Let’s imagine a situation when you want to alter the result, returned by some http handler to the client. Fortunately, Golang provides an easy mechanism for that, called a middleware. I’m going to dive directly to the source code, to save your time.
Imagine, we have this simple web server (here I’m using a chi router):
package main
import (
"bytes"
"github.com/go-chi/chi"
"io"
"log"
"net/http"
)
func main() {
r := chi.NewRouter()
r.Get("/", myFirstHandler)
http.ListenAndServe(":3000", r)
}
func myFirstHandler(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("This is a main page"))
}
When we run this application and visit a frontpage htttp://localhost:3000/, we can see this:
Now we got a new requirement to create another handler which should get all response data from myFirstHandler and add some modification on top.
We can do it easily in this way:
// Adds a new router handler with a middleware myMiddleware.
r.With(myMiddleware).Get("/other", myFirstHandler)
To be able to read a response from other handler, we have to implement our own ResponseWriter:
type MyResponseWriter struct {
http.ResponseWriter
buf *bytes.Buffer
}
// Here we are implementing a Write() function from ResponseWriter with our custom instructions.
func (myrw *MyResponseWriter) Write(p []byte) (int, error) {
return myrw.buf.Write(p)
}
And finally, let’s write our middleware:
func myMiddleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
// Create a response writer:
myResponseWriter := &MyResponseWriter{
ResponseWriter: w,
buf: &bytes.Buffer{},
}
// Here we are pssing our custom response writer to the next http handler.
next.ServeHTTP(myResponseWriter, r)
// Here we are adding our custom stuff to the response, which we received after http handler execution.
myResponseWriter.buf.WriteString(" and some additional modifications")
// And, finally, we are copiing everything back to the original response writer. if _, err := io.Copy(w, myResponseWriter.buf); err != nil {
log.`Printf("Failed to send out response: %v", err)
}
})
}
Now, if we run our server again and go to /other path, we will see this:
This was a silly example, which will never happen in real life, but, I hope you got an overview how you can play with http handlers and middlewares.
The source code you can found in this repository: https://github.com/alexsergivan/blog-examples/tree/master/middleware