Difficult to decode JSON from http form POST

andbelo

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Sep 28, 2015, 12:57:29 PM9/28/15

to golang-nuts

Hello golang-nuts,

I am having difficulties in decoding JSON from a http form POST and could not find the solution anywhere so I decided to ask for help here. It might be a silly mistake but I am not being able to sort this out.

The "page.html" I have is:

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>Title of page</title>
    </head>
    <body>
        <form enctype='application/json' method="post">
            <select name="Letter">
                <option value="Default"></option>
                <option value="A">A</option>
                <option value="B">B</option>
            </select>
            <input type='submit' value="Submit">
        </form>
    </body>
</html>

and the go server, compiled to server.exe, is:

package main


import (
 "encoding/json"
 "log"
 "net/http"
)


type t struct {
 Letter string
}


func page(w http.ResponseWriter, r *http.Request) {
 log.Printf("r: %v\n", r)
 if r.Method == "POST" {
 dec := json.NewDecoder(r.Body)
 v := &t{}
 e := dec.Decode(v)
 if e != nil {
 log.Println(e)
 }
 log.Printf("v: %#v\n", v)
 }
 http.ServeFile(w, r, "page.html")
}


func main() {
 http.HandleFunc("/", page)
 log.Fatal(http.ListenAndServe(":8082", nil))
}

When I run this in a terminal, access http://localhost:8082/, fill and submit the form, I have:

C:\>server.exe

2015/09/28 11:50:52 r: &{GET / HTTP/1.1 1 1 map[Cache-Control:[max-age=0] Accept:[text/html,applicat
ion/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8] User-Agent:[Mozilla/5.0 (Windows NT 6.1; W
OW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.89 Safari/537.36] Https:[1] Accept-Enc
oding:[gzip, deflate, sdch] Accept-Language:[en-US,en;q=0.8,pt;q=0.6] Connection:[keep-alive]] 0x8b0
a10 0 [] false localhost:8082 map[] map[] <nil> map[] [::1]:53593 / <nil> <nil>}

2015/09/28 11:51:20 r: &{POST / HTTP/1.1 1 1 map[Content-Type:[application/x-www-form-urlencoded] Ca
che-Control:[max-age=0] User-Agent:[Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, l
ike Gecko) Chrome/44.0.2403.89 Safari/537.36] Https:[1] Accept:[text/html,application/xhtml+xml,appl
ication/xml;q=0.9,image/webp,*/*;q=0.8] Origin:[http://localhost:8082] Referer:[http://localhost:808
2/] Accept-Encoding:[gzip, deflate] Accept-Language:[en-US,en;q=0.8,pt;q=0.6] Connection:[keep-alive
] Content-Length:[8]] 0xc0821b1bc0 8 [] false localhost:8082 map[] map[] <nil> map[] [::1]:53593 / <
nil> <nil>}

2015/09/28 12:21:28 r.Body: &{0xc082204c00 <nil> <nil> false true {0 0} false false false}

2015/09/28 12:21:28 invalid character 'L' looking for beginning of value

2015/09/28 12:21:28 v: &main.t{Letter:""}

If someone could give a hint, that would be very appreciated.

Thanks a lot!

Henrik Johansson

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Sep 28, 2015, 1:12:23 PM9/28/15

to andbelo, golang-nuts

But an ordinary form post like that will not generate valid JSON.

To parse a form you have to use the ParseForm(req) method or send the payload as actual json.

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Andy Balholm

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Sep 28, 2015, 1:32:00 PM9/28/15

to andbelo, golang-nuts

Your browser apparently does not support a value of application/json for the enctype attribute (I don’t know if any actually do). So it falls back to the default, which is application/x-www-form-urlencoded. You can see that in the Content-Type header that your program logged.

In other words, HTML forms don’t generate JSON, so you need to deal with the encoding that they do generate.

Andy

Matt Silverlock

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Sep 28, 2015, 5:38:40 PM9/28/15

to golang-nuts, and...@gmail.com

> In other words, HTML forms don’t generate JSON, so you need to deal with the encoding that they do generate.

This.

One approach would be to have a struct with tags for gorilla/schema (http://www.gorillatoolkit.org/pkg/schema - or similar) and encoding/json — decode the PostForm into the struct, and then you can turn the struct into JSON for storage or re-transmission as you see fit.

André Beló

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Sep 28, 2015, 6:23:09 PM9/28/15

to golang-nuts

Thank you for your comments!

As this is not really a go issue, I will parse the form from the Post, and then loop over the key:values. It will be easier for me to workaround in the code of my go server.

I thought that if the browser was able to generate that as mentioned somewhere in W3C a simple JSON decoding would be the best solution. It came out too good to be true!

Thank you again and best regards.

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