Mockk framework: Could not initialize class io.mockk.impl.JvmMockKGateway

If you are looking for a mock framework for Kotlin tests, Mockk is definitely on first on the list. I wrote my first test following a simple example on the official website (https://mockk.io/#dsl-examples). In my case, I am testing an export class using a parser to deserialize a file into an object:

@Test
fun `test export excluding sub-folders`() {
    // given
    val path = "my/new/path"
    val parser = mockk()
    every { parser.buildObject("$path/data.xml") } returns MyObject("first name", "last name", "title", "manager")

    // when
    var myObject = exporter.parse(
            parser,
            getResourceFile(path),
            mutableListOf()
        )

    // then
    verify { parser.buildObject("$path/data.xml") }
    confirmVerified(parser) // verifies all the verify calls were made
    assertEquals("first name", myObject.firstName)
    assertEquals("last name", myObject.lastName)
    assertEquals("title", myObject.title)
    assertEquals("manager", myObject.manager)

}

Wunderbar! I ran my test… and it failed:

java.lang.NoClassDefFoundError: Could not initialize class io.mockk.impl.JvmMockKGateway

What in the world is that?! I checked my pom file and verified I had the latest version:

<dependency>
  <groupId>io.mockk</groupId>
  <artifactId>mockk</artifactId>
  <version>1.9.3</version>
  <scope>test</scope>
</dependency>

After a while, I figured out the issue resides in the IDE (https://github.com/mockk/mockk/issues/254). Mockk has issue when running on JDK 11. I went to Intellij IDEA -> File -> Project Structure, and changed the Project SDK to JDK 8 (which I use for my current project). You can probably find another version that would work for your own project.

Browsing folders with Kotlin

Kotlin offers significant improvements to Java. But coming from a Java background, it can sometimes get tricky. I was looking at browsing folders and I found Kotlin’s method walkTopDown:

fun File.walkTopDown(): FileTreeWalk

I used it recursively to find pom.xml files.

fun findPomFiles(
        folder: File,
        pomTrees: MutableList,
        includeDependencyManagement: Boolean,
        searchSubFolders: Boolean
    ): MutableList {

        // find pom file in the current folder
        val pomFile = folder.listFiles().find { it.name == "pom.xml" }

        pomFile?.let {
            println("Found pom file in $folder")
            pomTrees.add(PomNode(pomParser.deserialize(it), includeDependencyManagement))
        }

        if (searchSubFolders) {
            folder.walkTopDown() // HERE IS THE METHOD
                .filter { it.isDirectory && it != folder && it.name != "target" && it.name != "src"} // ignore src and target folders for performance
                .forEach {
                    findPomFiles(it, pomTrees, includeDependencyManagement, searchSubFolders)
                }
        }

        return pomTrees
    }

I also found the function onEnter of FileTreeWalk that can be used to avoid exploring useless branches.

fun onEnter(function: (File) -> Boolean): FileTreeWalk

folder.walkTopDown() // HERE IS THE METHOD
    .onEnter { it.name != "target" && it.name != "src"}

However, the result was not what I expected. The program was looking at the folders multiple times. I realize walkTopDown is not made for exploring folders recursively.
What it actually does is to browse folders and for each folder, we can do whatever we need to.

So two possibilities:

A. keeping the code recursive and using listFiles() from File – like we used to do it in Java

    if (searchSubFolders) {
        folder.listFiles()
            .filter { it.isDirectory && it != folder && it.name != "target" && it.name != "src"} // ignore src and target folders for performance
            .forEach {
                findPomFiles(it, pomTrees, includeDependencyManagement, searchSubFolders)
            }
    }

B. using walkTopDown

fun findPomFiles(
        folder: File,
        pomTrees: MutableList,
        includeDependencyManagement: Boolean,
        searchSubFolders: Boolean
    ): MutableList {

        // find pom file in the current folder
        addPomNode(folder, includeDependencyManagement)?.let { pomTrees.add(it) }

        if (searchSubFolders) {
            folder.walkTopDown()
                .onEnter { it.name != "target" && it.name != "src" }
                .forEach {
                    addPomNode(it, includeDependencyManagement)?.let { pomNode -> pomTrees.add(pomNode) }
                }
        }

        return pomTrees
    }

    fun addPomNode(folder: File, includeDependencyManagement: Boolean): PomNode? {
        val pomFile = folder.listFiles()?.find { it.name == "pom.xml" }

        pomFile?.let {
            println("Found pom file in $folder")
            return PomNode(pomParser.deserialize(it), includeDependencyManagement)
        }

        return null
    }