Go 和 Colly笔记

Colly是Go下功能比较完整的一个HTTP客户端工具.

安装

使用GoLand作为开发环境

GOROOT: go目录放到了/opt/go, 所以GOROOT默认指向的也是/opt/go

GOPATH: 在Settings->Go->GOPATH里配置Global GOPATH, 指向 /home/milton/WorkGo

GOPROXY: 在Settings->Go->Go Modules下, 设置 Environments, GOPROXY=https://goproxy.cn

在GoLand内部的Terminal里查看环境变量, 命令 go env, 确认路径无误, 然后执行以下命令安装

# v1
go get -u github.com/gocolly/colly

# v2
go get -u github.com/gocolly/colly/v2

基础使用

增加import

import "github.com/gocolly/colly/v2"

调用

func main() {
	// Instantiate default collector
	c := colly.NewCollector(
		// Visit only domains: hackerspaces.org, wiki.hackerspaces.org
		colly.AllowedDomains("hackerspaces.org", "wiki.hackerspaces.org"),
	)

	// On every a element which has href attribute call callback
	c.OnHTML("a[href]", func(e *colly.HTMLElement) {
		link := e.Attr("href")
		// Print link
		fmt.Printf("Link found: %q -> %s
", e.Text, link)
		// Visit link found on page
		// Only those links are visited which are in AllowedDomains
		c.Visit(e.Request.AbsoluteURL(link))
	})

	// Before making a request print "Visiting ..."
	c.OnRequest(func(r *colly.Request) {
		fmt.Println("Visiting", r.URL.String())
	})

	// Start scraping on https://hackerspaces.org
	c.Visit("https://hackerspaces.org/")
}

　　

使用代理池

参考文档中的例子 http://go-colly.org/docs/examples/proxy_switcher/  这里的例子要注意两个问题

1. 初始化时, 需要设置AllowURLRevisit, 否则在访问同一URL时会直接跳过返回之前的结果

c := colly.NewCollector(colly.AllowURLRevisit())

2. 还需要设置禁用KeepAlive, 否则在多次访问同一网址时, 只会调用一次GetProxy, 这样达不到轮询代理池的效果, 相关信息 #392, #366 , #339 

c := colly.NewCollector(colly.AllowURLRevisit())

c.WithTransport(&http.Transport{
	DisableKeepAlives: true,
})

Golang里的协程同步(等价于Java中的锁)

Mutex

在Go程序中为解决Race Condition和Data Race问题, 使用Mutex来锁定资源只能同时被一个协程调用, 通过 &sync.Mutex() 创建一个全局变量, 在子方法里面通过Lock()和Unlock()锁定和释放资源. 注意defer关键字的使用.

import (
	"strconv"
	"sync"
)

var myBalance = &balance{amount: 50.00, currency: "GBP"}

type balance struct {
	amount   float64
	currency string
	mu       sync.Mutex
}

func (b *balance) Add(i float64) {
	b.mu.Lock()
	b.amount += i
	b.mu.Unlock()
}

func (b *balance) Display() string {
	b.mu.Lock()
	defer b.mu.Unlock()
	return strconv.FormatFloat(b.amount, 'f', 2, 64) + " " + b.currency
}

读写锁使用RWMutex, 在Mutex的基础上, 增加了RLock()和RUnlock()方法. 在Lock()时依然是互斥的, 但是RLock()与RLock()之间不互斥

import (
	"strconv"
	"sync"
)

var myBalance = &balance{amount: 50.00, currency: "GBP"}

type balance struct {
	amount   float64
	currency string
	mu       sync.RWMutex
}

func (b *balance) Add(i float64) {
	b.mu.Lock()
	b.amount += i
	b.mu.Unlock()
}

func (b *balance) Display() string {
	b.mu.RLock()
	defer b.mu.RUnlock()
	return strconv.FormatFloat(b.amount, 'f', 2, 64) + " " + b.currency
}

 Channel

Channel类似于Java中的Semaphore, 通过设置channel容量限制同时工作的协程数, channel满了之后协程会被阻塞

package main                                                                                                                                                           

import (
    "fmt"
    "time"
    "strconv"
)

func makeCakeAndSend(cs chan string) {
    for i := 1; i<=3; i++ {
        cakeName := "Strawberry Cake " + strconv.Itoa(i)
        fmt.Println("Making a cake and sending ...", cakeName)
        cs <- cakeName //send a strawberry cake
    }   
}

func receiveCakeAndPack(cs chan string) {
    for i := 1; i<=3; i++ {
        s := <-cs //get whatever cake is on the channel
        fmt.Println("Packing received cake: ", s)
    }   
}

func main() {
    cs := make(chan string)
    go makeCakeAndSend(cs)
    go receiveCakeAndPack(cs)

    //sleep for a while so that the program doesn’t exit immediately
    time.Sleep(4 * 1e9)
}

 可以设置channel的容量 

c := make(chan Type, n)