一:背景1. 讲故事 在项目中摸爬滚打几年,应该或多或少的见过有人把异常当做业务逻辑处理的情况(┬_┬),比如说判断一个数字是否为整数,就想当然的用try catch 包起来,再进行 int.Parse,如果抛异常就说明不是整数,简单粗暴,也不需要写正则或者其他逻辑,再比如一个字符串强制转化为Enum,直接用Enum.Parse,可能是因为对异常的开销不是特别了解,这种不好的使用习惯也许被官方发现了,后续给我们补了很多的Try前缀的方法,比如:int.TryParse , Enum.TryParse, dict.TryGetValue ,用代码展示如下: - <font size="3"> //原始写法
- var num = int.Parse("1");
- //使用try方式
- var result = 0;
- var b = int.TryParse("1", out result);
- </font>
复制代码用Try系列方法没毛病,但这写法让人吐槽,还要单独定义result变量,没撤,官方还得靠我们这些开发者给他们发扬光大,终于在C# 7.0 中新增了一个 out variables 语法糖。 - <font size="3">
- //try out 变量模式
- var c = int.TryParse("1", out int result2);
- </font>
复制代码这种 out 变量 模式就牛了,一个方法获取两个值,还没有抛异常的风险。 二:为什么要用tryxxx方法有了tryxxx方法之后,你就应该明白微软已经在提醒我们开发人员不要滥用异常,尤其在可预知可预见的场景下,毕竟他们知道异常的开销真的是太大了,不知者不怪哈。 1. 肉眼看得见的低性能为了让大家肉眼能看见,我们就用异常方法和tryxxx方法做一个性能比较,迭代50w次,看看各自的性能如何? - <font size="3">
- for (int i = 0; i < 3; i++)
- {
- var watch = Stopwatch.StartNew();
- for (int k = 0; k < 50000; k++)
- {
- try
- {
- var num = int.Parse("xxx");
- }
- catch (Exception ex) { }
- }
- watch.Stop();
- Console.WriteLine([ DISCUZ_CODE_127 ]quot;i={i + 1},耗费:{watch.ElapsedMilliseconds}");
- }
- Console.WriteLine("---------------------------------------------");
- for (int i = 0; i < 3; i++)
- {
- var watch = Stopwatch.StartNew();
- for (int k = 0; k < 50000; k++)
- {
- var num = int.TryParse("xxx", out int reuslt);
- }
- watch.Stop();
- Console.WriteLine([ DISCUZ_CODE_127 ]quot;i={i + 1},耗费:{watch.ElapsedMilliseconds}");
- }
- Console.ReadLine();
- </font>
复制代码看结果还挺吓人的,相差480倍, 好熟悉的一个数字。。。 南朝四百八十寺,多少楼台烟雨中 三: 异常的超强开销为什么异常有那么大的开销? 只有知己知彼才能心中有数,看过我多线程视频的朋友应该知道,线程的创建和销毁代价都是非常大的,其中有一项就是需要代码从用户态切换到了内核态,毕竟线程是操作系统层面的事情,和你CLR无关,CLR只是做了一层系统包装而已,其实很多人都想不到,我们用的 try catch finally 底层也是封装了操作系统层面的(Windows 结构化异常处理),也叫做SEH,什么意思? 就是当你throw之后,代码需要从用户态切换到内核态,这个开销是不会小的,还有一个开销来自于Exception中的StackTrace,这里面的值需要从当前异常的线程栈中去抓取调用堆栈,栈越深,开销就越大。 1. 从用户态到内核态大家肯定会说,甭那么玄乎,凡事都要讲个证据, Do more,Talk less, 这里我准备分两种情况讲解。 <1> 有catch情况准备在catch的时候阻塞住,然后抓它的dump文件。 - <font size="3">
- public static void Main(string[] args)
- {
- try
- {
- var num = int.Parse("xxx");
- }
- catch (Exception ex)
- {
- Console.WriteLine(ex.Message);
- Console.ReadLine();
- }
- }
- </font>
复制代码使用 !dumpstack 把当前 0号线程 的所有托管和非托管堆栈全部打出来,简化后如下: - <font size="3">
- 0:000> ~0s
- ntdll!NtReadFile+0x14:
- 00007fff`f805aa64 c3 ret
- 0:000> !dumpstack
- OS Thread Id: 0x2bf0 (0)
- Current frame: ntdll!NtReadFile+0x14
- Caller, Callee
- (MethodDesc 00007fffde3a40b8 +0x18 System.Console.ReadLine())
- (MethodDesc 00007fff810d59f8 +0xa5 ConsoleApp4.Program.Main(System.String[])), calling (MethodDesc 00007fffde3a40b8 +0 System.Console.ReadLine())
- 00000044433fc700 00007fffe07a29e0 clr!ExceptionTracker::CallCatchHandler+0x9c, calling clr!ExceptionTracker::CallHandler
- clr!ClrUnwindEx+0x40, calling ntdll!RtlUnwindEx
- ntdll!RtlRaiseException+0x4e, calling ntdll!RtlpCaptureContext
- clr!IL_Throw+0x114, calling clr!RaiseTheExceptionInternalOnly
- (MethodDesc 00007fffde4f95c0 System.Number.StringToNumber(System.String, System.Globalization.NumberStyles, NumberBuffer ByRef, System.Globalization.NumberFormatInfo, Boolean)), calling mscorlib_ni+0x53976a
- (MethodDesc 00007fffde3b5330 +0xae System.Number.ParseInt32(System.String, System.Globalization.NumberStyles, System.Globalization.NumberFormatInfo)), calling (MethodDesc 00007fffde4f95c0 +0 System.Number.StringToNumber(System.String, System.Globalization.NumberStyles, NumberBuffer ByRef, System.Globalization.NumberFormatInfo, Boolean))
- (MethodDesc 00007fffde1ebfa8 +0x2eb System.Globalization.NumberFormatInfo..ctor(System.Globalization.CultureData)), calling (MethodDesc 00007fffde1eba68 +0 System.Globalization.CultureData.GetNFIValues(System.Globalization.NumberFormatInfo))
- (MethodDesc 00007fff810d59f8 +0x49 ConsoleApp4.Program.Main(System.String[])), calling (MethodDesc 00007fffde3b1708 +0 System.Int32.Parse(System.String))
- </font>
复制代码因为是堆栈,所以执行流就要从后往前看,你会发现流程大概是这个样子 int.Parse -> CLR -> ntdll -> CLR -> Console.ReadLine,很显然 ntdll.dll 是操作系统层级的一个核心文件,这就从用户态切入到了内核态,如果不是很明白,我画一张简图吧。。。 <2>. 无catch处理大家肯定很好奇,如果无catch会是怎么样,大家也可以用windbg去挖一下。 - <font size="3">
- public static void Main(string[] args)
- {
- var num = int.Parse("xxx");
- }
- 0:000> !dumpstack
- OS Thread Id: 0xd68 (0)
- Current frame: ntdll!NtTerminateProcess+0x14
- Caller, Callee
- mscoreei!RuntimeDesc::ShutdownAllActiveRuntimes+0x285, calling KERNEL32!ExitProcessImplementation
- mscoreei!CLRRuntimeHostInternalImpl::ShutdownAllRuntimesThenExit+0x14, calling mscoreei!RuntimeDesc::ShutdownAllActiveRuntimes
- clr!EEPolicy::ExitProcessViaShim+0x9c
- clr!SafeExitProcess+0x9d, calling clr!EEPolicy::ExitProcessViaShim
- ntdll!KiUserExceptionDispatch+0x53, calling ntdll!NtRaiseException
- clr!RaiseTheExceptionInternalOnly+0x188426, calling clr!EEPolicy::HandleFatalError
- clr!IL_Throw+0x45, calling clr!LazyMachStateCaptureState
- (MethodDesc 00007fffde4f95c0 System.Number.StringToNumber(System.String, System.Globalization.NumberStyles, NumberBuffer ByRef, System.Globalization.NumberFormatInfo, Boolean)), calling mscorlib_ni+0x53976a
- (MethodDesc 00007fffde3b5330 +0xae System.Number.ParseInt32(System.String, System.Globalization.NumberStyles, System.Globalization.NumberFormatInfo)), calling (MethodDesc 00007fffde4f95c0 +0 System.Number.StringToNumber(System.String, System.Globalization.NumberStyles, NumberBuffer ByRef, System.Globalization.NumberFormatInfo, Boolean))
- (MethodDesc 00007fffde1ebfa8 +0x2eb System.Globalization.NumberFormatInfo..ctor(System.Globalization.CultureData)), calling (MethodDesc 00007fffde1eba68 +0 System.Globalization.CultureData.GetNFIValues(System.Globalization.NumberFormatInfo))
- (MethodDesc 00007fff810e59f8 +0x37 ConsoleApp4.Program.Main(System.String[])), calling (MethodDesc 00007fffde3b1708 +0 System.Int32.Parse(System.String))
- </font>
复制代码可以看到进程的退出逻辑给了托管程序入口 mscoreei.dll 而再也没有进入Main函数了, 为此我也补一张图给大家看看 2. 抓取线程调用栈当大家慌慌张张的看到异常的时候,第一眼会去看异常信息是什么? 第二眼会去看异常出在了哪一行代码,这就是线程的调用栈,这个信息非常重要,可以快捷的帮助我们找到问题解决问题,放在Exception的StackTrace中,先上一段代码。 - <font size="3">
- public static void Main(string[] args)
- {
- Run();
- Console.ReadLine();
- }
- public static void Run()
- {
- var ex = new FormatException("你的格式错误啦!!!");
- throw ex;
- }
- </font>
复制代码 <1> StackTrace何时塞入的到目前为止还没看到哪本书说到StackTrace是何时被塞入的? 由于水平有限,我也试着探测一下下。 从代码中可以看到不是在new的时候塞入的,那会是哪里呢? <2> 从CLR中寻找答案既然不在用户代码,那就到CLR中去看看,在windbg中用 dumpstack 去查看非托管堆栈。 - <font size="3">
- 0:000> !dumpstack
- OS Thread Id: 0x4090 (0)
- Current frame: ntdll!NtTerminateProcess+0x14
- Caller, Callee
- clr!EETypeHashTable::FindItem+0x532, calling clr!NgenHashTable<EEClassHashTable,EEClassHashEntry,4>::PersistedBucketList::GetBucket
- clr!JIT_StrCns+0xd0, calling clr!HelperMethodFrameRestoreState
- (MethodDesc 00007fff810f5a08 +0x70 ConsoleApp4.Program.Run()), calling clr!IL_Throw
- clr!IL_Throw+0x45, calling clr!LazyMachStateCaptureState
- (MethodDesc 00007fff810f5a08 +0x70 ConsoleApp4.Program.Run()), calling clr!IL_Throw
- (MethodDesc 00007fff810f59f8 +0x28 ConsoleApp4.Program.Main(System.String[])), calling 00007fff81200488 (stub for ConsoleApp4.Program.Run())
- </font>
复制代码从简化后的流程看,怀疑是由 clr!HelperMethodFrameRestoreState 处理的,为什么这么说呢? 因为我们定义的 FormatException ex 会传给CLR的,不信可以用 kb 看一看。 - <font size="3">
- 0:000> kb
- # RetAddr : Args to Child : Call Site
- 00 00007fff`e07a3181 : 00000000`e0434352 0000006d`4a7fe938 0000017b`30ad2d48 0000017b`2f081690 : KERNELBASE!RaiseException+0x68
- 01 00007fff`e07a45f4 : ffffffff`fffffffe 0000017b`2ef02542 00000000`0000000a 0000017b`2f040910 : clr!RaiseTheExceptionInternalOnly+0x31f
- 02 00007fff`811d0950 : 00000000`70000001 00007fff`810c4140 0000006d`4a7fedb8 0000006d`4a7fec78 : clr!IL_Throw+0x114
- 03 00007fff`811d08b8 : 0000017b`30ad2d30 00007fff`810c4140 00000000`00000000 00007fff`00000000 : 0x00007fff`811d0950
- 04 00007fff`e0736c93 : 0000017b`30ad2d30 00007fff`810c4140 00000000`00000000 00007fff`00000000 : 0x00007fff`811d08b8
- 05 00007fff`e0736b79 : 00000000`00000000 00007fff`e0737aae 0000006d`4a7fefb8 00000000`00000000 : clr!CallDescrWorkerInternal+0x83
- 06 00007fff`e0737410 : 0000006d`4a7fefb8 0000006d`4a7ff048 0000006d`4a7feeb8 00000000`00000001 : clr!CallDescrWorkerWithHandler+0x4e
- 07 00007fff`e08dcaf2 : 0000006d`4a7fee00 00000000`00000001 00000000`00000001 0000017b`2efcecf0 : clr!MethodDescCallSite::CallTargetWorker+0x102
- 08 00007fff`e08dd4b3 : 00000000`00000001 00000000`00000000 0000017b`30ad2d30 0000017b`30ad2d30 : clr!RunMain+0x25f
- 09 00007fff`e08dd367 : 0000017b`2f040910 0000006d`4a7ff420 0000017b`2f040910 0000017b`2f082770 : clr!Assembly::ExecuteMainMethod+0xb7
- 0a 00007fff`e08dccb3 : 00000000`00000000 0000017b`2ef00000 00000000`00000000 00000000`00000000 : clr!SystemDomain::ExecuteMainMethod+0x643
- 0b 00007fff`e08dcc31 : 0000017b`2ef00000 00007fff`e08de090 00000000`00000000 00000000`00000000 : clr!ExecuteEXE+0x3f
- 0c 00007fff`e08de0a4 : ffffffff`ffffffff 00007fff`e08de090 00000000`00000000 00000000`00000000 : clr!_CorExeMainInternal+0xb2
- 0d 00007fff`e1208a61 : 00000000`00000000 00007fff`00000091 00000000`00000000 0000006d`4a7ff9f8 : clr!CorExeMain+0x14
- 0e 00007fff`e133a4cc : 00000000`00000000 00007fff`e08de090 00000000`00000000 00000000`00000000 : mscoreei!CorExeMain+0x112
- 0f 00007fff`f5cc4034 : 00007fff`e1200000 00000000`00000000 00000000`00000000 00000000`00000000 : MSCOREE!CorExeMain_Exported+0x6c
- 10 00007fff`f8033691 : 00000000`00000000 00000000`00000000 00000000`00000000 00000000`00000000 : KERNEL32!BaseThreadInitThunk+0x14
- 11 00000000`00000000 : 00000000`00000000 00000000`00000000 00000000`00000000 00000000`00000000 : ntdll!RtlUserThreadStart+0x21
- </font>
复制代码其中第一行的 00 00007fffe07a3181 : 00000000e0434352 0000006d4a7fe938 0000017b30ad2d48 0000017b2f081690 : KERNELBASE!RaiseException+0x68中的第三个参数地址 0000017b30ad2d48` 就是我们的异常类,打印出来看一下。 - <font size="3">
- 0:000> !do 0000017b30ad2d48
- Name: System.FormatException
- MethodTable: 00007fffde285c38
- EEClass: 00007fffde3930e0
- Size: 160(0xa0) bytes
- File: C:\WINDOWS\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll
- Fields:
- MT Field Offset Type VT Attr Value Name
- 00007fffde2059c0 40002a2 8 System.String 0 instance 0000017b30ad4c80 _className
- 00007fffde282a50 40002a3 10 ...ection.MethodBase 0 instance 0000000000000000 _exceptionMethod
- 00007fffde2059c0 40002a4 18 System.String 0 instance 0000000000000000 _exceptionMethodString
- 00007fffde2059c0 40002a5 20 System.String 0 instance 0000017b30ad2de8 _message
- 00007fffde2883d8 40002a6 28 ...tions.IDictionary 0 instance 0000000000000000 _data
- 00007fffde205b70 40002a7 30 System.Exception 0 instance 0000000000000000 _innerException
- 00007fffde2059c0 40002a8 38 System.String 0 instance 0000000000000000 _helpURL
- 00007fffde205dd8 40002a9 40 System.Object 0 instance 0000017b30ad2e98 _stackTrace
- 00007fffde205dd8 40002aa 48 System.Object 0 instance 0000017b30ad2f28 _watsonBuckets
- 00007fffde2059c0 40002ab 50 System.String 0 instance 0000000000000000 _stackTraceString
- 00007fffde2059c0 40002ac 58 System.String 0 instance 0000000000000000 _remoteStackTraceString
- 00007fffde2085a0 40002ad 88 System.Int32 1 instance 0 _remoteStackIndex
- 00007fffde205dd8 40002ae 60 System.Object 0 instance 0000000000000000 _dynamicMethods
- 00007fffde2085a0 40002af 8c System.Int32 1 instance -2146233033 _HResult
- 00007fffde2059c0 40002b0 68 System.String 0 instance 0000000000000000 _source
- 00007fffde2831f8 40002b1 78 System.IntPtr 1 instance 0 _xptrs
- 00007fffde2085a0 40002b2 90 System.Int32 1 instance -532462766 _xcode
- 00007fffde21e720 40002b3 80 System.UIntPtr 1 instance 0 _ipForWatsonBuckets
- 00007fffde1f5080 40002b4 70 ...ializationManager 0 instance 0000017b30ad2e18 _safeSerializationManager
- 00007fffde205dd8 40002a1 100 System.Object 0 shared static s_EDILock
- >> Domain:Value 0000017b2efe0af0:NotInit <<
- 0:000> !do 0000017b30ad2e98
- Name: System.SByte[]
- MethodTable: 00007fffde20dde8
- EEClass: 00007fffde390920
- Size: 120(0x78) bytes
- Array: Rank 1, Number of elements 96, Type SByte (Print Array)
- Content: .........../{...P.......@..Jm....Z.........................Jm....Y..............................
- Fields:
- None
- </font>
复制代码此时 _stackTrace 已经有值了,毕竟Console上已经打印出来了。 最后补充一下大家也可以通过 !threads 去找异常的线程,如下图的中 System.FormatException 0000017b30ad2d48,然后通过 !printexception 去打印这个地址 0000017b30ad2d48 上异常对象。 - <font size="3">
- 0:000> !threads
- ThreadCount: 2
- UnstartedThread: 0
- BackgroundThread: 1
- PendingThread: 0
- DeadThread: 0
- Hosted Runtime: no
- Lock
- ID OSID ThreadOBJ State GC Mode GC Alloc Context Domain Count Apt Exception
- 0 1 80c 0000016816f508f0 2a020 Preemptive 0000016818CCE3B8:0000016818CCFFD0 0000016816ef0b10 0 MTA System.FormatException 0000017b30ad2d48
- 6 2 12d8 0000016816f7b0e0 2b220 Preemptive 0000000000000000:0000000000000000 0000016816ef0b10 0 MTA (Finalizer)
- 0:000> !printexception 0000017b30ad2d48
- Exception object: 0000017b30ad2d48
- Exception type: System.FormatException
- Message: 你的格式错误啦!!!
- InnerException: <none>
- StackTrace (generated):
- SP IP Function
- 0000001F8F7FEE90 00007FFF811E0951 ConsoleApp4!ConsoleApp4.Program.Run()+0x71
- 0000001F8F7FEEE0 00007FFF811E08B9 ConsoleApp4!ConsoleApp4.Program.Main(System.String[])+0x29
- StackTraceString: <none>
- HResult: 80131537
- </font>
复制代码 三:总结不要把异常当做业务逻辑处理,这开销有可能你承受不起,把那些真正不可期的情况留给异常吧,如: TimeoutException。。。
|