聊聊golang的panic與recover

序

本文主要研究一下golang的panic與recover

panic與recover

recover在以下三種狀況下返回nil

• panic參數爲nil
• goroutine沒有發生panic
• recover不是在defer func中調用

實例

實例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)
}

這裏fc爲nil，於是產生runtime panic，而後被defer的recover捕獲

實例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") 
    }()
}

這裏defer2捕獲了panic，defer1就捕獲不到panic了

實例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")
}

這裏defer2產生了panic，"覆蓋"了原始的panic，defer1捕獲的是最後一個panic即defer2產生的panic

實例4

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

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

    panic("main panic")
}

這裏defer2產生了panic，defer1沒有捕獲，產生的panic依次顯示了main panic及defer2 panic

輸出

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

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

實例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")
}

輸出

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

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

實例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")
}

輸出

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

小結

• recover必須在defer中調用才能夠，多個defer出現了panic沒有recover，則panic信息會列出全部的painc信息
• 若是defer也出現了panic則如有被recover則recover的是最後一個panic，非defer的panic被"覆蓋"
• 若是defer的recover以後出現了panic，則最後panic會出現被recover的那個panic信息

doc

• Handling_panics