日期输出从json.Encoder哪里来？

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?

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"}

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"}
   

2. 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"}
   

3. 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)
           }
       }
   }