KotlinでmockStaticを使おうとしてちょっとハマった話

kotlin 書いてて、mockito-inline の mockStatic を使おうとして
少し時間無駄にしたので、他の人が同じことで時間を無駄にしないように書いとく

tl;dr

@JvmStatic のAnnotationをつける必要がある
普通に考えれば当たり前なんだけど、つけ忘れてた時にエラー出てなんでだっけ？ってなる

object ExampleObject {
    @JvmStatic //これ
    fun static(): String {
        return "static"
    }
}

なぜか

じゃないとバイトコードになった時に static にならないので結果としてmock出来ずに失敗する

// ================com/example/springkotlin/utils/ExampleObject.class =================
// class version 52.0 (52)
// access flags 0x31
public final class com/example/springkotlin/utils/ExampleObject {


  // access flags 0x19
  public final static static()Ljava/lang/String;
  @Lkotlin/jvm/JvmStatic;()
  @Lorg/jetbrains/annotations/NotNull;() // invisible
   L0
    LINENUMBER 6 L0
    LDC "static"
    ARETURN
   L1
    MAXSTACK = 1
    MAXLOCALS = 0

  // access flags 0x11
  public final notStatic()Ljava/lang/String;
  @Lorg/jetbrains/annotations/NotNull;() // invisible
   L0
    LINENUMBER 10 L0
    LDC "notStatic"
    ARETURN
   L1
    LOCALVARIABLE this Lcom/example/springkotlin/utils/ExampleObject; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x2
  private <init>()V
   L0
    LINENUMBER 3 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this Lcom/example/springkotlin/utils/ExampleObject; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x19
  public final static Lcom/example/springkotlin/utils/ExampleObject; INSTANCE
  @Lorg/jetbrains/annotations/NotNull;() // invisible

  // access flags 0x8
  static <clinit>()V
   L0
    LINENUMBER 3 L0
    NEW com/example/springkotlin/utils/ExampleObject
    DUP
    INVOKESPECIAL com/example/springkotlin/utils/ExampleObject.<init> ()V
    ASTORE 0
    ALOAD 0
    PUTSTATIC com/example/springkotlin/utils/ExampleObject.INSTANCE : Lcom/example/springkotlin/utils/ExampleObject;
    RETURN
    MAXSTACK = 2
    MAXLOCALS = 1

  @Lkotlin/Metadata;(mv={1, 6, 0}, k=1, d1={"\u0000\u0014\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\u0008\u0002\n\u0002\u0010\u000e\n\u0002\u0008\u0002\u0008\u00c6\u0002\u0018\u00002\u00020\u0001B\u0007\u0008\u0002\u00a2\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0004J\u0008\u0010\u0005\u001a\u00020\u0004H\u0007\u00a8\u0006\u0006"}, d2={"Lcom/example/springkotlin/utils/ExampleObject;", "", "()V", "notStatic", "", "static", "spring-kotlin"})
  // compiled from: ExampleObject.kt
}

エラー

エラーとしてはこんなのが出るけどぱっと見ですぐに気がつきにくい

when() requires an argument which has to be 'a method call on a mock'.
For example:
    when(mock.getArticles()).thenReturn(articles);

Also, this error might show up because:
1. you stub either of: final/private/equals()/hashCode() methods.
   Those methods *cannot* be stubbed/verified.
   Mocking methods declared on non-public parent classes is not supported.
2. inside when() you don't call method on mock but on some other object.

org.mockito.exceptions.misusing.MissingMethodInvocationException: 
when() requires an argument which has to be 'a method call on a mock'.
For example:
    when(mock.getArticles()).thenReturn(articles);

Also, this error might show up because:
1. you stub either of: final/private/equals()/hashCode() methods.
   Those methods *cannot* be stubbed/verified.
   Mocking methods declared on non-public parent classes is not supported.
2. inside when() you don't call method on mock but on some other object.

コード

package com.example.springkotlin.utils

object ExampleObject {
    @JvmStatic
    fun static(): String {
        return "static"
    }

    fun notStatic(): String {
        return "notStatic"
    }
}

テスト

package com.example.springkotlin.utils

import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import org.mockito.Mockito.mockStatic

class ExampleObjectTest {
    @Test
    fun test() {
       // 雑
        val mocked = mockStatic(ExampleObject::class.java)
        mocked.`when`<String>(ExampleObject::static).thenReturn("mocked")
        assertEquals("mocked", ExampleObject.static())
        mocked.close()
    }
}