Objective-C使用@try @catch @finally来捕获并处理异常。处理异常需要用到NSException类，它是所有异常的基类。你可以直接使用NSException类来捕获异常，也可以继承一个新的类。
Objective-C是C语言的扩充，它的异常处理机制是通过C标准库提供两个特殊的函数setjmp()和longjmp()函数实现的。如果对C的异常处理机制和setjmp、longjmp函数不了解的，建议先阅读：C语言异常处理机制。
先来看看下面的例子：
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import
int main (int argc, const char * argv[])
{
@autoreleasepool
{
@try {
NSException *e = [NSException
exceptionWithName:@"FileNotFoundException"
reason:@"File Not Found on System"
userInfo:nil];
@throw e;
}
@catch (NSException *exception) {
if ([[exception name] isEqualToString:NSInvalidArgumentException]) {
NSLog(@"%@", exception);
} else {
@throw exception;
}
}
@finally {
NSLog(@"finally");
}
}
return 0;
}
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例子很简单，在@try中抛出一个自定义的FileNotFoundException类型的异常，然后在@catch中判断捕获的异常是不是NSInvalidArgumentException类型，如果不是，将异常再次抛出。最后总是会执行@finally语句，一般异常处理的善后工作都放这里来做。
如何才能了解它内部的工作流程，@try @catch @finally的定义无法查看。幸运的是我们可以通过Clang生成C的中间代码来了解try/catch原理。想了解Clang推荐阅读：编译器Clang介绍。
以上面的代码为例，使用文本编辑器将代码保存到main.m文件中，文件名可随便定义。打开终端输入：clang -rewrite-objc main.m 命令编译。
得到一份main.cpp文件：
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struct objc_selector; struct objc_class;
struct __rw_objc_super { struct objc_object *object; struct objc_object *superClass; };
ifndef _REWRITER_typedef_Protocol
typedef struct objc_object Protocol;
define _REWRITER_typedef_Protocol
endif
define __OBJC_RW_DLLIMPORT extern
__OBJC_RW_DLLIMPORT struct objc_object *objc_msgSend(struct objc_object *, struct objc_selector *, ...);
__OBJC_RW_DLLIMPORT struct objc_object *objc_msgSendSuper(struct objc_super *, struct objc_selector *, ...);
__OBJC_RW_DLLIMPORT struct objc_object *objc_msgSend_stret(struct objc_object *, struct objc_selector *, ...);
__OBJC_RW_DLLIMPORT struct objc_object *objc_msgSendSuper_stret(struct objc_super *, struct objc_selector *, ...);
__OBJC_RW_DLLIMPORT double objc_msgSend_fpret(struct objc_object *, struct objc_selector *, ...);
__OBJC_RW_DLLIMPORT struct objc_object *objc_getClass(const char *);
__OBJC_RW_DLLIMPORT struct objc_class *class_getSuperclass(struct objc_class *);
__OBJC_RW_DLLIMPORT struct objc_object *objc_getMetaClass(const char *);
__OBJC_RW_DLLIMPORT void objc_exception_throw(struct objc_object *);
__OBJC_RW_DLLIMPORT void objc_exception_try_enter(void *);
__OBJC_RW_DLLIMPORT void objc_exception_try_exit(void *);
__OBJC_RW_DLLIMPORT struct objc_object *objc_exception_extract(void *);
__OBJC_RW_DLLIMPORT int objc_exception_match(struct objc_class *, struct objc_object *);
__OBJC_RW_DLLIMPORT void objc_sync_enter(struct objc_object *);
__OBJC_RW_DLLIMPORT void objc_sync_exit(struct objc_object *);
__OBJC_RW_DLLIMPORT Protocol *objc_getProtocol(const char *);
ifndef __FASTENUMERATIONSTATE
struct __objcFastEnumerationState {
unsigned long state;
void **itemsPtr;
unsigned long *mutationsPtr;
unsigned long extra[5];
};
__OBJC_RW_DLLIMPORT void objc_enumerationMutation(struct objc_object *);
define __FASTENUMERATIONSTATE
endif
ifndef __NSCONSTANTSTRINGIMPL
struct __NSConstantStringImpl {
int *isa;
int flags;
char *str;
long length;
};
ifdef CF_EXPORT_CONSTANT_STRING
extern "C" __declspec(dllexport) int __CFConstantStringClassReference[];
else
__OBJC_RW_DLLIMPORT int __CFConstantStringClassReference[];
endif
define __NSCONSTANTSTRINGIMPL
endif
ifndef BLOCK_IMPL
define BLOCK_IMPL
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
// Runtime copy/destroy helper functions (from Block_private.h)
ifdef __OBJC_EXPORT_BLOCKS
extern "C" __declspec(dllexport) void _Block_object_assign(void *, const void *, const int);
extern "C" __declspec(dllexport) void _Block_object_dispose(const void *, const int);
extern "C" __declspec(dllexport) void *_NSConcreteGlobalBlock[32];
extern "C" __declspec(dllexport) void *_NSConcreteStackBlock[32];
else
__OBJC_RW_DLLIMPORT void _Block_object_assign(void *, const void *, const int);
__OBJC_RW_DLLIMPORT void _Block_object_dispose(const void *, const int);
__OBJC_RW_DLLIMPORT void *_NSConcreteGlobalBlock[32];
__OBJC_RW_DLLIMPORT void *_NSConcreteStackBlock[32];
endif
endif
define __block
define __weak
define OFFSETOFIVAR(TYPE, MEMBER) ((long long) &((TYPE *)0)->MEMBER)
static __NSConstantStringImpl __NSConstantStringImpl_main_m_0 attribute ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"FileNotFoundException",21};
static __NSConstantStringImpl __NSConstantStringImpl_main_m_1 attribute ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"File Not Found on System",24};
static __NSConstantStringImpl __NSConstantStringImpl_main_m_2 attribute ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"%@",2};
static __NSConstantStringImpl __NSConstantStringImpl_main_m_3 attribute ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"finally",7};
//
// main.c
// TestBlock
//
// Created by xxxx on 13-6-2.
// Copyright (c) 2013 xxxx. All rights reserved.
//
include
int main (int argc, const char * argv[])
{
@autoreleasepool
{
/* @try scope begin /
{
struct _objc_exception_data
{
int buf[18/32-bit i386*/];
char *pointers[4];
} _stack;
id volatile _rethrow = 0;
objc_exception_try_enter(&_stack);
if (!_setjmp(_stack.buf)) /* @try block continue */
{
NSException *e = ((NSException *(*)(id, SEL, NSString *, NSString *, NSDictionary *))(void *)objc_msgSend)(objc_getClass("NSException"), sel_registerName("exceptionWithName:reason:userInfo:"), (NSString *)&__NSConstantStringImpl_main_m_0, (NSString *)&__NSConstantStringImpl_main_m_1, (NSDictionary *)((void *)0));
objc_exception_throw(e);
} /* @catch begin */ else {
id _caught = objc_exception_extract(&_stack);
objc_exception_try_enter (&_stack);
if (_setjmp(_stack.buf))
_rethrow = objc_exception_extract(&_stack);
else { /* @catch continue */
if (objc_exception_match((struct objc_class *)objc_getClass("NSException"), (struct objc_object *)_caught)) {
NSException *exception = _caught;
if (((BOOL (*)(id, SEL, NSString *))(void *)objc_msgSend)((id)((NSString *(*)(id, SEL))(void *)objc_msgSend)((id)exception, sel_registerName("name")), sel_registerName("isEqualToString:"), (NSString *)NSInvalidArgumentException)) {
NSLog((NSString *)&__NSConstantStringImpl_main_m_2, exception);
} else {
objc_exception_throw( exception);
}
} /* last catch end */ else {
_rethrow = _caught;
objc_exception_try_exit(&_stack);
}
} /* @catch end */
}
/* @finally */
{
if (!_rethrow) objc_exception_try_exit(&_stack);
NSLog((NSString *)&__NSConstantStringImpl_main_m_3);
if (_rethrow) objc_exception_throw(_rethrow);
}
} /* @try scope end */
}
return 0;
}
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文件中信息量太大，咱们只看main函数部分，下面的代码把main函数的代码作了注释说明：
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include
int main (int argc, const char * argv[])
{
@autoreleasepool
{
/**
* try/catch的作用域从这里开始
/
/ @try scope begin /
{
/*
* 首先定义一个_objc_exception_data类型的结构体，用来保存异常现场的数据。
/
struct _objc_exception_data
{
/*
* buf变量就是c语言中的jmp_buf
* jmp_buf的定义可在setjmp.h文件中找到：
*
* #define _JBLEN (10 + 16 + 2)
* #define _JBLEN_MAX _JBLEN
*
* typedef int jmp_buf[_JBLEN];
/
int buf[18/32-bit i386*/];
/**
* pointers[0]用来存储通过@throw抛出的异常对象，
* pointers[1]存储下一个_stack数据。
*/
char *pointers[4];
} _stack;
/**
* _rethrow保存可能在@catch中再次抛出的异常对象。
*/
id volatile _rethrow = 0;
/**
* 因为异常处理支持嵌套，_stack会被存储在一个全局的栈中，这个栈用单链表的存储结构表示。
* objc_exception_try_enter函数将_stack压栈。
*/
objc_exception_try_enter(&_stack);
/**
* _setjmp是C的函数，用于保存当前程序现场。
* _setjmp需要传入一个jmp_buf参数，保存当前需要用到的寄存器的值。
* _setjmp()它能返回两次，第一次是初始化时，返回0，第二次遇到_longjmp()函数调用会返回，返回值由_longjmp的第二个参数决定。
* 如果对_setjmp()和_longjmp()概念不太了解的，请参考C语言的异常处理机制。
*
* 下面_setjmp()初始化返回0，然后执行if{}中也就是@try{}中的代码。
*/
if (!_setjmp(_stack.buf)) /* @try block continue */
{
/**
* 创建一个NSException对象，对应代码：
*
* NSException *e = [NSException
* exceptionWithName:@"FileNotFoundException"
* reason:@"File Not Found on System"
* userInfo:nil];
*/
NSException *e = ((NSException *(*)(id, SEL, NSString *, NSString *, NSDictionary *))(void *)objc_msgSend)(objc_getClass("NSException"), sel_registerName("exceptionWithName:reason:userInfo:"), (NSString *)&__NSConstantStringImpl_main_m_0, (NSString *)&__NSConstantStringImpl_main_m_1, (NSDictionary *)((void *)0));
/**
* 抛出异常对象，对应代码：@throw e;
*
* objc_exception_throw函数实现步骤如下：
* 1. 把e对象保存到_stack->pointers[0]中使其在@catch{}中能被捕获。
* 2. 将_stack从全局栈中弹出。
* 3. 调用_longjmp()跳转到前面if语句中的_setjmp()位置。_longjmp()使得_setjmp()函数第二次返回，
* 返回值为1，所以会执行else{}中也就是@catch{}中的代码。
*/
objc_exception_throw(e);
} /* @catch begin */ else {
/**
* objc_exception_extract函数从_stack->pointers[0]中取得上面抛出的异常对象。
*/
id _caught = objc_exception_extract(&_stack);
/**
* 这里为何再次调用objc_exception_try_enter对_stack压栈？先保留这个疑问，继续看下面的代码。
*/
objc_exception_try_enter (&_stack);
/**
* 在@catch中设置一个跳转位置
*/
if (_setjmp(_stack.buf))
/**
* 如果@catch{}中再次抛出异常，在这里捕获。
*/
_rethrow = objc_exception_extract(&_stack);
else { /* @catch continue */
/**
* objc_exception_match函数判断_caught对象是否是需要捕获的目标对象。对应代码：
*
* @catch (NSException *exception) {
*/
if (objc_exception_match((struct objc_class *)objc_getClass("NSException"), (struct objc_object *)_caught)) {
NSException *exception = _caught;
/**
* 比较捕获的异常是不是NSInvalidArgumentException类型。对应代码：
*
* if ([[exception name] isEqualToString:NSInvalidArgumentException]) {
* NSLog(@"%@", exception);
*
*/
if (((BOOL (*)(id, SEL, NSString *))(void *)objc_msgSend)((id)((NSString *(*)(id, SEL))(void *)objc_msgSend)((id)exception, sel_registerName("name")), sel_registerName("isEqualToString:"), (NSString *)NSInvalidArgumentException)) {
NSLog((NSString *)&__NSConstantStringImpl_main_m_2, exception);
} else {
/**
* 抛出异常对象，然后跳转到前面@catch中的if语句中的_setjmp()位置。
* 这就解释了前面为什么要在@catch中再次将_stack压栈和调用_setjmp()的原因。
* 在当前@catch中，如果不设置一个跳转点来捕获@catch中抛出的异常，那么程序就直接跳转到全局栈的下一个@catch中，而下面的@finally{}代码就无法执行。
* 在@catch中设置跳转点就是为了最后总能执行@finally中的代码。
*/
objc_exception_throw( exception);
}
} /* last catch end */ else {
/**
* 如果异常对象没被处理，先将其保存到_rethrow变量。
* objc_exception_try_exit函数将_stack从全局栈中弹出。
*/
_rethrow = _caught;
objc_exception_try_exit(&_stack);
}
} /* @catch end */
}
/* @finally */
{
if (!_rethrow) objc_exception_try_exit(&_stack);
NSLog((NSString *)&__NSConstantStringImpl_main_m_3);
/**
* _rethrow是前面@catch中没有被处理的或被捕获的异常对象，
* 最后，_rethrow异常对象被抛到全局栈的下一个@catch中。
*/
if (_rethrow) objc_exception_throw(_rethrow);
}
} /* @try scope end */
}
return 0;
}
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以上代码中还涉及了objc_exception_try_enter、objc_exception_extract、objc_exception_throw、objc_exception_try_exit等函数，这些函数可以在苹果开源的objc4的objc-exception.mm文件中找到，objc4源码可在这里下载。下面代码只显示部分方便阅读：
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typedef struct {
int version;
void (throw_exc)(id); // version 0
void (try_enter)(void ); // version 0
void (try_exit)(void ); // version 0
id (extract)(void ); // version 0
int (match)(Class, id); // version 0
} objc_exception_functions_t;
static objc_exception_functions_t xtab;
// forward declaration
static void set_default_handlers();
/*
- Exported functions
*/
// get table; version tells how many
void objc_exception_get_functions(objc_exception_functions_t *table) {
// only version 0 supported at this point
if (table && table->version == 0)
*table = xtab;
}
// set table
void objc_exception_set_functions(objc_exception_functions_t *table) {
// only version 0 supported at this point
if (table && table->version == 0)
xtab = *table;
}
/*
- The following functions are
- synthesized by the compiler upon encountering language constructs
*/
void objc_exception_throw(id exception) {
if (!xtab.throw_exc) {
set_default_handlers();
}
if (PrintExceptionThrow) {
_objc_inform("EXCEPTIONS: throwing %p (%s)",
exception, object_getClassName(exception));
void* callstack[500];
int frameCount = backtrace(callstack, 500);
backtrace_symbols_fd(callstack, frameCount, fileno(stderr));
}
OBJC_RUNTIME_OBJC_EXCEPTION_THROW(exception); // dtrace probe to log throw activity.
xtab.throw_exc(exception);
_objc_fatal("objc_exception_throw failed");
}
void objc_exception_try_enter(void *localExceptionData) {
if (!xtab.throw_exc) {
set_default_handlers();
}
xtab.try_enter(localExceptionData);
}
void objc_exception_try_exit(void *localExceptionData) {
if (!xtab.throw_exc) {
set_default_handlers();
}
xtab.try_exit(localExceptionData);
}
id objc_exception_extract(void *localExceptionData) {
if (!xtab.throw_exc) {
set_default_handlers();
}
return xtab.extract(localExceptionData);
}
int objc_exception_match(Class exceptionClass, id exception) {
if (!xtab.throw_exc) {
set_default_handlers();
}
return xtab.match(exceptionClass, exception);
}
// quick and dirty exception handling code
// default implementation - mostly a toy for use outside/before Foundation
// provides its implementation
// Perhaps the default implementation should just complain loudly and quit
extern void _objc_inform(const char *fmt, ...);
typedef struct { jmp_buf buf; void *pointers[4]; } LocalData_t;
typedef struct _threadChain {
LocalData_t *topHandler;
objc_thread_t perThreadID;
struct _threadChain *next;
}
ThreadChainLink_t;
static ThreadChainLink_t ThreadChainLink;
static ThreadChainLink_t *getChainLink() {
// follow links until thread_self() found (someday) XXX
objc_thread_t self = thread_self();
ThreadChainLink_t *walker = &ThreadChainLink;
while (walker->perThreadID != self) {
if (walker->next != NULL) {
walker = walker->next;
continue;
}
// create a new one
// XXX not thread safe (!)
// XXX Also, we don't register to deallocate on thread death
walker->next = (ThreadChainLink_t *)malloc(sizeof(ThreadChainLink_t));
walker = walker->next;
walker->next = NULL;
walker->topHandler = NULL;
walker->perThreadID = self;
}
return walker;
}
static void default_try_enter(void *localExceptionData) {
LocalData_t *data = (LocalData_t *)localExceptionData;
ThreadChainLink_t *chainLink = getChainLink();
data->pointers[1] = chainLink->topHandler;
chainLink->topHandler = data;
if (PrintExceptions) _objc_inform("EXCEPTIONS: entered try block %p\n", chainLink->topHandler);
}
static void default_throw(id value) {
ThreadChainLink_t *chainLink = getChainLink();
LocalData_t *led;
if (value == nil) {
if (PrintExceptions) _objc_inform("EXCEPTIONS: objc_exception_throw with nil value\n");
return;
}
if (chainLink == NULL) {
if (PrintExceptions) _objc_inform("EXCEPTIONS: No handler in place!\n");
return;
}
if (PrintExceptions) _objc_inform("EXCEPTIONS: exception thrown, going to handler block %p\n", chainLink->topHandler);
led = chainLink->topHandler;
chainLink->topHandler = (LocalData_t *)
led->pointers[1]; // pop top handler
led->pointers[0] = value; // store exception that is thrown
if TARGET_OS_WIN32
longjmp(led->buf, 1);
else
_longjmp(led->buf, 1);
endif
}
static void default_try_exit(void *led) {
ThreadChainLink_t *chainLink = getChainLink();
if (!chainLink || led != chainLink->topHandler) {
if (PrintExceptions) _objc_inform("EXCEPTIONS: *** mismatched try block exit handlers\n");
return;
}
if (PrintExceptions) _objc_inform("EXCEPTIONS: removing try block handler %p\n", chainLink->topHandler);
chainLink->topHandler = (LocalData_t *)
chainLink->topHandler->pointers[1]; // pop top handler
}
static id default_extract(void *localExceptionData) {
LocalData_t *led = (LocalData_t *)localExceptionData;
return (id)led->pointers[0];
}
static int default_match(Class exceptionClass, id exception) {
//return [exception isKindOfClass:exceptionClass];
Class cls;
for (cls = _object_getClass(exception); nil != cls; cls = _class_getSuperclass(cls))
if (cls == exceptionClass) return 1;
return 0;
}
static void set_default_handlers() {
objc_exception_functions_t default_functions = {
0, default_throw, default_try_enter, default_try_exit, default_extract, default_match };
// should this always print?
if (PrintExceptions) _objc_inform("EXCEPTIONS: *** Setting default (non-Foundation) exception mechanism\n");
objc_exception_set_functions(&default_functions);
}
void exception_init(void)
{
// nothing to do
}
void _destroyAltHandlerList(struct alt_handler_list *list)
{
// nothing to do
}
// !OBJC2
else
// OBJC2
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