Scala中Mock第三方Object出现Cannot mock/spy class PQR错误的解决方法

Scala类调用第三方库的object PQR方法，单元测试时用Mockito Mock该方法报错：

Cannot mock/spy class PQR
Mockito cannot mock/spy because :

• final class
  org.mockito.exceptions.base.MockitoException:
  Cannot mock/spy class PQR
  Mockito cannot mock/spy because :
• final class

相关代码：

class XYZ {
  def test() : Unit = {
    val ret = PQR.test2("abc") // 第三方依赖，PQR是object
    // ...其他逻辑
  }
}

class XYZSpec extends AnyFlatSpec with Matchers with MockitoSugar {
  it should "create object" in {
    val pqr = mock[PQR.type]
    when(pqr.test2(any[String])) thenReturn "def"
  }
}

解决方案 #

方法一：依赖注入+封装第三方调用（推荐） #

Scala的object是单例且默认被标记为final类，Mockito无法直接Mock。最合理的做法是对第三方依赖做一层封装，通过依赖注入实现测试时的替换：

1. 定义特质抽象第三方方法：

trait PQRClient {
  def test2(input: String): String
}

2. 实现生产环境的客户端，调用第三方PQR：

class DefaultPQRClient extends PQRClient {
  override def test2(input: String): String = PQR.test2(input)
}

3. 修改XYZ类，注入PQRClient（默认使用生产实现）：

class XYZ(pqrClient: PQRClient = new DefaultPQRClient) {
  def test(): Unit = {
    val ret = pqrClient.test2("abc")
    // ...其他逻辑
  }
}

4. 测试时Mock这个特质：

class XYZSpec extends AnyFlatSpec with Matchers with MockitoSugar {
  it should "execute test logic correctly" in {
    val mockPqrClient = mock[PQRClient]
    when(mockPqrClient.test2(any[String])).thenReturn("def")
    
    val xyz = new XYZ(mockPqrClient)
    xyz.test()
    
    verify(mockPqrClient).test2("abc")
  }
}

方法二：使用PowerMock（侵入性强，不推荐） #

如果无法修改原代码结构，可借助PowerMock绕过final类限制，但它会修改类加载机制，增加测试维护成本：

1. 引入PowerMock依赖（sbt示例）：

libraryDependencies ++= Seq(
  "org.powermock" % "powermock-module-junit4" % "2.0.9" % Test,
  "org.powermock" % "powermock-api-mockito2" % "2.0.9" % Test
)

2. 编写测试代码：

import org.powermock.api.mockito.PowerMockito
import org.powermock.core.classloader.annotations.PrepareForTest
import org.scalatestplus.junit.JUnitSuite
import org.junit.runner.RunWith
import org.powermock.modules.junit4.PowerMockRunner

@RunWith(classOf[PowerMockRunner])
@PrepareForTest(Array(classOf[PQR]))
class XYZSpec extends JUnitSuite with Matchers with MockitoSugar {
  it should "mock PQR object" in {
    PowerMockito.mockStatic(classOf[PQR])
    when(PQR.test2(any[String])).thenReturn("def")
    
    val xyz = new XYZ()
    xyz.test()
    
    PowerMockito.verifyStatic(classOf[PQR])
    PQR.test2("abc")
  }
}

方法三：用ScalaMock替代Mockito #

ScalaMock对Scala语言特性（如object、特质）支持更友好，可直接Mock单例对象：

1. 引入ScalaMock依赖（sbt示例）：

libraryDependencies += "org.scalamock" %% "scalamock" % "5.2.0" % Test

2. 编写测试代码：

import org.scalamock.scalatest.MockFactory

class XYZSpec extends AnyFlatSpec with Matchers with MockFactory {
  it should "mock PQR object correctly" in {
    val mockPQR = mockObject[PQR]
    (mockPQR.test2 _).expects("abc").returning("def")
    
    val xyz = new XYZ()
    xyz.test()
  }
}

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