Why is it possible for a JvmStatic method to access and modify state?

Question

I would've assumed that a method annotated with @JvmStatic would be unable to access, let alone modify, a class property.

Based on the following this is not the case. This code clearly updates state, printing From main - 11.

object Foo {
    var bar = 5

    @JvmStatic
    fun updateBar(updateVal : Int) {
        println("bar = $bar")
        bar += updateVal
        println("bar = $bar")
    }
}

fun main() {
    Foo.updateBar(6)
    println("From main - ${Foo.bar}")
}

Answer 1

I was also surprised by this. In fact, because Foo is an object, the field bar is actually a static field on the JVM. That's why it's not a problem to access it from a static method.

The decompiled Java code of your class is the following:

public final class Foo {
   private static int bar;
   public static final Foo INSTANCE;

   public final int getBar() {
      return bar;
   }

   public final void setBar(int var1) {
      bar = var1;
   }

   @JvmStatic
   public static final void updateBar(int updateVal) {
      String var1 = "bar = " + bar;
      boolean var2 = false;
      System.out.println(var1);
      bar += updateVal;
      var1 = "bar = " + bar;
      var2 = false;
      System.out.println(var1);
   }

   private Foo() {
   }

   static {
      Foo var0 = new Foo();
      INSTANCE = var0;
      bar = 5;
   }
}

However, I would avoid doing this. It's unexpected and doesn't really convey the intent.