HTTP Middleware in Go with httptest.ResponseRecorder

A technique for creating “post-process” HTTP middleware in Go.


You need to add a bit of extra “post-process” functionality or logic beyond what an existing http.Handler offers, but don’t have the ability to modify the existing handler, perhaps because it’s provided by a third party package. For example, how might you add an HTTP response header to the handler’s HTTP response before the response is sent to the client?

While the http.Handler wrapper technique is commonly utilized to invoke code before and/or after invoking an http.Handler’s ServeHTTP, method, it doesn’t enable post-processing the http.Handler’s handling of the http.ResponseWriter.

In other words, how could you write some middleware that wraps and adds to an http.Handler’s existing functionality after the http.Handler has already processed a request and produced an HTTP response, thereby modifying the HTTP response before it’s sent to the client?


Use httptest.ResponseRecorder to record the http.Handler’s response; this provides a hook through which the response can be modified as needed.

A basic example

In the following example, WrappedHandler invokes the http.Handler it’s passed, but uses an httptest.ResponseRecorder to record the http.Handler’s response and add an X-Foo header before writing to the the http.ResponseWriter:

// WrappedHandler uses the handler it's passed, but
// adds an 'X-Foo: bar' header to the response.
func WrappedHandler(handler http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Use an httptest.ResponseRecorder to invoke handler
        // and record its response:
        rec := httptest.NewRecorder()

        // Invoke handler with the http.ResponseRecorder
        handler.ServeHTTP(rec, r)

        // Store the response in a 'res' var
        res := rec.Result()

        // Copy the recorded headers to the http.ResponseWriter
        for k, v := range res.Header {
            k = http.CanonicalHeaderKey(k)
            w.Header()[k] = v

        // Set the custom 'X-Foo' header
        w.Header()["X-Foo"] = "bar"

        // Write the recorded status code to the http.ResponseWriter

        // Write the recorded body to the http.ResponseWriter

A real world example

Concourse pull request 5897 introduces what is perhaps a more realistic example via its token.StoreAccessToken function.

token.StoreAccessToken provides middleware that records /sky/issuer/token requests’ response from a dex server.Server and does some stuff with its HTTP response, including storing some of the response data in the Concourse database, as well as modifying the access_token field in its JSON response body.