Go协程同步的几种方式

为什么要有协程同步？

func main() {    go func() {       fmt.Println("goroutine 1")    }()    go func() {       fmt.Println("goroutine 2")    }() }

执行以上代码块，发现没有打印结果，原因就是主协程main在goroutine 1和goroutine 2执行之前已经先行结束，协程没有同步。

1.让主协程休息一段时间，等待其他协程执行完成后再结束

func main() {    go func() {       fmt.Println("goroutine 1")    }()    go func() {       fmt.Println("goroutine 2")    }()    time.Sleep(time.Second) }

缺点是不知道需要等待多长时间

2.channel

func main() {    ch := make(chan struct{})    go func() {       fmt.Println("goroutine 1")       ch <- struct{}{}    }()    go func() {       fmt.Println("goroutine 2")       ch <- struct{}{}    }()    <-ch    <-ch }

channel和goroutine是分不开的，搭配干活不累。

3.sync.WaitGroup

Add() 用来添加计数Done() 用来在操作结束时调用，使计数减一Wait() 用来等待所有的操作结束，即计数变为 0，该函数会在计数不为 0 时等待，在计数为 0 时立即返回

func main() {    var wg sync.WaitGroup    wg.Add(2)    go func() {       fmt.Println("goroutine 1")       wg.Done()    }()    go func() {       fmt.Println("goroutine 2")       wg.Done()    }()    wg.Wait() }

关于WaitGroup源码如下：

// A WaitGroup waits for a collection of goroutines to finish. // The main goroutine calls Add to set the number of // goroutines to wait for. Then each of the goroutines // runs and calls Done when finished. At the same time, // Wait can be used to block until all goroutines have finished. // // A WaitGroup must not be copied after first use. type WaitGroup struct {    noCopy noCopy    // 64-bit value: high 32 bits are counter, low 32 bits are waiter count.    // 64-bit atomic operations require 64-bit alignment, but 32-bit    // compilers do not ensure it. So we allocate 12 bytes and then use    // the aligned 8 bytes in them as state, and the other 4 as storage    // for the sema.    state1 [3]uint32 }

一个WaitGroup等待多个goroutine执行完成，main的goroutine可以调用Add()方法设置需要等待的goroutine数量，之后每一个goroutine在运行结束时调用Done()，在这段时间内，我们可以使用Wait()阻塞main的goroutine直到所有的goroutine都执行完成，WaitGroup不能进行复制操作【struct里面有noCopy类型，禁止做值拷贝，只能通过指针来传递】