Handling Request Bodies

In this lesson, explore handling request bodies through one small Go program, and connect its output to the language rules behind it.

The request body is a stream of bytes. For JSON input, use json.Decoder on r.Body. Bound incoming body sizes to protect resources and reject malformed payloads.

In a real handler, check the Content-Type and ensure the payload contains exactly the expected JSON value when that is your contract.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"encoding/json"
	"fmt"
	"net/http"
	"net/http/httptest"
	"strings"
)

func main() {
	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		var input struct {
			Name string `json:"name"`
		}
		r.Body = http.MaxBytesReader(w, r.Body, 1024)
		if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
			http.Error(w, "invalid JSON", 400)
			return
		}
		fmt.Fprint(w, "Hello, ", input.Name)
	})
	rec := httptest.NewRecorder()
	request := httptest.NewRequest("POST", "/", strings.NewReader(`{"name":"Mina"}`))
	handler.ServeHTTP(rec, request)
	fmt.Print(rec.Body.String())
}

Output

Hello, Mina

The decoder reads the supplied JSON into a struct. The body size is capped, and the handler responds Hello, Mina.

• Avoid reading unlimited request bodies into memory.
• Validate required fields after successful decoding; valid JSON can still be semantically invalid.

Reject an empty name with a 400 response and test both valid and invalid bodies.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Handling Request Bodies in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.