c := make(chan int) // 创建一个无缓冲(unbuffered)的int类型的channel
c := make(chan int, 5) // 创建一个带缓冲的int类型的channel
c <- x // 向channel c中发送一个值
<- c // 从channel c中接收一个值
x = <- c // 从channel c接收一个值并将其存储到变量x中
x, ok = <- c // 从channel c中接收一个值。若channel关闭了,ok将置为false
for i := range c { ... } // 将for range与channel结合使用
close(c) // 关闭channel c
c := make(chan chan int) // 创建一个无缓冲的chan int类型的channel
func stream(ctx context.Context, out chan<- Value) error //将只发送(send-only)channel作为函数参数
func spawn(...) <-chan T //将只接收(receive-only)channel作为返回值
34.1 无缓冲channel
- 无缓冲channel的接收和发送操作是同步的,单方面的操作会让对应的goroutine陷入阻塞状态
- 发送动作一定在接收动作完成之前
- 接收动作一定在发送动作完成之前
var c = make(chan int)
var a string
func f() {
a = "hello, world"
<-c
}
func main() {
go f()
c <- 5
println(a) // 输出:hello, world
}
1.用户信号传递
(1) 一对一通知信号
main goroutine在调用spawn函数后一直阻塞在对这个通知信号的接收动作上
type signal struct{}
func worker() {
println("worker is working...")
time.Sleep(1 * time.Second)
}
func spawn(f func()) <-chan signal {
c := make(chan signal)
go func() {
println("worker start to work...")
f()
c <- signal(struct{}{})
}()
return c
}
func main() {
println("start a worker...")
c := spawn(worker)
<-c
fmt.Println("worker work done!")
}
(2) 一对多通知信号
main goroutine通过close(groupSignal)向所有worker goroutine广播“开始工作”的信号
type signal struct{}
func worker(i int) {
fmt.Printf("worker %d: is working...\n", i)
time.Sleep(1 * time.Second)
fmt.Printf("worker %d: works done\n", i)
}
func spawnGroup(f func(i int), num int, groupSignal <-chan signal) <-chan signal {
c := make(chan signal)
var wg sync.WaitGroup
for i := 0; i < num; i++ {
wg.Add(1)
go func(i int) {
<-groupSignal
fmt.Printf("worker %d: start to work...\n", i)
f(i)
wg.Done()
}(i + 1)
}
go func() {
wg.Wait()
c <- signal(struct{}{})
}()
return c
}
func main() {
fmt.Println("start a group of workers...")
groupSignal := make(chan signal)
c := spawnGroup(worker, 5, groupSignal)
time.Sleep(5 * time.Second)
fmt.Println("the group of workers start to work...")
close(groupSignal)
<-c
fmt.Println("the group of workers work done!")
}
通知一组worker goroutine退出
type signal struct{}
func worker(i int, quit <-chan signal) {
fmt.Printf("worker %d: is working...\n", i)
LOOP:
for {
select {
default:
// 模拟worker工作
time.Sleep(1 * time.Second)
case <-quit:
break LOOP
}
}
fmt.Printf("worker %d: works done\n", i)
}
func spawnGroup(f func(int, <-chan signal), num int, groupSignal <-chan signal) <-chan signal {
c := make(chan signal)
var wg sync.WaitGroup
for i := 0; i < num; i++ {
wg.Add(1)
go func(i int) {
fmt.Printf("worker %d: start to work...\n", i)
f(i, groupSignal)
wg.Done()
}(i + 1)
}
go func() {
wg.Wait()
c <- signal(struct{}{})
}()
return c
}
func main() {
fmt.Println("start a group of workers...")
groupSignal := make(chan signal)
c := spawnGroup(worker, 5, groupSignal)
fmt.Println("the group of workers start to work...")
time.Sleep(5 * time.Second)
// 通知workers退出
fmt.Println("notify the group of workers to exit...")
close(groupSignal)
<-c
fmt.Println("the group of workers work done!")
}
2.用于替代锁机制
传统的基于共享内存+锁模式的goroutine安全的计数器实现
type counter struct {
sync.Mutex
i int
}
var cter counter
func Increase() int {
cter.Lock()
defer cter.Unlock()
cter.i++
return cter.i
}
func main() {
for i := 0; i < 10; i++ {
go func(i int) {
v := Increase()
fmt.Printf("goroutine-%d: current counter value is %d\n", i, v)
}(i)
}
time.Sleep(5 * time.Second)
}
无缓存channel代替锁(通过通信来共享内存)
type counter struct {
c chan int
i int
}
var cter counter
func InitCounter() {
cter = counter{
c: make(chan int),
}
go func() {
for {
cter.i++
cter.c <- cter.i
}
}()
fmt.Println("counter init ok")
}
func Increase() int {
return <-cter.c
}
func init() {
InitCounter()
}
func main() {
for i := 0; i < 10; i++ {
go func(i int) {
v := Increase()
fmt.Printf("goroutine-%d: current counter value is %d\n", i, v)
}(i)
}
time.Sleep(5 * time.Second)
}
34.2 带缓冲channel
- 对带缓冲channel的发送操作在缓存区未满、接收操作在缓冲区非空的情况下是异步的(发送或接收无须阻塞等待)
- 对一个带缓冲的channel
- 在缓冲区无数据或有数据但未满的情况下,对其进行发送操作的goroutine不会阻塞
- 在缓冲区已满的情况下,对其进行发送操作的goroutine会阻塞
- 在缓冲区为空的情况下,对其进行接收操作的goroutine亦会阻塞
1.用作消息队列
- 无论是单收单发、还是多收多发,带缓存channel的收发性能都要好于无缓存channel
- 对于带缓冲channel而言,选择适当容量会在一定程度上提升收发性能
2.用作计数信号量
同时允许处于活动状态的最大goroutine数量为3
var active = make(chan struct{}, 3)
var jobs = make(chan int, 10)
func main() {
go func() {
for i := 0; i < 8; i++ {
jobs <- (i + 1)
}
close(jobs)
}()
var wg sync.WaitGroup
for j := range jobs {
wg.Add(1)
go func(j int) {
active <- struct{}{}
log.Printf("handle job: %d\n", j)
time.Sleep(2 * time.Second)
<-active
wg.Done()
}(j)
}
wg.Wait()
}
3.len(channel)的应用
- len(channel)语义
- 当s为无缓冲channel时,len(s)总是返回0
- 当s为带缓冲channel时,len(s)返回当前channel s中尚未被读取的元素个数
使用select原语的default分支语义,当channel空的时候,tryRecv不会阻塞;当channel满的时候,trySend也不会阻塞
- 有一个问题是这种方法改变了channel的状态:接收或者发送了一个元素
- 特定的场景下,可以用len(channel)来实现
- 多发送单接收的场景,即有多个发送者,但只有一个接收者。可以在接收者goroutine中根据len(channel)是否大于0来判断channel中是否有数据需要接收
- 多接收单发送的场景,即有多个接收者,但只有一个发送者。可以在发送者goroutine中根据len(channel)是否小于cap(channel)
func producer(c chan<- int) {
var i int = 1
for {
time.Sleep(2 * time.Second)
ok := trySend(c, i)
if ok {
fmt.Printf("[producer]: send [%d] to channel\n", i)
i++
continue
}
fmt.Printf("[producer]: try send [%d], but channel is full\n", i)
}
}
func tryRecv(c <-chan int) (int, bool) {
select {
case i := <-c:
return i, true
default:
return 0, false
}
}
func trySend(c chan<- int, i int) bool {
select {
case c <- i:
return true
default:
return false
}
}
func consumer(c <-chan int) {
for {
i, ok := tryRecv(c)
if !ok {
fmt.Println("[consumer]: try to recv from channel, but the channel is empty")
time.Sleep(1 * time.Second)
continue
}
fmt.Printf("[consumer]: recv [%d] from channel\n", i)
if i >= 3 {
fmt.Println("[consumer]: exit")
return
}
}
}
func main() {
c := make(chan int, 3)
go producer(c)
go consumer(c)
select {} // 故意阻塞在此
}
34.3 nil channel的妙用
对没有初始化的channel(nil channel)进行读写操作将会发生阻塞
func main() {
c1, c2 := make(chan int), make(chan int)
go func() {
time.Sleep(time.Second * 5)
c1 <- 5
close(c1)
}()
go func() {
time.Sleep(time.Second * 7)
c2 <- 7
close(c2)
}()
var ok1, ok2 bool
for {
select {
case x := <-c1:
ok1 = true
fmt.Println(x)
case x := <-c2:
ok2 = true
fmt.Println(x)
}
if ok1 && ok2 {
break
}
}
fmt.Println("program end")
}
显式地将c1或c2置为nil,利用对一个nil channel执行获取操作,该操作将被阻塞的特性,已经被置为nil的c1或c2的分支将再也不会被select选中执行
func main() {
c1, c2 := make(chan int), make(chan int)
go func() {
time.Sleep(time.Second * 5)
c1 <- 5
close(c1)
}()
go func() {
time.Sleep(time.Second * 7)
c2 <- 7
close(c2)
}()
for {
select {
case x, ok := <-c1:
if !ok {
c1 = nil
} else {
fmt.Println(x)
}
case x, ok := <-c2:
if !ok {
c2 = nil
} else {
fmt.Println(x)
}
}
if c1 == nil && c2 == nil {
break
}
}
fmt.Println("program end")
}
34.4 与select结合使用的一些惯用法
1.利用default分支避免阻塞
// $GOROOT/src/time/sleep.go
func sendTime(c interface{}, seq uintptr) {
// 无阻塞地向c发送当前时间
...
select {
case c.(chan Time) <- Now():
default:
}
}
2.实现超时机制
要注意timer使用后的释放
- timer实质上是由Go运行时自行维护的,不是操作系统的定时器资源。Go启动了一个单独goroutine来维护一个“最小堆”,定期唤醒并读取堆顶的timer对象,执行完毕后删除
- timer.Timer实则是在这个最小堆中添加一个timer对象实例,而调用timer.Stop方法则是从堆中删除对应的timer对象
func worker() {
select {
case <- c:
//...
case <-time.After(30 * time.Second):
return
}
}
3.实现心跳机制
func wokrer() {
heartbeat := time.NewTicker(30 * time.second)
defer heartbeat.Stop()
for {
select {
case <-c:
//处理业务逻辑
case <- heartbeat.C:
//处理心跳
}
}
}
标签:读书笔记,int,goroutine,chan,34,func,time,Go,channel
From: https://www.cnblogs.com/brynchen/p/18031170