JSON APIs in Go

In this lesson, explore json apis in go through one small Go program, and connect its output to the language rules behind it.

Define structs with JSON tags to control external field names. json.Marshal encodes values, and json.Unmarshal decodes them.

Only exported fields are encoded by the standard JSON package. Keep internal-only fields private or omit them with json:"-".

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"
)

type Task struct {
	ID     int    `json:"id"`
	Title  string `json:"title"`
	Secret string `json:"-"`
}

func main() {
	task := Task{ID: 5, Title: "Study Go", Secret: "hidden"}
	data, err := json.Marshal(task)
	if err != nil {
		panic(err)
	}
	fmt.Println(string(data))
}

Output

{"id":5,"title":"Study Go"}

Marshal encodes the two exported, enabled fields as {"id":5,"title":"Study Go"}. Secret is omitted because its tag is -.

• Unexported struct fields are ignored by encoding/json.
• Do not directly decode untrusted JSON into privileged database fields without validation.

Add a completed boolean field with a JSON tag and print the updated representation.

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

You have practiced JSON APIs in Go 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.