);
   }
   // $FF: synthetic method
   // $FF: bridge method
   @NotNull
   public static Sample copy$default(Sample var0, int var1, int var2, Object var3) {
   if((var2 & 1) != 0) {
   var1 = var0.counter;
   }
   return var0.copy(var1);
   }
   public String toString() {
   return "Sample(counter=" + this.counter + ")";
   }
   public int hashCode() {
   return this.counter;
   }
   public boolean equals(Object var1) {
   if(this != var1) {
   if(var1 instanceof Sample) {
   Sample var2 = (Sample)var1;
   if(this.counter == var2.counter) {
   return true;
   }
   }
   return false;
   } else {
   return true;
   }
   }
   }
   @Metadata(
   k = 1,
   mv = {1, 1, 10},
   bv = {1, 0, 2},
   d1 = {"\u0000 \n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\b\n\u0002\b\u0006\n\u0002\u0010\u000b\n\u0002\b\u0003\n\u0002\u0010\u000e\n\u0000\b\u0086\b\u0018\u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0010\
   d2 = {"LSample;", "", "counter", "", "(I)V", "getCounter", "()I", "component1", "copy", "equals", "", "other", "hashCode", "toString", "", "production sources for module app"}
   ))
   

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10. @Metadata(
    k = 1,
    mv = {1, 1, 10},
    bv = {1, 0, 2},
    d1 = {"\u0000 \n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0
    d2 = {"LSample;", "", "counter", "", "(I)V", "getCounter", "()I", "compon
    ))
    

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11. @Metadata
    k
    mv
    bv
    d1
    d2
    xs
    xi
    pn
    

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12. @Metadata
    1. kind
    2. metadata version
    3. bytecode version
    4. data1
    5. data2
    6. extra string
    7. extra int
    8. package name
    

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13. 8. package name fully qualified package (if != from Java)
    (ie. @JvmPackageName)
    (no classes!)
    

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14. 7. extra int
    0 -> multi-file class facade or part
    1 -> compiled by a pre-release version
    2 -> Kotlin script source file (.kts)
    

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16. “it depends” mostly Protocol Buffers
    4. data1
    5. data2
    6. extra string
    

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17. 2. metadata version
    3. bytecode version
    ...boooring
    

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18. 1. kind
    1 -> Class
    2 -> File
    3 -> Synthetic class
    4 -> Multi-file class facade
    5 -> Multi-file class part
    

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19. * nullability (also nested)
    * secondary constructors
    * companion object name
    * properties list
    * delegated types
    * internal visibility
    * sealed class hierarchy
    * type aliases (declaration and usage)
    * generic types (also in/out /*, reified)
    * classes (data/sealed/object/companion)
    * property (var/getter/setter/const/lateinit)
    * value parameter (default/crossinline/noinline)
    * function (operator/infix/inline/tailrec/external)
    

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21. ( )
    

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22. square/moshi
    

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23. square/moshi
    *
    * not the actual logo
    

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24. google/room
    

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25. google/room
    

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26. google/room
    

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27. jetbrains/kotlinx-metadata-jvm
    

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28. kotlin-metadata roadmap
    - use jetbrains/kotlinx-metadata-jvm (shadowed)
    - wrap Types/Elements in idiomatic Kotlin API
    - expose type mapping Java <-> Kotlin
    - KotlinPoet integration
    - documentation–HAHAHA JUST KIDDING
    

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29. goo.gl/nEcGmR -> JetBrains/kotlinx-metadata-jvm
    goo.gl/V26u5C -> Takhion/kotlin-metadata
    goo.gl/yWgvor -> Exploring Moshi’s Kotlin Code Gen
    goo.gl/mZdccu -> Moshi Kotlin Code Gen: An Open Source Story
    goo.gl/FPKqZ7 -> Λrrow annotation processors
    goo.gl/UDNmvh -> Moshi annotation processor
    goo.gl/GBHMqs -> Room annotation processor
    

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30. Eugenio
    Marle!i
    @workingkills
    bit.ly/kotlin-metadata-apt
    

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