How to create a retrofit.Response object during Unit Testing with Retrofit 2

The retrofit.Response class has static factory methods to create instances:

public static <T> Response<T> success(T body) {
  /* ... */
}

public static <T> Response<T> success(T body, com.squareup.okhttp.Response rawResponse) {
  /* ... */
}

public static <T> Response<T> error(int code, ResponseBody body) {
  /* ... */
}

public static <T> Response<T> error(ResponseBody body, com.squareup.okhttp.Response rawResponse) {
  /* ... */
}

For example:

Account account = ...;
retrofit.Response<Account> aResponse = retrofit.Response.success(account);

Or:

retrofit.Response<Account> aResponse = retrofit.Response.error(
  403, 
  ResponseBody.create(
    MediaType.parse("application/json"),
    "{\"key\":[\"somestuff\"]}"
  )
);

Note: In latest Retrofit version (2.7.1) for Kotlin, it recommends to use extension method like this:

Response.error(
  400,
  "{\"key\":[\"somestuff\"]}"
    .toResponseBody("application/json".toMediaTypeOrNull())
)

This falls under Effective Java Item 1: Consider static factory methods instead of constructors.

A Kotlin + Mockito + okhttp3 example using Response.Builder

val mockResponse: Response<MyResponseReturnType> =
            Response.success(mock<MyResponseReturnType>(),
                    okhttp3.Response.Builder()
                            .code(200)
                            .message("Response.success()")
                            .protocol(Protocol.HTTP_1_1)
                            .request(Request.Builder().url("http://test-url/").build())
                            .receivedResponseAtMillis(1619053449513)
                            .sentRequestAtMillis(1619053443814)
                            .build())

Heres how to mock just the retrofit responses

First you need to add these dependencies in build.gradle:

// mock websever for testing retrofit responses
testImplementation "com.squareup.okhttp3:mockwebserver:4.6.0"
testImplementation "com.nhaarman.mockitokotlin2:mockito-kotlin:2.2.0"

Mock a successful 200 response:

val mockResponseBody = Mockito.mock(MoviesResponse::class.java)
val mockResponse = Response.success(mockResponseBody) 

Mock an unsuccessful response (eg 400, 401, 404):

val errorResponse = 
    "{\n" +
    "  \"type\": \"error\",\n" +
    "  \"message\": \"What you were looking for isn't here.\"\n"
    + "}"
val errorResponseBody = errorResponse.toResponseBody("application/json".toMediaTypeOrNull())
val mockResponse = Response.error<String>(400, errorResponseBody)

No need to create a mock webserver and all that extra work.