如何在Kotlin中用Mockito mock类私有成员并测试方法调用

方案一：重构代码（推荐） #

直接调整Employee类的设计，让依赖和状态更易于测试，避免依赖反射操作私有变量：

class Employee constructor(
    employeeName: String?,
    // 注入PromotedPosition依赖，测试时可直接传入mock对象
    val promotedPosition: PromotedPosition? = null
) {
    var isEligibleForPromotion = false // 改为可外部访问的变量，或提供setter方法

    init {
        try {
            // 原有的晋升资格检查逻辑
        } catch(e: Exception) {}
    }
    
    fun givePromotion(employeeName: String?) {
        if(isEligibleForPromotion) {
            promotedPosition?.promote(employeeName)
        }
    }
}

对应的JUnit测试用例（基于Mockito + JUnit 5）：

import org.junit.jupiter.api.Test
import org.mockito.Mockito.*

class EmployeeTest {

    @Test
    fun `givePromotion 应在符合资格时调用promote方法`() {
        // 1. Mock PromotedPosition实例
        val mockPromotedPosition = mock(PromotedPosition::class.java)
        // 2. 创建Employee实例并注入mock对象
        val employee = Employee("testName", mockPromotedPosition)
        // 3. 设置晋升资格为true
        employee.isEligibleForPromotion = true

        // 执行目标方法
        employee.givePromotion("testName")

        // 验证promote方法是否被正确调用
        verify(mockPromotedPosition).promote("testName")
    }

    @Test
    fun `givePromotion 不应在不符合资格时调用promote方法`() {
        val mockPromotedPosition = mock(PromotedPosition::class.java)
        val employee = Employee("testName", mockPromotedPosition)
        employee.isEligibleForPromotion = false

        employee.givePromotion("testName")

        // 验证promote方法从未被调用
        verify(mockPromotedPosition, never()).promote(anyString())
    }
}

方案二：不修改原代码，用反射控制私有变量 #

如果无法调整原Employee类的代码，可以通过反射工具修改私有变量的取值：

使用Spring的ReflectionTestUtils工具 #

import org.junit.jupiter.api.Test
import org.mockito.Mockito.*
import org.springframework.test.util.ReflectionTestUtils

class EmployeeTest {

    @Test
    fun `通过反射设置资格为true时，givePromotion应调用promote`() {
        val mockPromotedPosition = mock(PromotedPosition::class.java)
        val employee = Employee("testName")

        // 修改私有变量isEligibleForPromotion为true
        ReflectionTestUtils.setField(employee, "isEligibleForPromotion", true)
        // 为私有变量promotedPosition注入mock对象
        ReflectionTestUtils.setField(employee, "promotedPosition", mockPromotedPosition)

        employee.givePromotion("testName")

        verify(mockPromotedPosition).promote("testName")
    }

    @Test
    fun `通过反射设置资格为false时，givePromotion不应调用promote`() {
        val mockPromotedPosition = mock(PromotedPosition::class.java)
        val employee = Employee("testName")

        ReflectionTestUtils.setField(employee, "isEligibleForPromotion", false)
        ReflectionTestUtils.setField(employee, "promotedPosition", mockPromotedPosition)

        employee.givePromotion("testName")

        verify(mockPromotedPosition, never()).promote(anyString())
    }
}

使用Kotlin原生反射 #

import org.junit.jupiter.api.Test
import org.mockito.Mockito.*
import kotlin.reflect.full.declaredMemberProperties
import kotlin.reflect.jvm.isAccessible

class EmployeeTest {

    @Test
    fun `通过Kotlin反射设置资格为true时，givePromotion应调用promote`() {
        val mockPromotedPosition = mock(PromotedPosition::class.java)
        val employee = Employee("testName")

        // 修改私有变量isEligibleForPromotion的值
        val eligibleProp = Employee::class.declaredMemberProperties.find { it.name == "isEligibleForPromotion" }!!
        eligibleProp.isAccessible = true
        eligibleProp.setter.call(employee, true)

        // 为私有变量promotedPosition注入mock对象
        val positionProp = Employee::class.declaredMemberProperties.find { it.name == "promotedPosition" }!!
        positionProp.isAccessible = true
        positionProp.setter.call(employee, mockPromotedPosition)

        employee.givePromotion("testName")

        verify(mockPromotedPosition).promote("testName")
    }
}

注意事项 #

• 优先选择方案一，重构后的代码更符合面向对象设计原则，测试逻辑清晰且维护成本低。
• 反射方案属于“侵入式”测试，一旦类中私有变量名称修改，测试会直接失败，后续维护成本较高。

内容的提问来源于stack exchange，提问作者Coding_yatra