How do I get GoLang's http.HandleFunc() to work without error?

Question

I am trying to create a GoLang http.HandleFunc() that will listen for post requests and parse a json file. The Handler seems to fail as it starts and returns an error:

2017/01/24 13:35:08 listen tcp :8080: bind: Only one usage of each socket address (protocol/network address/port) is normally permitted.

I am assuming that it is throwing a fatal error.

Below is my function:

 http.HandleFunc("/", func(w http.ResponseWriter, request *http.Request) {

        //debug
        fmt.Fprintf(w, "Hello, %q", html.EscapeString(request.URL.Path))

        var m Message

        if request.Body == nil {
            http.Error(w, "Please send a request body", 400)                
            return
        }

        err := json.NewDecoder(request.Body).Decode(&m)
        if err != nil {
            http.Error(w, err.Error(), 400)
            return
        }
 })
 log.Fatal(http.ListenAndServe(":8080", nil))

How can I get this Handler to work and not get the error?

Your code snippet doesn't make any sense; why is there a defer res.Body.Close() call "in the middle on nowhere"? — user142162
– user142162, Commented Jan 24, 2017 at 18:51
This error indicates that there is another process or thread listening on the port already. Do you perhaps have another program on your computer serving to port 8080? — Jordan Lewis
– Jordan Lewis, Commented Jan 24, 2017 at 18:51
@TimCooper I'm not sure where it is supposed to go? Any suggestions? — antman1p
– antman1p, Commented Jan 24, 2017 at 19:16
@JordanLewis I ran netstat -nc TCP | find "8080" and got nothing back — antman1p
– antman1p, Commented Jan 24, 2017 at 19:17
I figured it out. The Handler continues to run as a process on Windows until you either shut down Eclipse, or kill the process. After you do this, you can run the handler without error. How can I get the handler to stop in the code for testing purposes? — antman1p
– antman1p, Commented Jan 24, 2017 at 20:36

daplho · Accepted Answer · 2017-01-24 19:22:24Z

1

I've remedied several of the problems with your code and made best guesses around missing information. Here is sample working code that will accept a message that has "name" and "text" fields:

package main

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

type Message struct {
  Name string `json:"name"`
  Text string `json:"text"`
}

func main() {
  http.HandleFunc("/", func(w http.ResponseWriter, request *http.Request) {
    fmt.Fprintf(w, "Hello, %q", html.EscapeString(request.URL.Path))

    var m Message

    if request.Body == nil {
      http.Error(w, "Please send a request body", 400)
      return
    }

    err := json.NewDecoder(request.Body).Decode(&m)
    if err != nil {
      http.Error(w, err.Error(), 400)
      return
    }

    log.Println(m)
  })

  log.Fatal(http.ListenAndServe(":8080", nil))
}

answered Jan 24, 2017 at 19:22

daplho

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4 Comments

antman1p Over a year ago

I cannot find much difference. I see that you added the main function and the struct, which I have. I was only showing the Handler fucntion and log, not the whole of my code.

daplho Over a year ago

Understood. I completed the code so that I can run it and verify that it works - that information might make for a better question.

antman1p Over a year ago

OK, thanks. Please let me know if you are getting the error.

daplho Over a year ago

I am not getting the error, as indicated in comments above you must have a process hanging onto that port.

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