golang: Marshal []os.FileInfo into JSON

Question

Basically, what I want to achieve is to get the content of a directory via os.ReadDir() and then encode the result into json.

Directly doing json.Marshal() cause no exception but gave me an empty result.

So I tried this:

func (f *os.FileInfo) MarshalerJSON() ([]byte, error) {
    return f.Name(), nil
}

Then Go tells me that os.FileInfo() is an interface and cannot be extended this way.

What's the best way to do this?

Answer 1

Pack the data into a struct that can be serialized:

http://play.golang.org/p/qDeg2bfik_

type FileInfo struct {
    Name    string
    Size    int64
    Mode    os.FileMode
    ModTime time.Time
    IsDir   bool
}

func main() {
    dir, err := os.Open(".")
    if err != nil {
        log.Fatal(err)
    }
    entries, err := dir.Readdir(0)
    if err != nil {
        log.Fatal(err)
    }

    list := []FileInfo{}

    for _, entry := range entries {
        f := FileInfo{
            Name:    entry.Name(),
            Size:    entry.Size(),
            Mode:    entry.Mode(),
            ModTime: entry.ModTime(),
            IsDir:   entry.IsDir(),
        }
        list = append(list, f)
    }

    output, err := json.Marshal(list)
    if err != nil {
        log.Fatal(err)
    }
    log.Println(string(output))

}

Answer 2

Here is a different version that makes the usage seemingly simple. Though you cannot unmarshall it back to the object. This is only applicable if you are sending it to client-end or something.

http://play.golang.org/p/vmm3temCUn

Usage

output, err := json.Marshal(FileInfo{entry})    

output, err := json.Marshal(FileInfoList{entries})

Code

type FileInfo struct {
    os.FileInfo
}

func (f FileInfo) MarshalJSON() ([]byte, error) {
    return json.Marshal(map[string]interface{}{
        "Name":    f.Name(),
        "Size":    f.Size(),
        "Mode":    f.Mode(),
        "ModTime": f.ModTime(),
        "IsDir":   f.IsDir(),
    })
}

type FileInfoList struct {
    fileInfoList []os.FileInfo
}

//This is inefficient to call multiple times for the same struct
func (f FileInfoList) MarshalJSON() ([]byte, error) {
    fileInfoList := make([]FileInfo, 0, len(f.fileInfoList))
    for _, val := range f.fileInfoList {
        fileInfoList = append(fileInfoList, FileInfo{val})
    }

    return json.Marshal(fileInfoList)
}