8. 标准库(Go Tutorial)

标准库

• 1 文档与源代码
• 2 记录日志
• 3 编码解码
• 4 输入和输出

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 main

import (
    "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 main

import (
    "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 main

import (
    "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 main

import (
    "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 main

import (
    "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 main

import (
    "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 main

import (
    "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)
    }
}