ButterUser Guide


Butter is a library that helps ease HTTP requests through a stack of middleware.

Butter is a replacement for web::WebClient providing an extensible chain of middleware for making repeated HTTP requests and processing the responses. The adoption of the Middleware pattern allows you to seamlessly enhance and modify the behaviour of your HTTP requests.

Features above and beyond Fantom's default WebClient:

  • Instances may be re-used for multiple requests
  • Automatic support for Cookies and sessions
  • Throws Err on 40x & 50x response codes (configurable)
  • Utility classes for commonly used HTTP headers
  • Proper support for 303 redirects

Butter was inspired by Ruby's Faraday library.

Quick Start

  1. Create a text file called Example.fan
    using afButter
    class Example {
        Void main() {
            butter   := Butter.churnOut()
            response := butter.get(`http://www.fantomfactory.org/`)
  2. Run Example.fan as a Fantom script from the command line:
    C:\> fan Example.fan
    <!DOCTYPE html>
            <title>Home :: Fantom-Factory</title>


An instance of Butter wraps a stack of Middleware classes. When a HTTP request is made through Butter, each piece of middleware is called in turn. Middleware classes may either pass the request on to the next piece of middleware, or return a response. At each step, the middleware classes have the option of modifying the request and / or response objects.

The ordering of the middleware stack is important.

The last piece of middleware MUST return a response. These middleware classes are called Terminators. The default terminator is the HttpTerminator which makes an actual HTTP request to the interweb. (When testing this could be substituted with a mock terminator that returns mocked / canned responses.)

To create a Butter instance, call the static Butter.churnOut() method, optionally passing in a custom list of middleware:

middlewareStack := [

butter := Butter.churnOut(middlewareStack)

Or to use the default stack of middleware bundled with Butter, just churn and go:

html := Butter.churnOut.get(`http://www.fantomfactory.org/`).body.str

Butter Dishes

Because functionality is encapsulated in the middleware, you need to access these classes to configure them. Use the Butter.findMiddleware() method to do this:

butter := Butter.churnOut()
((FollowRedriectsMiddleware) butter.findMiddleware(FollowRedriectsMiddleware#)).enabled = false
((ErrOn5xxMiddleware) butter.findMiddleware(ErrOn5xxMiddleware#)).enabled = false

As you can see, this code is quite verbose. To combat this, there are two alternative means of getting hold of middleware:

Dynamic Stylie

If you make dynamic invocation method calls on the Butter class, you can retrieve instances of middleware. The dynamic methods have the same simple name as the middleware type. If the type name ends with Middleware, it may be omitted. Example:

butter := Butter.churnOut()
butter->followRedriects->enabled = true
butter->errOn5xx->enabled = true

Should instances of the same middleware class be in the stack more than once (or should it contain 2 middleware classes with the same name from different pods) then the just first one is returned.

Obviously, dynamic invocation should be used with caution.

Static Stylie

To call the middleware in a statically typed fashion, use a ButterDish class that holds your Butter instance and contains helper methods. There is a default ButterDish class with methods to access middleware in the default stack. Example:

butter := ButterDish(Butter.churnOut())
butter.followRedirects.enabled = true
butter.errOn5xx.enabled = true

When using other middleware, you are encouraged to create your own ButterDish that extends the default one.

Handling 404 and other Status Codes

If a 404, or a 4xx, status code is returned from a request then, by default, a BadStatusErr is thrown. The same goes for 500 and 5xx status codes. In general this is what you want, a fail fast approach to erroneous status codes. But during testing it is often desirable to disable the errors and check / verify the status codes yourself. To do so, just disable the required middleware:

using afButter

class TestStatusCodes {
    Void test404() {
        butter := ButterDish(Butter.churnOut())
        butter.errOn4xx.enabled = false
        res := butter.get(`http://www.google.com/404`)
        verifyEq(res.statusCode, 404)

    Void test500() {
        butter := ButterDish(Butter.churnOut())
        butter.errOn5xx.enabled = false
        res := butter.get(`http://www.example.com/500`)  // insert failing URL here
        verifyEq(res.statusCode, 500)

Calling RESTful Services

Butter has some convenience methods for calling RESTful services.


For a simple GET request:

butter   := ButterDish(Butter.churnOut())
response := butter.get(`http://example.org/`)


To send a POST request:

butter   := ButterDish(Butter.churnOut())
jsonObj  := ["wot" : "ever"]
response := butter.postJsonObj(`http://example.org/`, jsonObj)


To send a PUT request:

butter   := ButterDish(Butter.churnOut())
jsonObj  := ["wot" : "ever"]
response := butter.putJsonObj(`http://example.org/`, jsonObj)


To send a DELETE request:

butter   := ButterDish(Butter.churnOut())
response := butter.delete(`http://example.org/`)


For complete control over the HTTP requests, create a ButterRequest and set the headers and the body yourself:

butter   := ButterDish(Butter.churnOut())
request  := ButterRequest(`http://example.org/`) {
    it.method = "POST"
    it.headers.contentType = MimeType("application/json")
    it.body.str = """ {"wot" : "ever"} """
response := butter.sendRequest(req)


Butter can log all requests and responses to std out. To use, set the Butter logger to DEBUG.

For example:

Log.get("afButter").level = LogLevel.debug

Will produce the following output:

[debug] [afButter]

Butter Request:
GET http://www.fantomfactory.org/ HTTP/1.1
Host: www.fantomfactory.org
Accept-Encoding: gzip

[debug] [afButter]

Butter Response:
HTTP/1.1 200 OK
Date: Thu, 07 Apr 2016 15:49:57 GMT
Server: Wisp/1.0.67
Content-Length: 6604
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Type: text/html; charset=utf-8
Cache-Control: max-age=0, no-cache

<!DOCTYPE html>
    <title>Home :: Fantom-Factory</title>
    <meta charset="utf-8">
    <meta id="viewport" name="viewport" content="initial-scale=1.0">