Why does a method parameter cause NotSerializableException with Mockito?

Question

I have some Scala code which runs with spark, but I simplified them:

// Not Serializable
class Config

object FileReader extends FileReader
class FileReader extends Serializable {
  def read(config: Config): String = config.getClass.toString
}

object Task extends Task(FileReader)
class Task(fileReader: FileReader) extends Serializable {
  def execute(config: Config): Unit = {
    fileReader.read(config)
  }
}

Not the Config is not Serializable.

I want to write some tests for them, and the instance of Task need to be Serializable because it may be serialized and send to spark workers.

I use this function to check if an object can be serialized or not:

def checkSerializable(obj: AnyRef, name: String) = {
  println("### checking " + name + ": " + obj.getClass)
  new ObjectOutputStream(new ByteArrayOutputStream()).writeObject(obj)
  println(name + " ok")
}

First let's check if a normal Task instance can be serialized:

val task = new Task(new FileReader)
checkSerializable(task, "no-mockito")

Outputs:

### checking no-mockito: class Task
no-mockito ok

Seems OK.

But I want to mock the FileReader with Mockito, so my code will be:

val fileReader = Mockito.mock(classOf[FileReader])
val config = Mockito.mock(classOf[Config])
Mockito.when(fileReader.read(config)).thenReturn("mocked")
val task = new Task(fileReader)
checkSerializable(task, "with-mockito1")

It reports error that Config is not serializable:

### checking with-mockito1: class Task
java.io.NotSerializableException: Config$$EnhancerByMockitoWithCGLIB$$c7dcb0a5
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1165)
    at java.io.ObjectOutputStream.writeArray(ObjectOutputStream.java:1359)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1155)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1535)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)

This is strange because config is just some method parameters, not class fields!

I modified my code a little bit to make the config serializable:

val fileReader = Mockito.mock(classOf[FileReader])
val config = Mockito.mock(classOf[Config], Mockito.withSettings().serializable())
Mockito.when(fileReader.read(config)).thenReturn("mocked")
val task = new Task(fileReader)
checkSerializable(task, "with-mockito2")

It still fails, with another NotSerializableException:

### checking with-mockito2: class Task
java.io.NotSerializableException: org.mockito.internal.creation.DelegatingMethod
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1165)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1535)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1413)

Not sure why config instance will be included in task when working with Mockito? And how to avoid it?

Demo project: https://github.com/freewind/mockito-serialization-issue, you can just clone it and run demo/MockitoDemo.scala

---

Update:

Another notable thing is: if I remove this line

fileReader.read(config)

from Task, that means the Task will be:

class Task(fileReader: FileReader) extends Serializable {
  def execute(config: Config): Unit = {
    // removed this line: fileReader.read(config)
  }
}

no NotSerializableException will be thrown anymore (I didn't change the testing code)

Answer 1

You probably will need to make the FileReader mock serializable too try:

val fileReader = Mockito.mock(classOf[FileReader], Mockito.withSettings().serializable())