web server

建立一個簡單的 http 伺服器，使用 http.ListenAndServe 啟動一個 server 監聽 8081 port，
http.HandleFunc("/", handler) 代表 url 路徑以 / 開頭的 request 都會被 handler function 處理．
http.Request 的參數 r.URL.Path 可以取得 url 的路徑．

package main

import (
	"net/http"
)

func main() {
	http.HandleFunc("/", handler)
	http.ListenAndServe(":8081", nil)

}
func handler(w http.ResponseWriter, r *http.Request) {
	message := "Request URL Path is " + r.URL.Path
	w.Write([]byte(message))
}

使用 curl 指令測試，r.URL.Path 會印出 http://localhost:8081 之後的路徑

> curl http://localhost:8081/
Request URL Path is /

> curl http://localhost:8081/hello
Request URL Path is /hello

> curl http://localhost:8081/hello/Daniel
Request URL Path is /hello/Daniel

計算呼叫網站的總數

宣告一個全域變數 count 並在 handler 加上 count++ 每次呼叫網站時總數就加 1，
並另外開一個 count 的 function，當有人呼叫 /count 這 url 時就會顯示目前網站被呼叫的次數．

package main

import (
	"net/http"
	"strconv"
)

var count int

func main() {
	http.HandleFunc("/", handler)
	http.HandleFunc("/count", counter)
	http.ListenAndServe(":8081", nil)

}
func handler(w http.ResponseWriter, r *http.Request) {
	message := "Request URL Path is " + r.URL.Path
	count++
	w.Write([]byte(message))
}
func counter(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte(strconv.Itoa(count)))
}

使用 go build 建一個執行檔，並執行它

go build example.go
./example

一開始的總數是 0

> curl http://localhost:8081/count
0

然後再寫一隻 Main 程式，並使用 goroutine 發送 http.Get 1000 次，
所以 count 應該要是 1000

package main

import (
	"fmt"
	"io/ioutil"
	"net/http"
	"sync"
)

var count int
var w sync.WaitGroup

func main() {
	for i := 0; i < 1000; i++ {
		w.Add(1)
		go sendRequest()
	}
	w.Wait()
}
func sendRequest() {
	defer w.Done()
	resp, err := http.Get("http://localhost:8081/Hello")
	if err != nil {
		fmt.Println(err)
	}

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		fmt.Println(err)
	}

	fmt.Println(string(body))
}

執行結果

Request URL Path is /Hello
Request URL Path is /Hello
...

然後去 get count 的總數時會發現並不是 1000

> curl http://localhost:8081/count
986

在同時有很多 user 在使用網站時，在使用共用變數時，讀或寫也都會遇到 race condition 的問題．
所以在 handler 與 counter function 裡，都要加上之前介紹的 sync.Mutex 來 Lock 與 Unlock 會對共用變數有讀寫的操作．

package main

import (
	"net/http"
	"strconv"
	"sync"
)

var count int
var m sync.Mutex

func main() {
	http.HandleFunc("/", handler)
	http.HandleFunc("/count", counter)
	http.ListenAndServe(":8081", nil)

}
func handler(w http.ResponseWriter, r *http.Request) {
	message := "Request URL Path is " + r.URL.Path
	m.Lock()
	count++
	m.Unlock()
	w.Write([]byte(message))
}
func counter(w http.ResponseWriter, r *http.Request) {
	m.Lock()
	w.Write([]byte(strconv.Itoa(count)))
	m.Unlock()
}

取得 request 參數

使用 http.Request.URL.Query().Get("num") 取得 num 的參數．

package main

import (
	"net/http"
	"strconv"
	"sync"
)

var count int
var m sync.Mutex

func main() {
	http.HandleFunc("/", handler)
	http.HandleFunc("/count", counter)
	http.HandleFunc("/add", adder)
	http.ListenAndServe(":8081", nil)

}
func handler(w http.ResponseWriter, r *http.Request) {
	message := "Request URL Path is " + r.URL.Path
	m.Lock()
	count++
	m.Unlock()
	w.Write([]byte(message))
}
func adder(w http.ResponseWriter, r *http.Request) {
	num := r.URL.Query().Get("num")
	addNum, _ := strconv.Atoi(num)
	m.Lock()
	count += addNum
	m.Unlock()
	w.Write([]byte(num))
}
func counter(w http.ResponseWriter, r *http.Request) {
	m.Lock()
	w.Write([]byte(strconv.Itoa(count)))
	m.Unlock()
}

呼叫 add 並給參數 num = 5

> curl http://localhost:8081/add\?num\=5
5

取得 count 為 5

curl http://localhost:8081/count
5