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本文主要研究一下golang的panic与recover

panic与recover

recover在如下三种情况下返回nil

• panic参数为nil
• goroutine没有发生panic
• recover不是在defer func中调用

实例

实例1

var fc func() string

func protect(g func() string) {
    defer func() {
        fmt.Println("done") // Println executes normally even if there is a panic
        if x := recover(); x != nil {
            fmt.Printf("run time panic: %v", x)
        }
    }()
    fmt.Println("start")
    fmt.Println(g())
}
func main() {
    protect(fc)
}

这里fc为nil，因而产生runtime panic，然后被defer的recover捕获

实例2

func deferRecover() {
    defer func() {
        fmt.Println("defer1: recover")
        if err := recover(); err != nil {
            fmt.Println(err)
        } else {
            fmt.Println("defer1 recover nil")
        }
    }()

    defer func() {
        fmt.Println("defer2: recover ")
        if err := recover(); err != nil {
            fmt.Println(err)
        }
    }()

    panic("manual panic")

    defer func() { 
        fmt.Println("never be executed") 
    }()
}

这里defer2捕获了panic，defer1就捕获不到panic了

实例3

func paincInDefer() {
    defer func() {
        fmt.Println("defer1 recover")
        if err := recover(); err != nil {
            // main panic is override by defer2 panic
            fmt.Println(err)
        } else {
            fmt.Println("defer1 recover nil")
        }
    }()

    defer func() {
        fmt.Println("defer2 begin to panic")
        panic("defer2 panic")
    }()

    panic("main panic")
}

这里defer2产生了panic，"覆盖"了原始的panic，defer1捕获的是最后一个panic即defer2产生的panic

实例4

func paincInDeferNotRecover() {
    defer func() {
        fmt.Println("defer1")
    }()

    defer func() {
        fmt.Println("defer2 begin to panic")
        panic("defer2 panic")
    }()

    panic("main panic")
}

这里defer2产生了panic，defer1没有捕获，产生的panic依次显示了main panic及defer2 panic

输出

defer2 begin to panic
defer1
panic: main panic
        panic: defer2 panic

goroutine 1 [running]:
main.paincInDeferNotRecover.func2()

实例5

func multiPaincInDefer() {
    defer func() {
        fmt.Println("defer1")
        panic("defer3 panic")
    }()

    defer func() {
        fmt.Println("defer2 begin to panic")
        panic("defer2 panic")
    }()

    panic("main panic")
}

输出

defer2 begin to panic
defer1
panic: main panic
        panic: defer2 panic
        panic: defer3 panic

goroutine 1 [running]:
main.multiPaincInDefer.func1()

实例6

func panicAfterRecover() {
    defer func() {
        fmt.Println("defer1")
        if err := recover(); err != nil {
            panic("defer1 panic after recover")
        } else {
            fmt.Println("defer1 recover nil")
        }
    }()

    defer func() {
        fmt.Println("defer2 begin to panic")
        panic("defer2 panic")
    }()

    panic("main panic")
}

输出

defer2 begin to panic
defer1
panic: main panic
        panic: defer2 panic [recovered]
        panic: defer1 panic after recover

goroutine 1 [running]:
main.panicAfterRecover.func1()
        /defer.go:180 +0x10d
panic(0x10b2020, 0x10eaed0)
        /usr/local/go/src/runtime/panic.go:969 +0x1b9
main.panicAfterRecover.func2()
        /defer.go:188 +0x95
panic(0x10b2020, 0x10eaee0)
        /usr/local/go/src/runtime/panic.go:969 +0x1b9
main.panicAfterRecover()
        /defer.go:191 +0x68
main.main()
        /defer.go:215 +0x25
exit status 2

小结

• recover必须在defer中调用才可以，多个defer出现了panic没有recover，则panic信息会列出所有的painc信息
• 如果defer也出现了panic则若有被recover则recover的是最后一个panic，非defer的panic被"覆盖"
• 如果defer的recover之后出现了panic，则最后panic会出现被recover的那个panic信息

doc

• Handling_panics