[Go语言]我的第十一个Go语言程序_go程序 type source struct-优快云博客

本文介绍了一个使用Go语言实现的简单并发网页爬虫程序。该爬虫能够并行抓取网页，并通过递归方式深入挖掘链接，同时避免重复抓取同一页面。文章展示了如何利用Go的并发特性来提高爬虫效率。

Exercise: Web Crawler

In this exercise you'll use Go's concurrency features to parallelize a web crawler.

Modify the Crawl function to fetch URLs in parallel without fetching the same URL twice.

Hint: you can keep a cache of the URLs that have been fetched on a map, but maps alone are not safe for concurrent use!

package main

import (
	"fmt"
	"sync"
	"time"
)

type Fetcher interface {
	// Fetch returns the body of URL and
	// a slice of URLs found on that page.
	Fetch(url string) (body string, urls []string, err error)
}

type Fetched struct {
     v map[string]bool
	 mux sync.Mutex
}

var fetched = Fetched{v : make(map[string]bool)}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	if depth <= 0 {
		return
	}
	
	fetched.mux.Lock()
	defer fetched.mux.Unlock()
	if _, ok := fetched.v[url]; !ok {
	    body, urls, err := fetcher.Fetch(url)
		fetched.v[url] = true;
	    if err != nil {
	    	fmt.Println(err)
		    return
	    }
	    fmt.Printf("found: %s %q\n", url, body)
	    for _, u := range urls {
		    go Crawl(u, depth-1, fetcher)
	    }
	}
	
	return
}

func main() {
	Crawl("http://golang.org/", 4, fetcher)
	time.Sleep(time.Second)
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
	body string
	urls []string
}

func (f fakeFetcher) Fetch(url string) (string, []string, error) {
	if res, ok := f[url]; ok {
		return res.body, res.urls, nil
	}
	return "", nil, fmt.Errorf("not found: %s", url)
}

// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
	"http://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"http://golang.org/pkg/",
			"http://golang.org/cmd/",
		},
	},
	"http://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"http://golang.org/",
			"http://golang.org/cmd/",
			"http://golang.org/pkg/fmt/",
			"http://golang.org/pkg/os/",
		},
	},
	"http://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/",
		},
	},
	"http://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/",
		},
	},
}