Kotlin snippets for return a canned response from a resource file whenever a URL is request is fired

FakeResourceHttpURLConnection.kt

// Based on https://stackoverflow.com/questions/565535/mocking-a-url-in-java

package com.example

import java.io.InputStream
import java.net.HttpURLConnection
import java.net.URL

class FakeResourceHttpURLConnection(
  url: URL,
  private val resourceFileName: String
) : HttpURLConnection(url) {
  override fun usingProxy(): Boolean {
    return false
  }

  override fun connect() {
  }

  override fun disconnect() {
  }

  override fun getInputStream(): InputStream {
    return resourceToFile(resourceFileName).inputStream()
  }
  
  /** Returns resource file contents instead of hittin the network for the provided URL */
  private inline fun <reified T> T.resourceToFile(resourceName: String) =
      File(T::class.java.classLoader.getResource(resourceName)!!.toURI())
}

HttpUrlStreamHandler.kt

package com.example

import org.mockito.BDDMockito.given
import org.mockito.Mockito
import java.io.IOException
import java.net.URL
import java.net.URLConnection
import java.net.URLStreamHandler
import java.net.URLStreamHandlerFactory

/**
 * [URLStreamHandler] that allows us to control the [URLConnections][URLConnection] that are returned
 * by [URLs][URL] in the code under test.
 * Call [addFakeResponse] to return a canned responce from the provided resource file for the
 * provided [URL].
 */
class HttpUrlStreamHandler private constructor() : URLStreamHandler() {
  private var connections: MutableMap<URL, URLConnection> = mutableMapOf()

  @Throws(IOException::class) override fun openConnection(url: URL): URLConnection? {
    return connections[url]
  }

  fun resetConnections() {
    connections.clear()
  }

  fun addFakeResponse(url: URL, resource: String): HttpUrlStreamHandler {
    connections[url] = FakeResourceHttpURLConnection(url, resource)
    return this
  }

  companion object {
    private val urlStreamHandlerFactory = Mockito.mock(URLStreamHandlerFactory::class.java)
    val INSTANCE = HttpUrlStreamHandler()

    init {
      URL.setURLStreamHandlerFactory(urlStreamHandlerFactory)
      given(urlStreamHandlerFactory.createURLStreamHandler("https")).willReturn(INSTANCE)
      given(urlStreamHandlerFactory.createURLStreamHandler("http")).willReturn(INSTANCE)
    }
  }
}

SampleTest.kt

package com.example

import org.junit.Before
import org.junit.BeforeClass
import org.junit.Test
import java.net.URL

class SampleTest {
  @Before fun setUp() {
    httpUrlStreamHandler.resetConnections()
  }
  
  @Test fun `return canned URL response from resource`() {
    val url = URL("https://some.url.that.com/want/to/mock.html")
    httpUrlStreamHandler.addFakeResponse(url, "my_resource_file.json")
    val stream = url.openConnection() // returns contents of my_resource_file.json
    stream.use {
      // use the stream :)
    }
  }

  companion object {
    private val httpUrlStreamHandler: HttpUrlStreamHandler = HttpUrlStreamHandler.INSTANCE
  }
}