Unleash the secret power of Kotlin Metadata

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`. These "metadata" are hidden, but not lost, and they're a gold mine when trying to create tools designed from the ground up with Kotlin in mind. Join us to learn where to find them, how they're structured and how we can make the most out of them.

Video: https://youtu.be/dAQaZFyZ_hQ

Slides in Keynote (animated): https://goo.gl/5Z7kBc

Links:
+ https://goo.gl/INLPDl -> The Future of Kotlin
+ https://goo.gl/UiT6yh -> descriptors.proto

Transcript

1. View Slide

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5. List
   List
   MutableList
   MyTypeAlias
   ->
   ->
   ->
   ->
   List
   List
   List
   List
   

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6. class
   object
   data class
   sealed class
   ->
   ->
   ->
   ->
   class
   class
   class
   abstract class
   

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7. data class Sample(val counter: Int)
   

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11. 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, 6},
    bv = {1, 0, 1},
    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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12. @Metadata(
    k = 1,
    mv = {1, 1, 6},
    bv = {1, 0, 1},
    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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13. @Metadata
    k
    mv
    bv
    d1
    d2
    xs
    xi
    

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14. @Metadata
    1. kind
    2. metadata version
    3. bytecode version
    4. data1
    5. data2
    6. extra string
    7. extra int
    

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15. 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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17. “it depends”
    4. data1
    5. data2
    6. extra string
    

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

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

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20. * 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. warning: PLEASE TRY THIS AT HOME
    

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22. goo.gl/INLPDl -> The Future of Kotlin
    goo.gl/8oFRaU -> kotlin-metadata-samples (Coming Soon™)
    goo.gl/UiT6yh -> descriptors.proto
    

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23. bit.ly/unleash-kotlin-metadata
    Eugenio
    Marletti
    @workingkills
    

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