Reading Json Files with Go
JSON is a widely used format for representing structured data. Developers like it because it is easy to read, most common languages have a library for interacting with it, and most public APIs accept JSON in HTTP requests.
In this post, we’ll look at parsing a JSON file using Go! We will be using the io/ioutil package to open a json file on local disk, and encoding/json to parse the JSON data within the file into a memory structure.
Let’s assume we have the following JSON data representing an employee:
{
"Id": 1,
"firstName": "Steve",
"lastName": "Rogers",
"alias": "Captain America",
"Department": "Avengers"
}
We need a way of representing this data in memory when decoding it from JSON. For this, we will need to create a struct:
We will build up our go module as we progress through the article
Let’s start:
// main.go
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"log"
)
// Hero represents an hero
// ALL fields must be exportable! Otherwise the JSON parsing will fail.
type Hero struct {
Id int
FirstName string
LastName string
Alias string
Group string
}
func main() {
// Here we set the log prefix. When reading stack traces, this makes it easier to know
// where a failure occurred.
log.SetPrefix("main(): ")
// We first need to load the JSON file into memory
content, err := ioutil.ReadFile("./heros.json")
if err != nil {
log.Fatal("Error opening the file: ", err)
}
// Now we parse the JSON data into a memory structure. We know our JSON file will have more than one hero object,
// so we'll use make() to create a []Hero slice. Since go is a pass-by-value language, we will then use the address-of
// operator (&) to unmarshal our json data into the heros slice
heros := make([]Hero, 5)
err = json.Unmarshal(content, &heros)
if err != nil {
log.Fatal("Error occurred during unmarshal: ", err)
}
// now we can read/interact with the data. We'll loop over the array
// and print the values in memory
for i := 0; i < len(heros); i++ {
fmt.Println(heros[i].Id)
fmt.Println(heros[i].FirstName)
fmt.Println(heros[i].LastName)
fmt.Println(heros[i].Group)
}
}
Create a file named employees.json with similar data:
[
{
"Id": 1,
"firstName": "Steve",
"lastName": "Rogers",
"alias": "Captain America",
"group": "Avengers"
},
{
"Id": 2,
"firstName": "Clark",
"lastName": "Kent",
"alias": "Superman",
"group": "Justice League"
}
]
Now, we can run our main.go file:
11:40:47 ryan@xerxes json → go run main.go
1
Steve
Rogers
Avengers
2
Clark
Kent
Justice League
11:40:49 ryan@xerxes json
Pretty simple! That’s why I love go. We accomplished all of this with ~50 lines of code. Doing something similar in asp.net project would easily double that count and involve creating multiple files! (nothing against asp.net or c#, I think c# is a great language and use it daily)
Now, ‘go’ try this!