Kotlin Backstage

Kotlin Backstage
Mitchell Tilbook @sir_tilbrook
youtube.com/c/anzcoders

Kotlin Backstage
— What Kotlin generates in bytecode
— Impact of const
— Objects & Statics
— Impact of Scoping on method count

Primitives
val number: Int = 10
val number: Int? = 10

Primitives
private final static I number = 10
private final static L…/Integer; nullableNumber
@L…/Nullable;() // invisible

String Interpolation
val i = 10
val s = "i = $i" // evaluates to "i = 10"

String Interpolation
NEW StringBuilder
DUP
INVOKESPECIAL StringBuilder.<init> ()V
LDC "i = "
INVOKEVIRTUAL StringBuilder.append (LString;)LStringBuilder;
GETSTATIC ExtKt.i : I
INVOKEVIRTUAL StringBuilder.append (I)LStringBuilder;
INVOKEVIRTUAL StringBuilder.toString ()LString;

Classes
class Speaker(val talk: String)

Classes
public final class Speaker {
private final String; talk
@NotNull;() // invisible
public final getTalk()String;
GETFIELD Speaker.talk : String;
ARETURN
public <init>(String;)V
…
PUTFIELD Speaker.talk : String;
}

Classes
class Speaker(@JvmField val talk: String)

Classes
public final class Speaker {
public final String; talk
@JvmField;() // invisible
public <init>(String;)V { … }
}

Nulls
fun len(str: String) = str.length
fun maybeLen(str: String?) = str?.length ?: 0

No-Nulls
public final static len(String;)I
@NotNull;() // invisible, parameter 0
LDC "str"
INVOKESTATIC Intrinsics.checkParameterIsNotNull (Object;String;)V
INVOKEVIRTUAL String.length ()I
IRETURN
public final static maybeLen(String;)I
@Nullable;() // invisible, parameter 0
IFNULL L1
INVOKEVIRTUAL java/lang/String.length ()I
GOTO L2
ICONST_0
L2
IRETURN

Extension Functions
fun Context.color(@ColorRes color: Int) =
ContextCompat.getColor(this, color)

Extension Functions
public final static color(Context;I)I
@ColorRes;() // invisible, parameter 1
ALOAD 0
LDC "$receiver"
INVOKESTATIC checkParameterIsNotNull (Object; String;)V
ALOAD 0
ILOAD 1
INVOKESTATIC ContextCompat.getColor (Context;I)I
IRETURN

Extension Functions
fun Context.color(@ColorRes color: Int) =
ContextCompat.getColor(this, color)
// sort of becomes
fun color($receiver: Context, @ColorRes color: Int) =
ContextCompat.getColor($receiver, color)

Inline fun
inline fun inlineFun(f: () -> Unit) {
does()
f()
dat()
}
inlineFun { dis() }

Inline fun
public final inlineFun(Function0;)V
INVOKEVIRTUAL InlineFuns.does ()V
ALOAD 1
INVOKEINTERFACE Function0.invoke ()
INVOKEVIRTUAL InlineFuns.dat ()V
RETURN

Inline fun
inlineFun { dis() }
INVOKEVIRTUAL InlineFuns.does ()V
INVOKEVIRTUAL InlineFuns.dis ()V
INVOKEVIRTUAL InlineFuns.dat ()V

Collection Map, Filter &
Sum

Collection Map, Filter & Sum
class TopicFragment {
fun twoX(x: Int) = x * 2
val numbers = arrayOf(0, 1, 2, 3, 4, 5)
numbers.map(::twoX)
}

BIPUSH 10
ANEWARRAY Integer
INVOKESTATIC Integer.valueOf (I)Integer;
…
CHECKCAST [LObject;
NEW ArrayList
ARRAYLENGTH
INVOKESPECIAL ArrayList.<init> (I)V
CHECKCAST Collection
ARRAYLENGTH
IF_ICMPGE L9
CHECKCAST Number
INVOKEVIRTUAL Number.intValue ()I
CHECKCAST TopicFragment
INVOKESPECIAL TopicFragment.twoX (I)I
INVOKEINTERFACE Collection.add (LObject;)GOTO L8
CHECKCAST List

Integer[] numbers = new Integer[]{0, 1, 2, 3, 4, 5};
Object[] $receiver$iv = (Object[])numbers;
TopicFragment var3 = this;
Object[] $receiver$iv$iv = $receiver$iv;
Collection destination$iv$iv = (Collection)(
new ArrayList($receiver$iv.length)
);
for(int var6 = 0; var6 < $receiver$iv$iv.length; ++var6) {
Object item$iv$iv = $receiver$iv$iv[var6];
int p1 = ((Number)item$iv$iv).intValue();
Integer var13 = ((TopicFragment)var3).twoX(p1);
destination$iv$iv.add(var13);
}
List var10000 = (List)destination$iv$iv;

val numbers = intArrayOf(0, 1, 2, 3, 4, 5)

int[] numbers = new int[]{0, 1, 2, 3, 4, 5};
TopicFragment var3 = this;
int[] $receiver$iv$iv = numbers;
Collection destination$iv$iv = (Collection)(
new ArrayList(numbers.length));
int item$iv$iv = $receiver$iv$iv[var6];
Integer var13 = ((TopicFragment)var3).twoX(item$iv$iv);
}

intArrayOf(0, 1, 2, 3, 4, 5)
.filter(::isEven)
.map(::twoX)

int[] $receiver$iv = new int[]{0, 1, 2, 3, 4, 5};
// filter(::isEven)
int element$iv$iv = $receiver$iv$iv[var5];
if (((TopicFragment)var2).isEven(element$iv$iv)) {
destination$iv$iv.add(element$iv$iv);
}
}
// map(::twoX)
while(var14.hasNext()) {
Object item$iv$iv = var14.next();
int p1 = ((Number)item$iv$iv).intValue();
Integer var12 = ((TopicFragment)var2).twoX(p1);
}

intArrayOf(0, 1, 2, 3, 4, 5)
.asSequence()
.filter(::isEven)
.map(::twoX)
.toList()

final class TopicFragment$1
extends FunctionReference implements Function1 {
public synthetic bridge invoke(Object;)
public final invoke(I)Z
public final getOwner()KDeclarationContainer;
public final getName()String;
public final getSignature()String;
}

intArrayOf(0, 1, 2, 3, 4, 5)
.asSequence()
.filter { it % 2 == 0 }
.map { it * 2}
.toList()

INVOKESTATIC ArraysKt.asSequence ([I)LSequence;
GETSTATIC TopicFragment$1.INSTANCE : TopicFragment$1;
INVOKESTATIC SequencesKt.filter (LSequence;Function1;)
GETSTATIC TopicFragment$2.INSTANCE : TopicFragment$2;
INVOKESTATIC SequencesKt.map (LSequence;Function1;)
INVOKESTATIC SequencesKt.toList (LSequence;)List;

Data Classes
data class Speaker(
val description: String,
val talks: List<Talk>,
val profile: String
)

De-structure data class
val speaker = Speaker(
talks = arrayListOf(),
description = "Bytecode",
profile = ""
)
val (description, talks) = speaker

public final component1()String;
ALOAD 0
GETFIELD Speaker.description : String;
ARETURN
// signature ()LList<LTalk;>;
// declaration: java.util.List<Talk> component2()
public final component2()LList;
ALOAD 0
GETFIELD Speaker.talks : LList;
ARETURN

// signature …
// declaration: Speaker copy(java.lang.String, java.util.List<? extends Talk>)
public final copy(String;LList;)LSpeaker;
INVOKESTATIC checkParameterIsNotNull (Object;String;)V
INVOKESTATIC checkParameterIsNotNull (Object;String;)V
NEW Speaker
INVOKESPECIAL Speaker.<init> (String;LList;)V
ARETURN

val speaker2 = speaker.copy(
description = "Second speaker"
)

public static synthetic bridge copy$default(LSpeaker;String;LList;IObject;)LSpeaker;
ILOAD 3
ICONST_1
IAND
IFEQ L0
ALOAD 0
GETFIELD Speaker.description : String;
ASTORE 1
L0
ILOAD 3
ICONST_2
IAND
IFEQ L1
ALOAD 0
GETFIELD Speaker.talks : LList;
ASTORE 2
L1
ALOAD 0
ALOAD 1
ALOAD 2
INVOKEVIRTUAL Speaker.copy (String;LList;)LSpeaker;
ARETURN

public static Speaker copy$default(
Speaker var0,
String var1, List var2, String var3,
int var4, Object var5
) {
if ((var4 & 1) != 0) {
var1 = var0.description;
}
if ((var4 & 2) != 0) {
var2 = var0.talks;
}
if ((var4 & 4) != 0) {
var3 = var0.profile;
}
return var0.copy(var1, var2, var3);
}

Lambdas
— SAM (Single Access Method)
— Non-Capturing Lambdas
— Capturing Lambdas

SAMs
view.setOnClickListener {
print("Hello")
}

SAMs
final class Sam$1 implements View$OnClickListener {
public final onClick(Landroid/view/View;)V
LDC "Hello"
INVOKEVIRTUAL PrintStream.print (Object;)V
RETURN
<init>()V
INVOKESPECIAL Object.<init> ()V
RETURN
public final static Sam$1; INSTANCE
static <clinit>()V
NEW Sam$1
DUP
INVOKESPECIAL Sam$1.<init> ()V
PUTSTATIC Sam$1.INSTANCE : Sam$1;
RETURN

class Lmd {
private val lambda = {
"I'm a lambda"
}
}

public final class Lmd {
// signature Function0<String;>;
// declaration: Function0<java.lang.String>
private final Function0; lambda
public <init>()V
// …
ALOAD 0
GETSTATIC Lmd$lambda$1.INSTANCE : Lmd$lambda$1;
CHECKCAST Function0
PUTFIELD Lmd.lambda : Function0;
RETURN
}

final class Lmd$lambda$1
extends Lambda
implements Function0 {
<init>()V
INVOKESPECIAL Lambda.<init> (I)V
RETURN
public final static Lmd$lambda$1; INSTANCE
static <clinit>()V
NEW Lmd$lambda$1
DUP
INVOKESPECIAL Lmd$lambda$1.<init> ()V
PUTSTATIC Lmd$lambda$1.INSTANCE : Lmd$lambda$1;
RETURN
}

public synthetic bridge invoke()Object;
L0
LINENUMBER 3 L0
ALOAD 0
INVOKEVIRTUAL Lmd$lambda$1.invoke ()String;
ARETURN
// access flags 0x11
public final invoke()String;
L0
LINENUMBER 5 L0
LDC "I'm a lambda"
L1
ARETURN

class Lmd {
"I'm a lambda"
}
val value = lambda()
}

public <init>()V
…
ALOAD 0
GETFIELD Lmd.lambda : Function0;
INVOKEINTERFACE Function0.invoke ()Object;
CHECKCAST String
}
final class public Lmd$lambda$1 {
public synthetic bridge invoke()Object;
Lmd$lambda$1.invoke ()String;
}

Lambda s and Field
class Lmd (
private val speaker: Speaker
) {
speaker.name
}
}

final class Lmd$lambda$1 {
final synthetic Lmd; this$0
GETFIELD Lmd$lambda$1.this$0 : Lmd;
INVOKESTATIC Lmd.access$getSpeaker$p (Lmd;)LSpeaker;
INVOKEVIRTUAL Speaker.getName ()String;
ARETURN
<init>(Lmd;)V
PUTFIELD Lmd$lambda$1.this$0 : Lmd;
INVOKESPECIAL Lambda.<init> (I)V
RETURN
}

public final static synthetic access$getSpeaker$p(Lmd;)Speaker;
ALOAD 0
GETFIELD Lmd.speaker : Speaker;
ARETURN
}

class Lmd (
@JvmField protected
private val speaker: Speaker
){
speaker.name
}
}

class Lmd (
@Suppress("WARNINGS")
@JvmField
protected
private val speaker: Speaker = Speaker("speaker")
){
speaker.name
}
}

final class Lmd$lambda$1 {
GETFIELD Lmd$lambda$1.this$0 : LLmd;
GETFIELD Lmd.speaker : LSpeaker;
INVOKEVIRTUAL Speaker.getName ()String;
ARETURNdas.lambda : Function0;
RETURN
}

File values and
functions

// file name Objects
private val speaker = Speaker()
public final class ObjectsKt {
private final static Speaker; speaker
static <clinit>()V
NEW Speaker
DUP
INVOKESPECIAL Speaker.<init> ()V
PUTSTATIC ObjectsKt.speaker : LSpeaker;
RETURN
}

fun someStr() = "Some String"
// Becomes
public final static someStr() String;
LDC "Some String"
ARETURN
}

private val someStr = "Some String"

private final static String; someStr = "Some String"
static <clinit>()V
LDC "Some String"
PUTSTATIC ObjectsKt.someStr : String;
RETURN
}

private const val someStr = "Some String"

private final static String; someStr = "Some String"
}

Objects
object AnObject

public final class AnObject {
public final static AnObject; INSTANCE
private <init>()V
RETURN
static <clinit>()V
NEW AnObject
DUP
INVOKESPECIAL AnObject.<init> ()V
PUTSTATIC AnObject.INSTANCE : AnObject;
RETURN
}

object AnObject {
val name = "Mitchell"
}

private final static String; name = "Mitchell"
public final getName()String;
GETSTATIC AnObject.name : String;
ARETURN
static <clinit>()V
LDC "Mitchell"
PUTSTATIC AnObject.name : String;
RETURN
}

object AnObject {
const val name = "Mitchell"
}

public final static String; name = "Mitchell"
public final static LAnObject; INSTANCE
private <init>()V
RETURN
static <clinit>()V
NEW AnObject
DUP
INVOKESPECIAL AnObject.<init> ()V
PUTSTATIC AnObject.INSTANCE : LAnObject;

object AnObject {
fun speaker() = Speaker()
}

public final speaker() Speaker;
NEW Speaker
DUP
INVOKESPECIAL Speaker.<init> ()V
ARETURN

object AnObject {
@JvmStatic
fun speaker() = Speaker()
}
// With @JvmStatic
public final static speaker() Speaker;
@JvmStatic;()
}

Companion Object

companion object {
}
}

public final class TopicFragment {
public <init>()V
static <clinit>()V
NEW TopicFragment$Companion
DUP
ACONST_NULL
INVOKESPECIAL TopicFragment$Companion.<init> (
DefaultConstructorMarker;
)V
PUTSTATIC TopicFragment.Companion : TopicFragment$Companion;
RETURN
public final static TopicFragment$Companion; Companion
public final static INNERCLASS
TopicFragment$Companion TopicFragment Companion
}

public final class TopicFragment$Companion {
private <init>()V
RETURN
public synthetic <init>(DefaultConstructorMarker;)V
INVOKESPECIAL TopicFragment$Companion.<init> ()V
RETURN
public final static INNERCLASS
TopicFragment$Companion TopicFragment Companion
}

companion object {
val EXTRA_TOPIC_ID = "EXTRA_TOPIC_ID"
}
}

init {
EXTRA_TOPIC_ID
}
companion object {
val EXTRA_TOPIC_ID = "EXTRA_TOPIC_ID"
}
}

private final static String; EXTRA_TOPIC_ID =
"EXTRA_TOPIC_ID"
static <clinit>()V
NEW TopicFragment$Companion
…
LDC "EXTRA_TOPIC_ID"
PUTSTATIC TopicFragment
.EXTRA_TOPIC_ID : String;
RETURN
}

public <init>()V
GETSTATIC Companion : $Companion;
INVOKEVIRTUAL $Companion
.getEXTRA_TOPIC_ID ()String;
POP
…
}

public final class TopicFragment$Companion {
public final getEXTRA_TOPIC_ID()String;
L0
LINENUMBER 11 L0
INVOKESTATIC TopicFragment.access$getEXTRA_TOPIC_ID$cp ()String;
ARETURN
}
public final static synthetic access$getEXTRA_TOPIC_ID$cp()String;
GETSTATIC TopicFragment.EXTRA_TOPIC_ID : String;
ARETURN

init {
EXTRA_TOPIC_ID
}
companion object {
const val EXTRA_TOPIC_ID = "EXTRA_TOPIC_ID"
}
}

public <init>()V
// Now all we get is metadata
LINENUMBER 6 L1
RETURN
public final static String; EXTRA_TOPIC_ID =
"EXTRA_TOPIC_ID"

package io.github.droidkaigi.confsched2018.data.api.response.mapper
class LocalDateTimeAdapter : JsonAdapter<LocalDateTime>() {
companion object {
private val FORMATTER: DateTimeFormatter =
DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss")
fun parseDateString(dateString: String?): LocalDateTime =
LocalDateTime.parse(dateString, FORMATTER)
}
}

private object FormatCompanion {
@JvmField
val FORMATTER: DateTimeFormatter =
DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss")
}
class LocalDateTimeAdapter : JsonAdapter<LocalDateTime>() {
…
companion object {
fun parseDateString(dateString: String?): LocalDateTime =
LocalDateTime.parse(dateString, FORMATTER)
}
}

/* package private */ final class FormatCompanion {
public final static DateTimeFormatter; FORMATTER
}

companion object {
private const val EXTRA_TOPIC_ID = "EXTRA_TOPIC_ID"
fun newInstance(topicId: Int) {
}
}
}

Reducing Synthetic Methods
class TopicDetailViewModel
@Inject constructor(
private val repository: SessionRepository,
private val schedulerProvider: SchedulerProvider
) : ViewModel(), LifecycleObserver {
var topicId: Int = 0
val topicSessions: LiveDataSessions by lazy {
repository.topicSessions
.map {
it
.filter { it.key.id == topicId }
.map { it.key to it.value }
.first()
}
.toResult(schedulerProvider)
.toLiveData()
}

public final class TopicDetailViewModel
extends android/arch/lifecycle/ViewModel
implements android/arch/lifecycle/LifecycleObserver {
public final static synthetic access$getRepository
$p(LTopicDetailViewModel;)SessionRepository;
L0
LINENUMBER 20 L0
ALOAD 0
GETFIELD TopicDetailViewModel.repository : SessionRepository;
ARETURN
public final static synthetic access$getSchedulerProvider
$p(LTopicDetailViewModel;)SchedulerProvider;
L0
LINENUMBER 20 L0
ALOAD 0
GETFIELD TopicDetailViewModel.schedulerProvider : SchedulerProvider;
ARETURN

private val repository: SessionRepository,
private val schedulerProvider: SchedulerProvider
) : ViewModel(), LifecycleObserver {
var topicId: Int = 0
val topicSessions: LiveDataSessions by lazy {
repository.topicSessions
.map {
it
.filter { it.key.id == topicId }
.map { it.key to it.value }
.first()
}
.toResult(schedulerProvider)
.toLiveData()
}

@JvmField protected
val repository: SessionRepository,
@JvmField protected
val schedulerProvider: SchedulerProvider
)

// protected is effectively private in final classes
@Inject
@Suppress("warnings")
constructor(
@JvmField protected
val repository: SessionRepository,
@JvmField protected
val schedulerProvider: SchedulerProvider
)

Mitchell Tilbrook
@sir_tilbrook
Sydney Tech Talks
Moneytree
Android Developer
moneytree.jp

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