Basic HTTP Server
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Writing a Handler Function
An HTTP handler is a function matching func(w http.ResponseWriter, r *http.Request), where writing to w sends bytes back as the response body to whoever made the request.
Example: Writing a Handler Function
package main
import (
"fmt"
"net/http"
)
func helloHandler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "Hello from a Go HTTP handler!")
}
func main() {
http.HandleFunc("/hello", helloHandler)
fmt.Println("handler registered for /hello")
}Login to try C/C++/Java/PHP code in the editor
Testing a Handler Without a Real Server
Since starting a real listener would block forever in this sandbox, handlers can be exercised directly using httptest.NewRecorder and httptest.NewRequest, which simulate an HTTP request/response without opening any actual network socket.
Note: net/http/httptest is the standard way to test handlers without binding a real network port.
Example: Testing a Handler Without a Real Server
package main
import (
"fmt"
"net/http"
"net/http/httptest"
)
func helloHandler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "Hello, HTTP!")
}
func main() {
req := httptest.NewRequest(http.MethodGet, "/hello", nil)
rec := httptest.NewRecorder()
helloHandler(rec, req)
fmt.Println("status:", rec.Code)
fmt.Println("body:", rec.Body.String())
}Login to try C/C++/Java/PHP code in the editor
Setting a Status Code
w.WriteHeader(code) explicitly sets the HTTP status code for the response before any body is written -- omitting it defaults to 200 OK once the first write happens.
Note: Call WriteHeader before any call to Write/Fprintln, since the status line must go out first.
Example: Setting a Status Code
package main
import (
"fmt"
"net/http"
"net/http/httptest"
)
func notFoundHandler(w http.ResponseWriter, r *http.Request) {
w.WriteHeader(http.StatusNotFound)
fmt.Fprintln(w, "resource not found")
}
func main() {
req := httptest.NewRequest(http.MethodGet, "/missing", nil)
rec := httptest.NewRecorder()
notFoundHandler(rec, req)
fmt.Println("status:", rec.Code, "body:", rec.Body.String())
}Login to try C/C++/Java/PHP code in the editor
- Calling http.ListenAndServe in an interactive/CI sandbox, which blocks forever waiting for real network connections -- runnable examples here should call handlers directly instead of actually starting a listener.
- Registering the same route pattern twice with http.HandleFunc, which panics at startup.
- Forgetting to write an HTTP status code before writing the body when a non-200 response is needed -- WriteHeader must be called before Write.
- http.HandleFunc registers a function to handle requests matching a URL pattern.
- A handler function receives an http.ResponseWriter (to write the response) and an *http.Request (describing the incoming request).
- http.ListenAndServe starts the server, blocking and listening for connections on a given address.
- http.NewRequest plus calling a handler directly is a simple way to demonstrate handler logic without a real network listener.
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