dongwan0574
dongwan0574
2019-08-18 02:21

日期输出从json.Encoder哪里来?

已采纳

At the end of the JSON and Go blog post you'll find this sample program:

package main

import (
    "encoding/json"
    "log"
    "os"
)

func main() {
    dec := json.NewDecoder(os.Stdin)
    enc := json.NewEncoder(os.Stdout)
    for {
        var v map[string]interface{}
        if err := dec.Decode(&v); err != nil {
            log.Println(err)
            return
        }
        for k := range v {
            if k != "Name" {
                delete(v, k)
            }
        }
        if err := enc.Encode(&v); err != nil {
            log.Println(err)
        }
    }
}

I compiled this with go build json_decoder.go and then ran the program like so in bash:

echo '{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}' | json_decoder

and received this output:

{"Name":"Wednesday"}
2019/08/17 22:09:20 EOF

The first line of output is exactly what I'd expect. But where is the line 2019/08/17 22:09:20 EOF coming from?

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2条回答

  • douhuai2861 douhuai2861 2年前

    When EOF is reached the decoder returns io.EOF which is then is being output by the logger log.Println(err) with a timestamp prepended.

    You can check for EOF when decoding like this for example

    if err := dec.Decode(&v); err != nil {
        if err != io.EOF {
            log.Println(err)
        }
        return
    }
    

    Output:

    ➜ echo '{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}' | ./json_decoder
    {"Name":"Wednesday"}
    
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  • douque8855 douque8855 2年前
    log.Println(err)
    

    That logger writes to standard error and prints the date and time of each logged message.


    You are seeing both stdoutand stderr so:

    1. The solution is to redirect the stdout to a file (or to another pipe):

      echo '{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}' | ./jsonio > file.json
      

      Output (which is stderr):

      2019/08/17 20:35:40 EOF
      

      Then see the file content (which is stdout):

      cat file.json 
      
      {"Name":"Wednesday"}
      

    1. You may discard stderr (not recommended, but for test purpose) or redirect stderr:

      echo '{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}' | ./jsonio 2>/dev/null
      

      Output (which is stdout):

      {"Name":"Wednesday"}
      

    1. Or don't show the EOF at all (note: you'll see other errors like incorrect JSON errors in stderr):

      package main
      
      import (
          "encoding/json"
          "io"
          "log"
          "os"
      )
      
      func main() {
          dec := json.NewDecoder(os.Stdin)
          enc := json.NewEncoder(os.Stdout)
          for {
              var v map[string]interface{}
              if err := dec.Decode(&v); err != nil {
                  if err != io.EOF {
                      log.Println(err)
                  }
                  return
              }
              for k := range v {
                  if k != "Name" {
                      delete(v, k)
                  }
              }
              if err := enc.Encode(&v); err != nil {
                  log.Println(err)
              }
          }
      }
      
      
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