This is my first go project. All I want to do is read a file.json on my server, then make it available to others via a REST API. But I'm getting errors. Here's what I have so far.
main.go
package main
import (
"encoding/json"
"github.com/gorilla/mux"
"log"
"net/http"
"io/ioutil"
"fmt"
)
func GetDetail(w http.ResponseWriter, r *http.Request) {
b,_ := ioutil.ReadFile("file.json");
rawIn := json.RawMessage(string(b))
var objmap map[string]*json.RawMessage
err := json.Unmarshal(rawIn, &objmap)
if err != nil {
fmt.Println(err)
}
fmt.Println(objmap)
json.NewEncoder(w).Encode(objmap)
}
func main() {
router := mux.NewRouter()
router.HandleFunc("/detail", GetDetail).Methods("GET")
log.Fatal(http.ListenAndServe(":8000", router))
}
file.json
{
favourite_color:"blue",
attribute:{density:23,allergy:"peanuts",locations:["USA","Canada","Jamaica"]},
manufacture_year:1998
}
When I run go build; ./sampleproject, then go to my web browser at http://localhost:8000/detail, I get the error message:
invalid character 'f' looking for beginning of object key string
map[]
I've tried a few marshal methods, but they all give me different errors. I just need a working example to study from to better understand how all this works.
I should also mention that file.json does not have a fixed schema. It can change drastically at any minute to have a random set of data.
How do I get around the error invalid character f message and get my file.json to render at http://localhost:8000/detail?
As cerise mentioned, it's just formatting error in the JSON file. Must quote all properties.
Then the rest of the code works
Related
Following this tutorial I'm trying to read a json file in Golang. It says there are two ways of doing that:
unmarshal the JSON using a set of predefined structs
or unmarshal the JSON using a map[string]interface{}
Since I'll probably have a lot of different json formats I prefer to interpret it on the fly. So I now have the following code:
package main
import (
"fmt"
"os"
"io/ioutil"
"encoding/json"
)
func main() {
// Open our jsonFile
jsonFile, err := os.Open("users.json")
// if we os.Open returns an error then handle it
if err != nil {
fmt.Println(err)
}
fmt.Println("Successfully Opened users.json")
// defer the closing of our jsonFile so that we can parse it later on
defer jsonFile.Close()
byteValue, _ := ioutil.ReadAll(jsonFile)
var result map[string]interface{}
json.Unmarshal([]byte(byteValue), &result)
fmt.Println(result["users"])
fmt.Printf("%T\n", result["users"])
}
This prints out:
Successfully Opened users.json
[map[type:Reader age:23 social:map[facebook:https://facebook.com twitter:https://twitter.com] name:Elliot] map[name:Fraser type:Author age:17 social:map[facebook:https://facebook.com twitter:https://twitter.com]]]
[]interface {}
At this point I don't understand how I can read the age of the first user (23). I tried some variations:
fmt.Println(result["users"][0])
fmt.Println(result["users"][0].age)
But apparently, type interface {} does not support indexing.
Is there a way that I can access the items in the json without defining the structure?
Probably you want
fmt.Println(result["users"].(map[string]interface{})["age"])
or
fmt.Println(result[0].(map[string]interface{})["age"])
As the JSON is a map of maps the type of the leaf nodes is interface{} and so has to be converted to map[string]interface{} in order to lookup a key
Defining a struct is much easier. My top tip for doing this is to use a website that converts JSON to a Go struct definition, like Json-To-Go
I am trying to parse a json from a third party software. It returns a json like this
{
"top1/dir1": "10",
"top1/dir2": "20",
"top1/dir3": "30",
"top2/diff_val1": "40"
}
JSONLint says this is a valid json. But I could not figure how I can parse this with golang.
The code I used to parse the json file above (to be clear I took the code from another stackoverflow post).
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"log"
)
type mytype []map[string]string
func main() {
var data mytype
file, err := ioutil.ReadFile("t1.json")
if err != nil {
log.Fatal(err)
}
err = json.Unmarshal(file, &data)
if err != nil {
log.Fatal(err)
}
fmt.Println(data)
}
When I do a go run main.go, I get the below error
$ go run main.go
2016/06/19 22:53:57 json: cannot unmarshal object into Go value of type main.mytype
exit status 1
I did try to parse this format with another library - "github.com/Jeffail/gabs", but was unsuccessful. Since this is a valid json, I am pretty sure this can be parsed, but I am not sure how.
There is a Go package with methods for decoding JSON strings.
https://golang.org/pkg/encoding/json/#Unmarshal
Here is an example of usage:
package main
import (
"encoding/json"
"fmt"
)
func main() {
var jsonBlob = []byte(`[
{"Name": "Platypus", "Order": "Monotremata"},
{"Name": "Quoll", "Order": "Dasyuromorphia"}
]`)
type Animal struct {
Name string
Order string
}
var animals []Animal
err := json.Unmarshal(jsonBlob, &animals)
if err != nil {
fmt.Println("error:", err)
}
fmt.Printf("%+v", animals)
}
EDIT: As pointed out by Malik, the type of the value whose pointer you pass is wrong. In this case, your type should be map[string]interface{} (preferably, because a JSON field might not store a string) or map[string]string instead of []map[string]string. The brackets at the beginning are wrong: such would be an array of JSON objects.
It's just that you have a small typo in your program. You've declared mytype as a slice of maps, rather than just a map.
Just change:
type mytype []map[string]string
To:
type mytype map[string]string
See https://play.golang.org/p/pZQl8jV5TC for an example.
Jonathan's answer provides a good example of decoding JSON, and links the relevant package. You don't provide much detail on what exactly is going wrong with your parsing, but if I had to take a guess I would say you're probably not creating an appropriate struct to contain the JSON once it is unmarshalled. Because Go is statically typed, it expects data to adhere to explicitly defined formats.
If you don't want to go to the trouble of defining structs, you could just use an empty interface object, which is sort of a catch all in Go. Simply declare a variable with the type []interface{}, and then pass it into the JSON unmarshal function. Hope this helps!
I'm trying to work out a bit of code to pull in config from a JSON file.
When I attempt to build, I get this error
type ConfigVars is not an expression
Below is the config and program code I'm trying to work with. Every example I've found so far is similar to the below code. Any suggestion of what I'm doing incorrectly?
-- Config File
{"beaconUrl":"http://test.com/?id=1"}
-- Program Code
package main
import (
"encoding/json"
"fmt"
"os"
)
type ConfigVars struct {
BeaconUrl string
}
func main() {
configFile, err := os.Open("config.json")
defer configFile.Close()
if err != nil {
fmt.Println("Opening config file", err.Error())
}
jsonParser := json.NewDecoder(configFile)
if err = jsonParser.Decode(&ConfigVars); err != nil {
fmt.Println("Parsing config file", err.Error())
}
}
What you're doing there is trying to pass a pointer to the ConfigVars type (which obviously doesn't really mean anything). What you want to do is make a variable whose type is ConfigVars and pass a pointer to that instead:
var cfg ConfigVars
err = jsonParser.Decode(&cfg)
...
For others who come onto this problem, you may find that you've forgotten to initialize the variable during assignment using the := operator, as described in Point 3 at the end of this GoLang tutorial.
var cfg ConfigVars
err := jsonParser.Decode(&cfg)
I'm now learning Go myself and am stuck in getting and parsing HTML/XML. In Python, I usually write the following code when I do web scraping:
from urllib.request import urlopen, Request
url = "http://stackoverflow.com/"
req = Request(url)
html = urlopen(req).read()
, then I can get raw HTML/XML in a form of either string or bytes and proceed to work with it. In Go, how can I cope with it? What I hope to get is raw HTML data which is stored either in string or []byte (though it can be easily converted, that I don't mind which to get at all). I consider using gokogiri package to do web scraping in Go (not sure I'll indeed end up with using it!), but it looks like it requires raw HTML text before doing any work with it...
So how can I acquire such object?
Or is there any better way to do web scraping work in Go?
Thanks.
From the Go http.Get Example:
package main
import (
"fmt"
"io/ioutil"
"log"
"net/http"
)
func main() {
res, err := http.Get("http://www.google.com/robots.txt")
if err != nil {
log.Fatal(err)
}
robots, err := ioutil.ReadAll(res.Body)
res.Body.Close()
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s", robots)
}
Will return the contents of http://www.google.com/robots.txt into the string variable robots.
For XML parsing look into the Go encoding/xml package.
I'm getting an error of:
json.Unmarshal undefined (type interface {} has no field or method Unmarshal)
trying to convert a json byte slice into the generic interface{} type. I'm reading the docs for encoding/json and they give an example that shows this is valid. What gives?
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
)
func main() {
var json interface{}
data, _ := ioutil.ReadFile("testMusic.json")
json.Unmarshal(data, &json)
m := json.(map[string]interface{})
fmt.Printf("%+v", m)
}
You've defined a local variable json that masks the global symbol json referring to the JSON module. Renaming your local variable should allow your code to work.