r3dlight r3dlight - 3 months ago 42
JSON Question

golang sending json on multicast ip

I'm writting a piece of Go to send json data on multicast udp:

func send(a string, messages chan interface{}) {
addr, err := net.ResolveUDPAddr("udp", a)
CheckError(err)
c, err := net.DialUDP("udp", nil, addr)
CheckError(err)
for {
msg := <-messages
myjson, err := json.Marshal(msg)
if err != nil {
fmt.Println("Error encoding JSON")
return
}
//Write to bytes to multicast UDP
c.Write([]byte(myjson))

time.Sleep(2 * time.Second)
}
}


So my json is converted to an array of byte to make it work. Here is my "receiver" func:

func serveMulticastUDP(a string, messages chan interface{}) {

addr, err := net.ResolveUDPAddr("udp", a)
CheckError(err)
l, err := net.ListenMulticastUDP("udp", nil, addr)
l.SetReadBuffer(maxDatagramSize)
for {
b := make([]byte, maxDatagramSize)
n, src, err := l.ReadFromUDP(b)
if err != nil {
log.Fatal("ReadFromUDP failed:", err)
}
s := string(b[:n])//here is my problem, I want s to be map[string]interface before sending in my channel
messages<-s
log.Println(s)
log.Println(src)
log.Println(n)
//h(src, n, b)
}
}


How can convert a array of bytes to map[string]interface (json) ?

Answer

In your code you used json.Marshal() to convert your value to JSON text ([]byte).

The other direction ([]byte -> value) can be done using json.Unmarshal(). json.Unmarshal() expects a []byte so you don't even have to convert it to string.

See this example:

data := []byte(`{"key1":"value1","key2":123}`)

var m map[string]interface{}
if err := json.Unmarshal(data, &m); err != nil {
    panic(err)
}

fmt.Printf("%+v", m)

Output (try it on the Go Playground):

map[key1:value1 key2:123]

Notes:

The result of marshaling (json.Marshal()) is a value of type []byte so you don't need explicit conversion here:

c.Write([]byte(myjson))

You can simply write:

c.Write(myjson)

Also when unmarshaling, make sure you pass b[:n] to json.Unmarshal(), as the rest of the slice contains 0s (which your 2nd error suggests) but they are not part of the json text!