ioutil包二

ReadAll reads from r until an error or EOF and returns the data it read. A successful call returns err == nil, not err == EOF. Because ReadAll is defined to read from src until EOF, it does not treat an EOF from Read as an error to be reported.

ReadAll從r中讀取數據直到發生錯誤或遇到EOF，返回讀到的數據。調用成功返回err == nil，而不是err == EOF。由於ReadAll被定義爲從源讀取數據直到結束（EOF），不會將讀取過程當中遇到的EOF做爲error報告。

godoc中提供了一個ReadAll的Example，從一個strings.NewReader 構造的io.Reader中讀取全部數據：

r := strings.NewReader("Go is a general-purpose language designed with systems programming in mind.")

 

b, err := ioutil.ReadAll(r)

if err != nil {

    log.Fatal(err)

}

fmt.Printf("%s", b)

再看另一個比較實用的例子，發起一個http request，而後從response中讀取數據：

package main

 

import (

    "fmt"

    "io/ioutil"

    "log"

    "net/http"

)

 

func main() {

    resp,err := http.Get("http://www.baidu.com")

    if err != nil {

        log.Fatal(err)

    }

 

    bData,err := ioutil.ReadAll(resp.Body)

    if err != nil {

        log.Fatal(err)

    }

defer resp.Body.Close()

    fmt.Println(string(bData))

}

經過ioutil包的源碼能夠看到ReadFile和ReadAll調用了同一個函數readAll

func readAll(r io.Reader, capacity int64) (b []byte, err error)

 

func ReadAll(r io.Reader) ([]byte, error) {

    return readAll(r, bytes.MinRead)

}

func ReadFile(filename string) ([]byte, error) {

    f, err := os.Open(filename)

    if err != nil {

        return nil, err

    }

    defer f.Close()

    // It's a good but not certain bet that FileInfo will tell us exactly how much to

    // read, so let's try it but be prepared for the answer to be wrong.

    var n int64

 

    if fi, err := f.Stat(); err == nil {

        // Don't preallocate a huge buffer, just in case.

        if size := fi.Size(); size < 1e9 {

            n = size

        }

    }

    // As initial capacity for readAll, use n + a little extra in case Size is zero,

    // and to avoid another allocation after Read has filled the buffer. The readAll

    // call will read into its allocated internal buffer cheaply. If the size was

    // wrong, we'll either waste some space off the end or reallocate as needed, but

    // in the overwhelmingly common case we'll get it just right.

    return readAll(f, n+bytes.MinRead)

}

io.TempFile\io.TempDir

TempFile和TempDir在指定位置生成臨時文件和臨時文件夾。

如下來自godoc：

func TempDir(dir, prefix string) (name string, err error)

TempDir creates a new temporary directory in the directory dir with a name beginning with prefix and returns the path of the new directory. If dir is the empty string, TempDir uses the default directory for temporary files (see os.TempDir). Multiple programs calling TempDir simultaneously will not choose the same directory. It is the caller's responsibility to remove the directory when no longer needed.

TempDir在指定的dir目錄新建立以prefix爲前綴的臨時目錄，返回新臨時目錄的路徑。若是dir爲空字符串，TempDir使用默認的存放臨時文件的目錄（os.TempDir，操做系統臨時目錄）。多個程序同時調用TempDir不會返回相同目錄。調用者負責刪除再也不須要的臨時目錄。

Godoc中的Example，建立臨時目錄，退出時調用os.RemoveAll刪除:

content := []byte("temporary file's content")

dir, err := ioutil.TempDir("", "example")

if err != nil {

    log.Fatal(err)

}

defer os.RemoveAll(dir) // clean up

tmpfn := filepath.Join(dir, "tmpfile")

if err := ioutil.WriteFile(tmpfn, content, 0666); err != nil {

    log.Fatal(err)

}

若是dir指定的目錄不存在，返回err：

CreateFile d:/notexistdir: The system cannot find the file specified.

Godoc中關於TempFile的說明以下：

func TempFile(dir, prefix string) (f *os.File, err error)

TempFile creates a new temporary file in the directory dir with a name beginning with prefix, opens the file for reading and writing, and returns the resulting *os.File. If dir is the empty string, TempFile uses the default directory for temporary files (see os.TempDir). Multiple programs calling TempFile simultaneously will not choose the same file. The caller can use f.Name() to find the pathname of the file. It is the caller's responsibility to remove the file when no longer needed.

TempFile在dir目錄中新建立以prefix爲前綴的臨時文件，打開文件用於讀寫，返回os.File指針。若是dir爲空字符串，TempFile使用默認的存放臨時文件的目錄（os.TempDir，操做系統臨時目錄）。多個程序同時調用TempFile不會選擇相同的文件。調用者負責刪除再也不須要的臨時文件。

Godoc中關於TempFile的Example，在操做系統臨時目錄中建立臨時文件，退出時調用os.Remove刪除臨時文件：

content := []byte("temporary file's content")

tmpfile, err := ioutil.TempFile("", "example")

if err != nil {

    log.Fatal(err)

}

defer os.Remove(tmpfile.Name()) // clean up

if _, err := tmpfile.Write(content); err != nil {

    log.Fatal(err)

}

if err := tmpfile.Close(); err != nil {

    log.Fatal(err)

}

若是dir不存在，返回err：

open d:\notexistdir\tmp946090042: The system cannot find the path specified.

在Windows中，dir爲空字符串時，os.TempDir返回的目錄爲：

C:\Users\Dell\AppData\Local\Temp\

ioutil.ReadDir

ioutil.ReadDir能夠用於遍歷目錄。Godoc中關於ReadDir的說明以下：

func ReadDir(dirname string) ([]os.FileInfo, error)

ReadDir reads the directory named by dirname and returns a list of directory entries sorted by filename.

ReadDir返回dirname目錄的目錄條目列表，按照文件名排序。

Godoc中的Example：

files, err := ioutil.ReadDir(".")

if err != nil {

    log.Fatal(err)

}

for _, file := range files {

    fmt.Println(file.Name())

}

略做修改，便可迭代遍歷目錄：

package main

 

import (

    "log"

    "fmt"

    "io/ioutil"

)

 

func main() {

 

err := listFolder("d:/Tmp","")

if err != nil {

     log.Fatal(err)

}

    

}

 

func listFolder(dir string,indent string)(err error){

    fis ,err := ioutil.ReadDir(dir)

    if err != nil {

        return err

    }

    for _,fi := range fis {

        if fi.IsDir(){

            fmt.Printf("%sd:%s\r\n",indent,fi.Name())           

            err = listFolder(dir+fi.Name()+"/",indent+"\t")

            if err != nil {

                return err

            }

        }else{

            fmt.Printf("%sf:%s\r\n",indent,fi.Name())

        }

        

    }

    return 

}

ioutil.NopCloser

func NopCloser(r io.Reader) io.ReadCloser

NopCloser returns a ReadCloser with a no-op Close method wrapping the provided Reader r.

NopCloser函數將io.Reader接口適配爲io.ReadCloser接口，爲其封裝了一個無操做的Close方法。

其源碼很是簡單；

type nopCloser struct {

    io.Reader

}

 

func (nopCloser) Close() error { return nil }

 

// NopCloser returns a ReadCloser with a no-op Close method wrapping

// the provided Reader r.

func NopCloser(r io.Reader) io.ReadCloser {

    return nopCloser{r}

}

這裏使用了一個隱藏實現的技術，go標準庫大量使用。非導出結構體nopCloser有一個匿名成員io.Reader，nopCloser實現了Closer方法，從而nopCloser實現了io.ReadCloser接口；ioutil.NopCloser函數返回nopCloser實現了從io.Reader到io.ReadCloser的適配。對調用者來講，內部的實現細節被隱藏掉了。

Io.nopCloser在net包中有大量的使用，src\net\http\httputil中的一個例子：

// drainBody reads all of b to memory and then returns two equivalent

// ReadClosers yielding the same bytes.

//

// It returns an error if the initial slurp of all bytes fails. It does not attempt

// to make the returned ReadClosers have identical error-matching behavior.

func drainBody(b io.ReadCloser) (r1, r2 io.ReadCloser, err error) {

    if b == http.NoBody {

        // No copying needed. Preserve the magic sentinel meaning of NoBody.

        return http.NoBody, http.NoBody, nil

    }

    var buf bytes.Buffer

    if _, err = buf.ReadFrom(b); err != nil {

        return nil, b, err

    }

    if err = b.Close(); err != nil {

        return nil, b, err

    }

    return ioutil.NopCloser(&buf), ioutil.NopCloser(bytes.NewReader(buf.Bytes())), nil

}

Ioutil.NopCloser將bytes.NewReader構造的io.Reader接口封裝成一個io.ReadCloser接口。

ioutil.Discard

變量定義以下：

// Discard is an io.Writer on which all Write calls succeed

// without doing anything.

var Discard io.Writer = devNull(0)

ioutil.Discard是一個io.Writer變量，對它的全部寫調用都成功，不作任何操做。

func (devNull) Write(p []byte) (int, error) {

    return len(p), nil

}

 

func (devNull) WriteString(s string) (int, error) {

    return len(s), nil

}

ioutil.Discard在標準庫net包和測試用例中有大量使用，如下代碼節選自src\net\http\server.go

    // Per RFC 2616, we should consume the request body before

    // replying, if the handler hasn't already done so. But we

    // don't want to do an unbounded amount of reading here for

    // DoS reasons, so we only try up to a threshold.

    // TODO(bradfitz): where does RFC 2616 say that? See Issue 15527

    // about HTTP/1.x Handlers concurrently reading and writing, like

    // HTTP/2 handlers can do. Maybe this code should be relaxed?

    if w.req.ContentLength != 0 && !w.closeAfterReply {

        var discard, tooBig bool

 

        switch bdy := w.req.Body.(type) {

        case *expectContinueReader:

            if bdy.resp.wroteContinue {

                discard = true

            }

        case *body:

            bdy.mu.Lock()

            switch {

            case bdy.closed:

                if !bdy.sawEOF {

                    // Body was closed in handler with non-EOF error.

                    w.closeAfterReply = true

                }

            case bdy.unreadDataSizeLocked() >= maxPostHandlerReadBytes:

                tooBig = true

            default:

                discard = true

            }

            bdy.mu.Unlock()

        default:

            discard = true

        }

 

        if discard {

            _, err := io.CopyN(ioutil.Discard, w.reqBody, maxPostHandlerReadBytes+1)

            switch err {

            case nil:

                // There must be even more data left over.

                tooBig = true

            case ErrBodyReadAfterClose:

                // Body was already consumed and closed.

            case io.EOF:

                // The remaining body was just consumed, close it.

                err = w.reqBody.Close()

                if err != nil {

                    w.closeAfterReply = true

                }

            default:

                // Some other kind of error occurred, like a read timeout, or

                // corrupt chunked encoding. In any case, whatever remains

                // on the wire must not be parsed as another HTTP request.

                w.closeAfterReply = true

            }

        }

 

        if tooBig {

            w.requestTooLarge()

            delHeader("Connection")

            setHeader.connection = "close"

        }

    }

按RFC2616的規範，http server在回覆以前須要消耗掉全部的request body。

（原創隨筆，轉載請註明出處 http://www.cnblogs.com/majianguo/p/8016426.html）

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