defer

语法:

defer function_name() 

简单来讲，在defer所在函数执行完所有的代码之后，会自动执行defer的这个函数。

示例一（基本功能）

package main
import "fmt"

/*
D:\examples>go run helloworld.go
first
second

D:\examples>
*/
func main() {
    defer second()
    first()
}

func first() {
    fmt.Println("first")
}

func second() {
    fmt.Println("second")
}

示例二（函数局部性）

package main
import "fmt"

/*
D:\examples>go run helloworld.go
first
third
first
second

D:\examples>
*/
func main() {
    defer second()
    first()
    third()
}

func first() {
    fmt.Println("first")
}

func second() {
    fmt.Println("second")
}

func third() {
    defer first()
    fmt.Println("third")
}

示例三(栈特性)

package main
import "fmt"

/*
D:\examples>go run helloworld.go
first
third
second

D:\examples>
*/
func main() {
    defer second()
    defer third()
    first()
}

func first() {
    fmt.Println("first")
}

func second() {
    fmt.Println("second")
}

func third() {
    fmt.Println("third")
}

使用场景

主要用于资源需要释放的场景。比如打开一个文件，最后总是要关闭的。而在打开和关闭之间，会有诸多的处理，可能会有诸多的if-else、根据不同的情况需要提前返回。在传统语言中，return之前都需要一一调用close()。

而Go的defer就将事情变得简单了，open()之后，直接就用defer“注册”一个close()。伪代码：

f, = os.open(filename)
defer f.close()
do_something()
if (condition_a) {return}
do_something_again() 
if (condition_b) {return}
do_further_things()

panic & recover

先给出https://golang.org/pkg/builtin/上的函数说明。

panic

func panic(v interface{})

The panic built-in function stops normal execution of the current goroutine. When a function F calls panic, normal execution of F stops immediately. Any functions whose execution was deferred by F are run in the usual way, and then F returns to its caller. To the caller G, the invocation of F then behaves like a call to panic, terminating G’s execution and running any deferred functions. This continues until all functions in the executing goroutine have stopped, in reverse order. At that point, the program is terminated and the error condition is reported, including the value of the argument to panic. This termination sequence is called panicking and can be controlled by the built-in function recover.

recover

func recover() interface{}

The recover built-in function allows a program to manage behavior of a panicking goroutine. Executing a call to recover inside a deferred function (but not any function called by it) stops the panicking sequence by restoring normal execution and retrieves the error value passed to the call of panic. If recover is called outside the deferred function it will not stop a panicking sequence. In this case, or when the goroutine is not panicking, or if the argument supplied to panic was nil, recover returns nil. Thus the return value from recover reports whether the goroutine is panicking.

要点

• panic相当于一个运行时异常
• 遇到panic的时候，会停止当前函数剩下来的语句，但在退出该函数之前，会执行defer的语句
• 依据函数调用层次，panic依次终止每个函数，直至main()。

panic示例

package main
import "fmt"

/*
D:\examples>go run helloworld.go
f.1
g.1
h.1
h.defer()
g.defer()
panic: panic in h()

goroutine 1 [running]:
panic(0x495360, 0xc04203a230)
        C:/Go/src/runtime/panic.go:500 +0x1af
main.h()
        D:/examples/helloworld.go:54 +0x12b
main.g()
        D:/examples/helloworld.go:45 +0xee
main.f()
        D:/examples/helloworld.go:38 +0xab
main.main()
        D:/examples/helloworld.go:29 +0x1b
exit status 2

D:\examples>
*/
func main() {
    f() // Line Number: 29
}

func final_print(msg string) {
    fmt.Println(msg)
}

func f() {
    fmt.Println("f.1")
    g() // Line Number: 38
    fmt.Println("f.2")
}

func g() {
    defer final_print("g.defer()")
    fmt.Println("g.1")
    h() // Line Number: 45
    fmt.Println("g.2")
}

func h() {
    defer final_print("h.defer()")
    fmt.Println("h.1")
    panic("panic in h()") // Line Number: 52
    fmt.Println("h.2")
}

panic & defer & recover

recover相当于try-catch的catch部分，使得panic不再传递。而defer相当于try-catch-final的final部分。

package main
import "fmt"

/*
D:\examples>go run helloworld.go
f.1
g.1
h.1
h.defer()
g.defer()
panic in h()
f.2

D:\examples>
*/
func main() {
    f()
}

func final_print(msg string) {
    fmt.Println(msg)
}

func f() {
    fmt.Println("f.1")
    g()
    fmt.Println("f.2")
}

func g() {
    defer func() {
          str := recover()
          fmt.Println(str)
    }()

    defer final_print("g.defer()")
    fmt.Println("g.1")
    h()
    fmt.Println("g.2")
}

func h() {
    defer final_print("h.defer()")
    fmt.Println("h.1")
    panic("panic in h()")
    fmt.Println("h.2")
}

获取数组元素

接下来再给一个例子，获取数组元素，处理数组访问越界的问题。

package main
import "fmt"

/*
D:\examples>go run helloworld.go
a[0]=1[true]
a[1]=2[true]
a[2]=3[true]
a[3]=4[true]
a[4]=5[true]
runtime error: index out of range [set to default value -1]
a[5]=-1[false]
runtime error: index out of range [set to default value -1]
a[6]=-1[false]
runtime error: index out of range [set to default value -1]
a[7]=-1[false]
runtime error: index out of range [set to default value -1]
a[8]=-1[false]
runtime error: index out of range [set to default value -1]
a[9]=-1[false]

D:\examples>
*/
func main() {
    a := [5]int {1,2,3,4,5}

    for i := 0; i < 10; i++ {
        item, ok := get(i, a)
        fmt.Printf("a[%d]=%d[%v]\n", i, item, ok)
    }
}

func get(i int, a [5]int) (ret int, ok bool) {
    ok = true 

    defer func() {
        err := recover()
        if err != nil {
            fmt.Println(err, "[set to default value -1]")
            ret = -1
            ok = false
        }
    }() 

    ret = a[i]
    return
}