将HTTP响应正文写入文件后，出现EOF错误[重复]

Question

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• Golang read request body 2 answers

I'd like to save a JSON response to a text file before parsing it:

req, err := http.NewRequest("POST", url, body)
req.Header.Set("Authorization", "secret_key")
req.Header.Set("Content-Type", "application/json")

resp, err := client.Do(req)
if err != nil {
    log.Fatal(err)
}
defer resp.Body.Close()

f, err := os.Create("./response.json")
if err != nil {
    log.Fatal(err)
}
defer f.Close()
io.Copy(f, resp.Body)

var result JSONResult
if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
    log.Fatal(err)
}

It successfully writes the JSON to a file but then fails on the decoding step with an error that just says EOF. If I parse before writing to file it parses ok, but then the file is empty. Can someone please explain what's happening here? Thanks!

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Answer 1

http.Response.Body is of type io.ReadCloser, which can only be read once (as you can see it does not have a method to rewind).

So alternatively for decoding purposes you could read your just created file.

Or if the response is not large (or you could trim it with io.LimitReader) - you can read it into a buffer

(not tested, something along these lines):

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

var buf bytes.Buffer
tee := io.TeeReader(r.Body, &buf)

io.Copy(f, tee)

var result JSONResult
if err := json.NewDecoder(buf).Decode(&result); err != nil {
    log.Fatal(err)
}

References:

• https://golang.org/pkg/io/#TeeReader