Why Kotlin decompiler generates null.INSTANCE

Question

I am trying to understand few of the Kotlin features by checking how it looks in Java side.

So as an experiment, I tried with this:

val printKotlin = fun () {
    print("Hello Kotlin")
}

So the output for the above snippet is:

public final class FunAsVariableKt {
    private static final Function0 printKotlin;

    public static final Function0 getPrintKotlin() {
        return printKotlin;
    }

    static {
        printKotlin = (Function0)null.INSTANCE;
    }
}

How do I understand the static block of the above decompiled code? Why it is generating this non working code?

Answer 1

Use the Kotlin bytecode inspector to see what is being generated as JVM bytecode rather than trying to decompile bytecode to Java, which likely won't work for code generated by a different language compiler because it might not follow the expected patterns. A bytecode decompiler does not always generate working code.

JVM bytecode != Java language specific

Kotlin generated JVM bytecode != Java generated JVM bytecode

The NULL instance you are seeing is a misinterpretation by your decompiler, and you can see all references to INSTANCE in the bytecode are setting values correctly.

This is the actual output:

// ================Stackoverflow_53384931Kt.class =================
// class version 52.0 (52)
// access flags 0x31
public final class Stackoverflow_53384931Kt {


  // access flags 0x1A
  // signature Lkotlin/jvm/functions/Function0<Lkotlin/Unit;>;
  // declaration: kotlin.jvm.functions.Function0<kotlin.Unit>
  private final static Lkotlin/jvm/functions/Function0; printKotlin
  @Lorg/jetbrains/annotations/NotNull;() // invisible

  // access flags 0x19
  // signature ()Lkotlin/jvm/functions/Function0<Lkotlin/Unit;>;
  // declaration: kotlin.jvm.functions.Function0<kotlin.Unit> getPrintKotlin()
  public final static getPrintKotlin()Lkotlin/jvm/functions/Function0;
  @Lorg/jetbrains/annotations/NotNull;() // invisible
   L0
    LINENUMBER 3 L0
    GETSTATIC Stackoverflow_53384931Kt.printKotlin : Lkotlin/jvm/functions/Function0;
    ARETURN
   L1
    MAXSTACK = 1
    MAXLOCALS = 0

  // access flags 0x8
  static <clinit>()V
   L0
    LINENUMBER 3 L0
    GETSTATIC Stackoverflow_53384931Kt$printKotlin$1.INSTANCE : LStackoverflow_53384931Kt$printKotlin$1;
    CHECKCAST kotlin/jvm/functions/Function0
    PUTSTATIC Stackoverflow_53384931Kt.printKotlin : Lkotlin/jvm/functions/Function0;
    RETURN
    MAXSTACK = 1
    MAXLOCALS = 0

  @Lkotlin/Metadata; ... 
  // access flags 0x18
  final static INNERCLASS Stackoverflow_53384931Kt$printKotlin$1 null null
  // compiled from: stackoverflow-53384931.kt
}


// ================Stackoverflow_53384931Kt$printKotlin$1.class =================
// class version 52.0 (52)
// access flags 0x30
// signature Lkotlin/jvm/internal/Lambda;Lkotlin/jvm/functions/Function0<Lkotlin/Unit;>;
// declaration: Stackoverflow_53384931Kt$printKotlin$1 extends kotlin.jvm.internal.Lambda implements kotlin.jvm.functions.Function0<kotlin.Unit>
final class Stackoverflow_53384931Kt$printKotlin$1 extends kotlin/jvm/internal/Lambda  implements kotlin/jvm/functions/Function0  {


  // access flags 0x1041
  public synthetic bridge invoke()Ljava/lang/Object;
    ALOAD 0
    INVOKEVIRTUAL Stackoverflow_53384931Kt$printKotlin$1.invoke ()V
    GETSTATIC kotlin/Unit.INSTANCE : Lkotlin/Unit;
    ARETURN
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x11
  public final invoke()V
   L0
    LINENUMBER 4 L0
    LDC "Hello Kotlin"
    ASTORE 1
   L1
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 1
    INVOKEVIRTUAL java/io/PrintStream.print (Ljava/lang/Object;)V
   L2
   L3
    LINENUMBER 5 L3
    RETURN
   L4
    LOCALVARIABLE this LStackoverflow_53384931Kt$printKotlin$1; L0 L4 0
    MAXSTACK = 2
    MAXLOCALS = 2

  // access flags 0x0
  <init>()V
    ALOAD 0
    ICONST_0
    INVOKESPECIAL kotlin/jvm/internal/Lambda.<init> (I)V
    RETURN
    MAXSTACK = 2
    MAXLOCALS = 1

  // access flags 0x19
  public final static LStackoverflow_53384931Kt$printKotlin$1; INSTANCE

  // access flags 0x8
  static <clinit>()V
    NEW Stackoverflow_53384931Kt$printKotlin$1
    DUP
    INVOKESPECIAL Stackoverflow_53384931Kt$printKotlin$1.<init> ()V
    PUTSTATIC Stackoverflow_53384931Kt$printKotlin$1.INSTANCE : LStackoverflow_53384931Kt$printKotlin$1;
    RETURN
    MAXSTACK = 2
    MAXLOCALS = 0

  @Lkotlin/Metadata; ...
  OUTERCLASS Stackoverflow_53384931Kt null
  // access flags 0x18
  final static INNERCLASS Stackoverflow_53384931Kt$printKotlin$1 null null
  // compiled from: stackoverflow-53384931.kt
  // debug info: SMAP
  ...

}

Answer 2

You can use JD-GUI for this.

http://java-decompiler.github.io/

This worked better for me as compared to the inbuilt IntelliJ plugin for decompilation. You just need to feed it your *.class files.

Kotlin Code

//TryingOutLamdas.kt

class TryingOutLambdas {

    public fun performSomething()
    {
        takingAOnclickListener { x -> print("Hello $x").toString() }
    }
    
    fun takingAOnclickListener(onClickListener: (Int) -> (String))
    {
        onClickListener.invoke(6)
    }

} 

Converted bytecode for this using JD-GUI.

public final class TryingOutLambdas {


    public final void performSomething() {
        
       takingAOnclickListener(TryingOutLambdas$performSomething$1
        .INSTANCE);
    }

    public final void takingAOnclickListener(@NotNull Function1 onClickListener) {
        Intrinsics.checkParameterIsNotNull(onClickListener, "onClickListener");
        onClickListener.invoke(Integer.valueOf(6));
    }

    static final class TryingOutLambdas$performSomething$1 extends Lambda implements Function1<Integer, String> {
        public static final TryingOutLambdas$performSomething$1 INSTANCE = new TryingOutLambdas$performSomething$1();
        
        @NotNull
        public final String invoke(int x) {
          String str = "Hello " + x;
          boolean bool = false;
          System.out.print(str);
          return Unit.INSTANCE.toString();
        }
        
        TryingOutLambdas$performSomething$1() {
          super(1);
        }
    } 

} 

In case a lambda expression accesses any variables from the function it is declared, a new object implementing that interface will be created instead of creating a Singleton instance created above.