8. 标准库(Go Tutorial)

标准库

Go 标准库是一组核心包，用来扩展和增强语言的能力，会随着 Go 语言的升级而更新并保持向后兼容

8.1 文档与源代码

Go 语言的标准库里有超过 100 个包被分在了 38 个类别里面，详情请见Packages

8.2 记录日志

// This sample program demonstrates how to create customized loggers.package mainimport (    "io"    "io/ioutil"    "log"    "os")var (    Trace   *log.Logger // Just about anything    Info    *log.Logger // Important information    Warning *log.Logger // Be concerned    Error   *log.Logger // Critical problem)func init() {    file, err := os.OpenFile("errors.txt",        os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)    if err != nil {        log.Fatalln("Failed to open error log file:", err)    }    Trace = log.New(ioutil.Discard,        "TRACE: ",        log.Ldate|log.Ltime|log.Lshortfile)    Info = log.New(os.Stdout,        "INFO: ",        log.Ldate|log.Ltime|log.Lshortfile)    Warning = log.New(os.Stdout,        "WARNING: ",        log.Ldate|log.Ltime|log.Lshortfile)    Error = log.New(io.MultiWriter(file, os.Stderr),        "ERROR: ",        log.Ldate|log.Ltime|log.Lshortfile)}func main() {    Trace.Println("I have something standard to say")    Info.Println("Special Information")    Warning.Println("There is something you need to know about")    Error.Println("Something has failed")}

8.3 编码/解码

8.3.1 解码 json

// This sample program demonstrates how to decode a JSON response// using the json package and NewDecoder function.package mainimport (    "encoding/json"    "fmt"    "log"    "net/http")type (    // gResult maps to the result document received from the search.    gResult struct {        GsearchResultClass string `json:"GsearchResultClass"`        UnescapedURL       string `json:"unescapedUrl"`        URL                string `json:"url"`        VisibleURL         string `json:"visibleUrl"`        CacheURL           string `json:"cacheUrl"`        Title              string `json:"title"`        TitleNoFormatting  string `json:"titleNoFormatting"`        Content            string `json:"content"`    }    // gResponse contains the top level document.    gResponse struct {        ResponseData struct {            Results []gResult `json:"results"`        } `json:"responseData"`    })func main() {    uri := "http://ajax.googleapis.com/ajax/services/search/web?v=1.0&rsz=8&q=golang"    // Issue the search against Google.    resp, err := http.Get(uri)    if err != nil {        log.Println("ERROR:", err)        return    }    defer resp.Body.Close()    // Decode the JSON response into our struct type.    var gr gResponse    err = json.NewDecoder(resp.Body).Decode(&gr)    if err != nil {        log.Println("ERROR:", err)        return    }    fmt.Println(gr)    // Marshal the struct type into a pretty print    // version of the JSON document.    pretty, err := json.MarshalIndent(gr, "", "    ")    if err != nil {        log.Println("ERROR:", err)        return    }    fmt.Println(string(pretty))}

当需要处理的 JSON 文档以 string 的形式存在时，可以将 string 转换为 byte 切片([] byte)，并使用 json 包的 Unmarshal 函数进行反序列化的处理

// This sample program demonstrates how to decode a JSON string.package mainimport (    "encoding/json"    "fmt"    "log")// Contact represents our JSON string.type Contact struct {    Name    string `json:"name"`    Title   string `json:"title"`    Contact struct {        Home string `json:"home"`        Cell string `json:"cell"`    } `json:"contact"`}// JSON contains a sample string to unmarshal.var JSON = `{    "name": "Gopher",    "title": "programmer",    "contact": {        "home": "415.333.3333",        "cell": "415.555.5555"    }}`func main() {    // Unmarshal the JSON string into our variable.    var c Contact    err := json.Unmarshal([]byte(JSON), &c)    if err != nil {        log.Println("ERROR:", err)        return    }    fmt.Println(c)}

有时，无法为 JSON 的格式声明一个结构类型，而是需要更加灵活的方式来处理 JSON 文档

// This sample program demonstrates how to decode a JSON string.package mainimport (    "encoding/json"    "fmt"    "log")// JSON contains a sample string to unmarshal.var JSON = `{    "name": "Gopher",    "title": "programmer",    "contact": {        "home": "415.333.3333",        "cell": "415.555.5555"    }}`func main() {    // Unmarshal the JSON string into our map variable.    var c map[string]interface{}    err := json.Unmarshal([]byte(JSON), &c)    if err != nil {        log.Println("ERROR:", err)        return    }    fmt.Println("Name:", c["name"])    fmt.Println("Title:", c["title"])    fmt.Println("Contact")    fmt.Println("H:", c["contact"].(map[string]interface{})["home"])    fmt.Println("C:", c["contact"].(map[string]interface{})["cell"])}

8.3.2 编码 JSON

// This sample program demonstrates how to marshal a JSON string.package mainimport (    "encoding/json"    "fmt"    "log")func main() {    // Create a map of key/value pairs.    c := make(map[string]interface{})    c["name"] = "Gopher"    c["title"] = "programmer"    c["contact"] = map[string]interface{}{        "home": "415.333.3333",        "cell": "415.555.5555",    }    // Marshal the map into a JSON string.    data, err := json.MarshalIndent(c, "", "    ")    if err != nil {        log.Println("ERROR:", err)        return    }    fmt.Println(string(data))}

8.4 输入和输出

输出(Write 接口)

// Sample program to show how different functions from the// standard library use the io.Writer interface.package mainimport (    "bytes"    "fmt"    "os")// main is the entry point for the application.func main() {    // Create a Buffer value and write a string to the buffer.    // Using the Write method that implements io.Writer.    var b bytes.Buffer    b.Write([]byte("Hello "))    // Use Fprintf to concatenate a string to the Buffer.    // Passing the address of a bytes.Buffer value for io.Writer.    fmt.Fprintf(&b, "World!")    // Write the content of the Buffer to the stdout device.    // Passing the address of a os.File value for io.Writer.    b.WriteTo(os.Stdout)}

// Sample program to show how to write a simple version of curl using// the io.Reader and io.Writer interface support.package mainimport (    "io"    "log"    "net/http"    "os")// main is the entry point for the application.func main() {    // r here is a response, and r.Body is an io.Reader.    r, err := http.Get(os.Args[1])    if err != nil {        log.Fatalln(err)    }    // Create a file to persist the response.    file, err := os.Create(os.Args[2])    if err != nil {        log.Fatalln(err)    }    defer file.Close()    // Use MultiWriter so we can write to stdout and    // a file on the same write operation.    dest := io.MultiWriter(os.Stdout, file)    // Read the response and write to both destinations.    io.Copy(dest, r.Body)    if err := r.Body.Close(); err != nil {        log.Println(err)    }}