Golang bytes.buffer详解

原文：https://www.jianshu.com/p/e53083132a25

Buffer 介绍
Buffer 是 bytes 包中的一个 type Buffer struct{…}
A buffer is a variable-sized buffer of bytes with Read and Write methods. The zero value for Buffer is an empty buffer ready to use.
（是一个变长的 buffer，具有 Read 和Write 方法。 Buffer 的 零值 是一个 空的 buffer，但是可以使用）
Buffer 就像一个集装箱容器，可以存东西，取东西（存取数据）

创建缓冲器

func main() {
    buf1 := bytes.NewBufferString("hello")
    buf2 := bytes.NewBuffer([]byte("hello"))
    buf3 := bytes.NewBuffer([]byte{'h','e','l','l','o'})
    fmt.Printf("%v,%v,%v
",buf1,buf2,buf3)
    fmt.Printf("%v,%v,%v
",buf1.Bytes(),buf2.Bytes(),buf3.Bytes())

    buf4 := bytes.NewBufferString("")
    buf5 := bytes.NewBuffer([]byte{})
    fmt.Println(buf4.Bytes(),buf5.Bytes())
}

输出

hello,hello,hello
[104 101 108 108 111],[104 101 108 108 111],[104 101 108 108 111]
[] []

写入到缓冲器
buffer在new的时候是空的，也是可以直接Write的

Write

func (b *Buffer) Write(p []byte) (n int,err error)

func main() {
s := []byte(" world")
buf := bytes.NewBufferString("hello")
fmt.Printf("%v,%v
",buf.String(),buf.Bytes())
buf.Write(s)
fmt.Printf("%v,%v
",buf.String(),buf.Bytes())
}

结果

hello,[104 101 108 108 111]
hello world,[104 101 108 108 111 32 119 111 114 108 100]

WriteString

func (b *Buffer) WriteString(s string)(n int,err error)

func main() {
    s := " world"
    buf := bytes.NewBufferString("hello")
    fmt.Printf("%v,%v
",buf.String(),buf.Bytes())
    buf.WriteString(s)
    fmt.Printf("%v,%v
",buf.String(),buf.Bytes())
}

结果

hello,[104 101 108 108 111]
hello world,[104 101 108 108 111 32 119 111 114 108 100]

缓冲区原理介绍
go字节缓冲区底层以字节切片做存储，切片存在长度len与容量cap, 缓冲区写从长度len的位置开始写，当len>cap时，会自动扩容。缓冲区读会从内置标记off位置开始读(off始终记录读的起始位置)，当off==len时，表明缓冲区已全部读完
并重置缓冲区(len=off=0),此外当将要内容长度+已写的长度(即len) <= cap/2时，缓冲区前移覆盖掉已读的内容(off=0，len-=off)，从避免缓冲区不断扩容

func main() {
    byteSlice := make([]byte, 20) 
    byteSlice[0] = 1                                  // 将缓冲区第一个字节置1
    byteBuffer := bytes.NewBuffer(byteSlice)          // 创建20字节缓冲区 len = 20 off = 0
    c, _ := byteBuffer.ReadByte()                     // off+=1
    fmt.Printf("len:%d, c=%d
", byteBuffer.Len(), c) // len = 20 off =1   打印c=1
    byteBuffer.Reset()                                // len = 0 off = 0
    fmt.Printf("len:%d
", byteBuffer.Len())          // 打印len=0
    byteBuffer.Write([]byte("hello byte buffer"))     // 写缓冲区  len+=17
    fmt.Printf("len:%d
", byteBuffer.Len())          // 打印len=17
    byteBuffer.Next(4)                                // 跳过4个字节 off+=4
    c, _ = byteBuffer.ReadByte()                      // 读第5个字节 off+=1
    fmt.Printf("第5个字节:%d
", c)                    // 打印:111(对应字母o)    len=17 off=5
    byteBuffer.Truncate(3)                            // 将未字节数置为3        len=off+3=8   off=5
    fmt.Printf("len:%d
", byteBuffer.Len())          // 打印len=3为未读字节数  上面len=8是底层切片长度
    byteBuffer.WriteByte(96)                          // len+=1=9 将y改成A
    byteBuffer.Next(3)                                // len=9 off+=3=8
    c, _ = byteBuffer.ReadByte()                      // off+=1=9    c=96
    fmt.Printf("第9个字节:%d
", c)                    // 打印:96
}

 demo1

package main

import "bytes"
import "log"

func main(){
    buf := &bytes.Buffer{}
    buf.WriteString("abcdefg")

    str, _ := buf.ReadString('?')
    log.Println("str = ", str)
    log.Println("buff = ", buf.String())
}

输出：

[root@localhost]# go run buffer.go 
2020/06/05 19:12:42 str =  abcdefg
2020/06/05 19:12:42 buff =  

 demo2

// MyBuffer2 project main.go
package main

import (
    "bytes"
    "log"
    "os"
)

func main() {
    log.SetFlags(log.Lshortfile)
    buff := bytes.NewBufferString("123456789")
    log.Println("buff = ", buff.String())

    s := make([]byte, 4)
    n, _ := buff.Read(s)
    log.Println("buff = ", buff.String())
    log.Println("s = ", string(s))
    log.Println("n = ", n)

    n, _ = buff.Read(s)
    log.Println("buff = ", buff.String())
    log.Println("s = ", string(s))
    log.Println("n = ", n)

    n, _ = buff.Read(s)
    log.Println("buff = ", buff.String())
    log.Println("s = ", string(s))
    log.Println("n = ", n)

    buff.Reset()
    buff.WriteString("abcdefg")
    log.Println("buff = ", buff.String())

    b, _ := buff.ReadByte()
    log.Println("b = ", string(b))
    log.Println("buff = ", buff.String())

    b, _ = buff.ReadByte()
    log.Println("b = ", string(b))
    log.Println("buff = ", buff.String())

    bs, _ := buff.ReadBytes('e')
    log.Println("bs = ", string(bs))
    log.Println("buff = ", buff.String())

    buff.Reset()
    buff.WriteString("编译输出GO")
    r, l, _ := buff.ReadRune()
    log.Println("r = ", r, ", l = ", l, ", string(r) = ", string(r))

    buff.Reset()
    buff.WriteString("qwer")
    str, _ := buff.ReadString('?')
    log.Println("str = ", str)
    log.Println("buff = ", buff.String())

    buff.WriteString("qwer")
    str, _ = buff.ReadString('w')
    log.Println("str = ", str)
    log.Println("buff = ", buff.String())

    file, _ := os.Open("doc.go")
    buff.Reset()
    buff.ReadFrom(file)
    log.Println("doc.go = ", buff.String())

    buff.Reset()
    buff.WriteString("中国人")
    cbyte := buff.Bytes()
    log.Println("cbyte = ", cbyte)
}

输出：

C:/Go/bin/go.exe build -i [D:/golang/src/MyBuffer2]
成功: 进程退出代码 0.
D:/golang/src/MyBuffer2/MyBuffer2.exe  [D:/golang/src/MyBuffer2]
main.go:13: buff =  123456789
main.go:17: buff =  56789
main.go:18: s =  1234
main.go:19: n =  4
main.go:22: buff =  9
main.go:23: s =  5678
main.go:24: n =  4
main.go:27: buff =  
main.go:28: s =  9678
main.go:29: n =  1
main.go:33: buff =  abcdefg
main.go:36: b =  a
main.go:37: buff =  bcdefg
main.go:40: b =  b
main.go:41: buff =  cdefg
main.go:44: bs =  cde
main.go:45: buff =  fg
main.go:50: r =  32534 , l =  3 , string(r) =  编
main.go:55: str =  qwer
main.go:56: buff =  
main.go:60: str =  qw
main.go:61: buff =  er
main.go:66: doc.go =  // MyBuffer2 project doc.go

/*
MyBuffer2 document
*/
package main

main.go:71: cbyte =  [228 184 173 229 155 189 228 186 186]
成功: 进程退出代码 0.