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# 13.3 从错误中恢复 (Recover) | ||
# 13.3 从 panic 中恢复 (Recover) | ||
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正如名字一样,这个(recover)内建函数被用于从 panic 或 错误场景中恢复:让程序可以从 panicking 重新获得控制权,停止终止过程进而恢复正常执行。 | ||
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`recover` 只能在 defer 修饰的函数(参见 6.4 节)中使用:用于取得 panic 调用中传递过来的错误值,如果是正常执行,调用 `recover` 会返回 nil,且没有其它效果。 | ||
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<u>总结</u>:panic 会导致栈被展开直到 defer 修饰的 recover() 被调用或者程序中止。 | ||
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下面例子中的 protect 函数调用函数参数 g 来保护调用者防止从 g 中抛出的运行时 panic,并展示 panic 中的信息: | ||
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```go | ||
func protect(g func()) { | ||
defer func() { | ||
log.Println(“done”) | ||
// Println executes normally even if there is a panic | ||
if err := recover(); err != nil { | ||
log.Printf(“run time panic: %v”, err) | ||
} | ||
}() | ||
log.Println(“start”) | ||
g() // possible runtime-error | ||
} | ||
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``` | ||
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这跟 Java 和 .NET 这样的语言中的 catch 块类似。 | ||
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log 包实现了简单的日志功能:默认的 log 对象向标准错误输出中写入并打印每条日志信息的日期和时间。除了 `Println` 和 `Printf` 函数,其它的致命性函数都会在写完日志信息后调用 os.Exit(1),那些退出函数也是如此。而 Panic 效果的函数会在写完日志信息后调用 panic;可以在程序必须中止或发生了临界错误时使用它们,就像当 web 服务器不能启动时那样(参见 15.4 节中的例子)。 | ||
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log 包用那些方法(methods)定义了一个 Logger 接口类型,如果你想自定义日志系统的话可以参考(参见 [http://golang.org/pkg/log/#Logger](http://golang.org/pkg/log/#Logger))。 | ||
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这是一个展示 panic,defer 和 recover 怎么结合使用的完整例子: | ||
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示例 13.3 [panic_recover.go](examples/chapter_13/panic_recover.go): | ||
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```go | ||
// panic_recover.go | ||
package main | ||
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import ( | ||
"fmt" | ||
) | ||
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func badCall() { | ||
panic("bad end") | ||
} | ||
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func test() { | ||
defer func() { | ||
if e := recover(); e != nil { | ||
fmt.Printf("Panicing %s\r\n", e) | ||
} | ||
}() | ||
badCall() | ||
fmt.Printf("After bad call\r\n") // <-- wordt niet bereikt | ||
} | ||
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func main() { | ||
fmt.Printf("Calling test\r\n") | ||
test() | ||
fmt.Printf("Test completed\r\n") | ||
} | ||
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``` | ||
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输出: | ||
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``` | ||
Calling test | ||
Panicing bad end | ||
Test completed | ||
``` | ||
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`defer-panic-recover` 在某种意义上也是一种像 `if`,`for` 这样的控制流机制。 | ||
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Go 标准库中许多地方都用了这个机制,例如,json 包中的解码和 regexp 包中的 Complie 函数。Go 库的原则是即使在包的内部使用了 panic,在它的对外接口(API)中也必须用 recover 处理成返回显式的错误。 | ||
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## 链接 | ||
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- [目录](directory.md) | ||
- 上一节:[错运行时异常和 panic](13.2.md) | ||
- 下一节:[自定义包中的错误处理和 panicking](13.4.md) |
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# 自定义包中的错误处理和 panicking | ||
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