ioutil包二

（原创随笔，转载请注明出处 http://www.cnblogs.com/majianguo/p/8016426.html）

ioutil包实现了一些I/O实用功能，导出了7个函数和1个变量：

func NopCloser(r io.Reader) io.ReadCloser

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

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

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

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

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

func WriteFile(filename string, data []byte, perm os.FileMode) error

var Discard io.Writer = devNull(0)

前一篇随笔《Golang文件I/O一》介绍了用ReadFile和WriteFile读写文件，本文介绍ioutil包的其他功能。

ioutil.ReadAll

ReadFile和WriteFile可一次性读写文件的全部数据，非常便捷，除去错误判断，只需一行代码即可搞定。ReadAll与ReadFile类似，可一次性从io.Reader接口读取所有内容。

以下是来自godoc的内容：

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

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从r中读取数据直到发生错误或遇到EOF，返回读到的数据。调用成功返回err == nil，而不是err == EOF。因为ReadAll被定义为从源读取数据直到结束（EOF），不会将读取过程中遇到的EOF作为error报告。

godoc中提供了一个ReadAll的Example，从一个strings.NewReader  构造的io.Reader中读取所有数据：

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)

再看另外一个比较实用的例子，发起一个http request，然后从response中读取数据：

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))

}

通过ioutil包的源码可以看到ReadFile和ReadAll调用了同一个函数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在指定位置生成临时文件和临时文件夹。

以下来自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目录新创建以prefix为前缀的临时目录，返回新临时目录的路径。如果dir为空字符串，TempDir使用默认的存放临时文件的目录（os.TempDir，操作系统临时目录）。多个程序同时调用TempDir不会返回相同目录。调用者负责删除不再需要的临时目录。

Godoc中的Example，创建临时目录，退出时调用os.RemoveAll删除:

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)

}

如果dir指定的目录不存在，返回err：

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

Godoc中关于TempFile的说明如下：

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目录中新创建以prefix为前缀的临时文件，打开文件用于读写，返回os.File指针。如果dir为空字符串，TempFile使用默认的存放临时文件的目录（os.TempDir，操作系统临时目录）。多个程序同时调用TempFile不会选择相同的文件。调用者负责删除不再需要的临时文件。

Godoc中关于TempFile的Example，在操作系统临时目录中创建临时文件，退出时调用os.Remove删除临时文件：

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)

}

如果dir不存在，返回err：

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

在Windows中，dir为空字符串时，os.TempDir返回的目录为：

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

ioutil.ReadDir

ioutil.ReadDir可以用于遍历目录。Godoc中关于ReadDir的说明如下：

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目录的目录条目列表，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())

}

略作修改，即可迭代遍历目录：

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函数将io.Reader接口适配为io.ReadCloser接口，为其封装了一个无操作的Close方法。

其源码非常简单；

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}

}

这里使用了一个隐藏实现的技术，go标准库大量使用。非导出结构体nopCloser有一个匿名成员io.Reader，nopCloser实现了Closer方法，从而nopCloser实现了io.ReadCloser接口；ioutil.NopCloser函数返回nopCloser实现了从io.Reader到io.ReadCloser的适配。对调用者来说，内部的实现细节被隐藏掉了。

Io.nopCloser在net包中有大量的使用，src\net\http\httputil中的一个例子：

// 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将bytes.NewReader构造的io.Reader接口封装成一个io.ReadCloser接口。

ioutil.Discard

变量定义如下：

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

// without doing anything.

var Discard io.Writer = devNull(0)

ioutil.Discard是一个io.Writer变量，对它的所有写调用都成功，不做任何操作。

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

    return len(p), nil

}

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

    return len(s), nil

}

ioutil.Discard在标准库net包和测试用例中有大量使用，以下代码节选自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的规范，http server在回复之前需要消耗掉所有的request body。

（原创随笔，转载请注明出处 http://www.cnblogs.com/majianguo/p/8016426.html）