更改以JSON对象的字节表示形式归档

Question

I write A object to a file f.

a := A{42}
bytes, _ := json.MarshalIndent(a, "", "\t")
f.Write(bytes)

Where A looks like:

type A struct {
    A int `json:"a"`
} 

Then I change field of this object and write it to the file:

a.A = 666
f.Write(bytes)

As a result, I see only

{
    "a": 42
}{
    "a": 42
}

While I expected:

{
    "a": 42
}{
    "a": 666
}

I know that I can overcome it by using json.MarshalIndent again. But I need to do a lot of (~10^6) writings to the file, so using json.MarshalIndent again and again seems to be a heavy task.

How can I directly change a bytes variable?

Code is located at https://play.golang.org/p/8CMpwehMidR

Answer 1

You have no choice but to marshal repeatedly. Use a *json.Encoder for improved ergonomics and efficiency:

package main

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

type A struct {
    A int `json:"a"`
}

func main() {
    f, err := os.Create("foo.json")
    if err != nil {
        log.Fatal(err)
    }
    defer f.Close()

    enc := json.NewEncoder(f)
    enc.SetIndent("", "\t")

    a := A{42} // Using a pointer may improve performance if A is large.
    enc.Encode(a)

    a.A = 666
    enc.Encode(a)
}

Wrapping the file with a buffered writer may also improve performance, depending on how quickly you can compute successive values for the As and how fast the disk is.