java.lang.NullPointerException when upgraded to kotlin 2.0

[hanrw]

gradle config

plugins {
    id("org.springframework.boot") version "3.3.0"
    id("io.spring.dependency-management") version "1.1.5"
    kotlin("jvm") version "2.0.0"
    kotlin("plugin.spring") version "2.0.0"
}

group = "com.kotlin.npe"
version = "0.0.1-SNAPSHOT"

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(21)
    }
}

repositories {
    mavenLocal()

    mavenCentral()
}

dependencies {
    implementation("org.springframework.boot:spring-boot-starter")
    implementation("org.springframework.boot:spring-boot-starter-data-jpa")
    testImplementation("org.springframework.boot:spring-boot-starter-test")
    testImplementation("org.jetbrains.kotlin:kotlin-test-junit5")
    testImplementation("org.mockito.kotlin:mockito-kotlin:5.3.1")
    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
}

kotlin {
    compilerOptions {
        freeCompilerArgs.addAll("-Xjsr305=strict")
    }
}

tasks.withType<Test> {
    useJUnitPlatform()
}

test

@ExtendWith(MockitoExtension::class)
class UserStatusServiceTest {
    @InjectMocks
    lateinit var target: UserStatusServiceImpl

    @Mock
    lateinit var userStatusRepository: UserStatusRepository

    @Test
    fun addStatus() {
        whenever(
            userStatusRepository.findByUserId(
                1L,
            )
        ).thenReturn(
            Optional.empty()
        )

        target.addStatus(1L)

        val suspension = UserStatus()
        verify(userStatusRepository).save(suspension)
    }
}

service

interface UserStatusService {
    fun addStatus(userId: Long)
}

class UserStatusServiceImpl(private val userStatusRepository: UserStatusRepository) :
    UserStatusService {
    override fun addStatus(userId: Long) {
        val optional = userStatusRepository.findByUserId(userId)
        if (optional.isEmpty) {
            println("[userId: $userId] adding a new status")

            userStatusRepository.save(UserStatus())
        } else {
            println("[userId: $userId] status already exists")
        }
    }
}

repository

@Repository
interface UserStatusRepository : JpaRepository<UserStatus, Long>,
    JpaSpecificationExecutor<UserStatus> {
    fun findByUserId(userId: Long): Optional<UserStatus>
}

data class UserStatus(
    val userId: Long = 1L
)

and got error

WARNING: If a serviceability tool is in use, please run with -XX:+EnableDynamicAgentLoading to hide this warning
WARNING: If a serviceability tool is not in use, please run with -Djdk.instrument.traceUsage for more information
WARNING: Dynamic loading of agents will be disallowed by default in a future release
[userId: 1] adding a new status

java.lang.NullPointerException
	at com.kotlin.npe.reporduce.UserStatusServiceImpl.addStatus(UserStatusService.kt:11)
	at com.kotlin.npe.reporduce.UserStatusServiceTest.addStatus(UserStatusServiceTest.kt:31)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)

here's reproducing repo and it works after downgrading to kotlin 1.9.24
archive.zip

[lasficw]

We have the same Problem, is there something we can do about it?

[Vladimir-Buttn]

+1 here

[heliming]

+1

[silviuburceadev]

I am facing the same issue. My setup:

@ExtendWith(MockitoExtension::class)
internal class MyTest {
  @Mock
  private lateinit var myRepository: MyRepository

  @Mock
  private lateinit var myService: MyService

  // depends on myRepository and myService
  @InjectMocks
  private lateinit var myCompositeService: MyCompositeService
}

MyRepository is a org.springframework.data.repository.CrudRepository, no annotations, and MyService is a classic @Service class.

Even though I'm checking that myRepository is not null, both in the test class, before calling the method tested and inside the method, before the repository does a save. And yet, myRepository.save fails with NPE.

Edit:
The root cause for me was that the CrudRepository's save method was not mocked, so by default mockito seem to return null for it, which Kotlin doesn't like, as the method returns a non-null object. The solution was:

`when`(myRepository.save(notNull(MyEntity::class.java))).thenAnswer(AdditionalAnswers.returnsFirstArg<MyEntity>)

This would be nice to be auto-configured, somehow. I am guessing that the Kotlin compiler is able to be more strict in this case, compared to Kotlin 1.9, which was more lenient and the build was passing.

[silviuburceadev]

@hanrw I have tested my finding on your reproducible project. You have a commented-out line there, replace it with:

whenever(userStatusRepository.save(Mockito.notNull(UserStatus::class.java)))
    .thenAnswer(AdditionalAnswers.returnsFirstArg<UserStatus>())

... and it works.