Parse JSON in Go - json

Question

This is an example of JSON output when calling 'ListObjects' for AWS S3
{
"Contents": [{
"ETag": "9e2bc2894b23742b7bb688c646c6fee9",
"Key": "DSC-0237.jpg",
"LastModified": "2017-09-06 21:53:15 +0000 UTC",
"Owner": {
"DisplayName": "demo-user",
"ID": "a9e2f170a6880f1d61852df8e523e88ca2a2b7abd093476cc93f1239ab5063c6"
},
"Size": 117904,
"StorageClass": "STANDARD"
}, {
"ETag": "\"9e2bc2894b23742b7bb688c646c6fee9\"",
"Key": "DSC-0238.jpg",
"LastModified": "2017-09-06 21:52:24 +0000 UTC",
"Owner": {
"DisplayName": "demo-user",
"ID": "a9e2f170a6880f1d61852df8e523e88ca2a2b7abd093476cc93f1239ab5063c6"
},
"Size": 117904,
"StorageClass": "STANDARD"
}, {
"ETag": "\"9e2bc2894b23742b7bb688c646c6fee9\"",
"Key": "DSC-0239.jpg",
"LastModified": "2017-09-06 21:53:01 +0000 UTC",
"Owner": {
"DisplayName": "demo-user",
"ID": "a9e2f170a6880f1d61852df8e523e88ca2a2b7abd093476cc93f1239ab5063c6"
},
"Size": 117904,
"StorageClass": "STANDARD"
}],
"IsTruncated": false,
"Marker": "",
"MaxKeys": 5,
"Name": "test-bucket-x011pp3",
"Prefix": ""
}
How do I parse this in Go? Chiefly I am interested in collecting:
Bucket Name
Key
Size
Owner's DisplayName
LastModified
I am coming from Python and in Python it would be something really simple like:
json_result = json.loads(json_string)
bucket_name = json_result['Name']
for idx, obj in enumerate(json_result['Contents']):
key = obj['Key']
size = obj['Size']
lastmod = obj['LastModified']
owner= obj['Owner']['DisplayName']
Thank you for the help!

Answer 1

It is something like this
package main
import (
"bytes"
"encoding/json"
"fmt"
"log"
)
var jsonData = []byte(`
{
"Contents": [{
"ETag": "9e2bc2894b23742b7bb688c646c6fee9",
"Key": "DSC-0237.jpg",
"LastModified": "2017-09-06 21:53:15 +0000 UTC",
"Owner": {
"DisplayName": "demo-user",
"ID": "a9e2f170a6880f1d61852df8e523e88ca2a2b7abd093476cc93f1239ab5063c6"
},
"Size": 117904,
"StorageClass": "STANDARD"
}, {
"ETag": "\"9e2bc2894b23742b7bb688c646c6fee9\"",
"Key": "DSC-0238.jpg",
"LastModified": "2017-09-06 21:52:24 +0000 UTC",
"Owner": {
"DisplayName": "demo-user",
"ID": "a9e2f170a6880f1d61852df8e523e88ca2a2b7abd093476cc93f1239ab5063c6"
},
"Size": 117904,
"StorageClass": "STANDARD"
}, {
"ETag": "\"9e2bc2894b23742b7bb688c646c6fee9\"",
"Key": "DSC-0239.jpg",
"LastModified": "2017-09-06 21:53:01 +0000 UTC",
"Owner": {
"DisplayName": "demo-user",
"ID": "a9e2f170a6880f1d61852df8e523e88ca2a2b7abd093476cc93f1239ab5063c6"
},
"Size": 117904,
"StorageClass": "STANDARD"
}],
"IsTruncated": false,
"Marker": "",
"MaxKeys": 5,
"Name": "test-bucket-x011pp3",
"Prefix": ""
}`,
)
type Response struct {
Contents []*Content
IsTruncated bool
Marker string
MaxKeys int
Name string
Prefix string
}
type Content struct {
ETag string
Key string
LastModified string
Owner *Owner
Size int
StroageClass string
}
type Owner struct {
DisplayName string
ID string
}
func main() {
resp := &Response{}
if err := json.NewDecoder(bytes.NewBuffer(jsonData)).Decode(resp); err != nil {
log.Fatal(err)
}
fmt.Printf("%#v", resp)
}
Try it in the Go Playground
And you should read this https://golang.org/pkg/encoding/json/

Answer 2

See https://golang.org/pkg/encoding/json
You annotate a struct with JSON fields that should be unmarshalled from the buffer of JSON, something like this:
type AWSObject struct {
Size int `json:"Size"`
Key string `json:"key"`
Owner AWSObjectOwner `json:"Owner"`
}
type AWSObjectOwner struct {
DisplayName `json:"DisplayName"`
}
var awsObjects []AWSObject
err = json.Unmarshal(jsonBuffer, &awsObjects)
if err != nil {
fmt.Printf("Error unmarshaling objects: " + err.Error() + "\n")
...
}

Answer 3

Using quicktype, I generated your model, marshaling code, and usage instructions:
// To parse this JSON data, add this code to your project and do:
//
// r, err := UnmarshalListObjectsResponse(bytes)
// bytes, err = r.Marshal()
package main
import "encoding/json"
func UnmarshalListObjectsResponse(data []byte) (ListObjectsResponse, error) {
var r ListObjectsResponse
err := json.Unmarshal(data, &r)
return r, err
}
func (r *ListObjectsResponse) Marshal() ([]byte, error) {
return json.Marshal(r)
}
type ListObjectsResponse struct {
Contents []Content `json:"Contents"`
IsTruncated bool `json:"IsTruncated"`
Marker string `json:"Marker"`
MaxKeys int64 `json:"MaxKeys"`
Name string `json:"Name"`
Prefix string `json:"Prefix"`
}
type Content struct {
ETag string `json:"ETag"`
Key string `json:"Key"`
LastModified string `json:"LastModified"`
Owner Owner `json:"Owner"`
Size int64 `json:"Size"`
StorageClass string `json:"StorageClass"`
}
type Owner struct {
DisplayName string `json:"DisplayName"`
ID string `json:"ID"`
}
As others suggested, it's probably best to use the AWS SDK for Go, but this method may be useful in general the next time you need to parse arbitrary JSON.