Objective-C try/catch異常處理機制原理。

　　Objective-C使用@try @catch @finally來捕獲並處理異常。處理異常須要用到NSException類，它是全部異常的基類。你能夠直接使用NSException類來捕獲異常，也能夠繼承一個新的類。

　　Objective-C是C語言的擴充，它的異常處理機制是經過C標準庫提供兩個特殊的函數setjmp()和longjmp()函數實現的。若是對C的異常處理機制和setjmp、longjmp函數不瞭解的，建議先閱讀：C語言異常處理機制。

 

先來看看下面的例子：

#import <Foundation/Foundation.h>

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;
}

 

 

　　例子很簡單，在@try中拋出一個自定義的FileNotFoundException類型的異常，而後在@catch中判斷捕獲的異常是否是NSInvalidArgumentException類型，若是不是，將異常再次拋出。最後老是會執行@finally語句，通常異常處理的善後工做都放這裏來作。

　　如何才能瞭解它內部的工做流程，@try @catch @finally的定義沒法查看。幸運的是咱們能夠經過Clang生成C的中間代碼來了解try/catch原理。想了解Clang推薦閱讀：編譯器Clang介紹。

　　以上面的代碼爲例，使用文本編輯器將代碼保存到main.m文件中，文件名可隨便定義。打開終端輸入：clang -rewrite-objc main.m 命令編譯。

獲得一份main.cpp文件：

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 <Foundation/Foundation.h>

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;
}

 

文件中信息量太大，我們只看main函數部分，下面的代碼把main函數的代碼做了註釋說明：

#include <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
    @autoreleasepool
    {
        /**
         * try/catch的做用域從這裏開始
         */
        /* @try scope begin */
        {
            /**
             * 首先定義一個_objc_exception_data類型的結構體，用來保存異常現場的數據。
             */
            struct _objc_exception_data
            {
                /**
                 * buf變量就是c語言中的jmp_buf
                 * jmp_buf的定義可在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]用來存儲經過@throw拋出的異常對象，
                 * pointers[1]存儲下一個_stack數據。
                 */
                char *pointers[4];
            } _stack;
            
            /**
             * _rethrow保存可能在@catch中再次拋出的異常對象。
             */
            id volatile _rethrow = 0;
            
            /**
             * 由於異常處理支持嵌套，_stack會被存儲在一個全局的棧中，這個棧用單鏈表的存儲結構表示。
             * objc_exception_try_enter函數將_stack壓棧。
             */
            objc_exception_try_enter(&_stack);
            
            /**
             * _setjmp是C的函數，用於保存當前程序現場。
             * _setjmp須要傳入一個jmp_buf參數，保存當前須要用到的寄存器的值。
             * _setjmp()它能返回兩次，第一次是初始化時，返回0，第二次遇到_longjmp()函數調用會返回，返回值由_longjmp的第二個參數決定。
             * 若是對_setjmp()和_longjmp()概念不太瞭解的，請參考C語言的異常處理機制。
             *
             * 下面_setjmp()初始化返回0，而後執行if{}中也就是@try{}中的代碼。
             */
            if (!_setjmp(_stack.buf)) /* @try block continue */
            {
                /**
                 * 建立一個NSException對象，對應代碼：
                 * 
                 *             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));
                
                /**
                 * 拋出異常對象，對應代碼：@throw e;
                 * 
                 * objc_exception_throw函數實現步驟以下：
                 * 1. 把e對象保存到_stack->pointers[0]中使其在@catch{}中能被捕獲。
                 * 2. 將_stack從全局棧中彈出。
                 * 3. 調用_longjmp()跳轉到前面if語句中的_setjmp()位置。_longjmp()使得_setjmp()函數第二次返回，
                 * 返回值爲1，因此會執行else{}中也就是@catch{}中的代碼。
                 */
                objc_exception_throw(e);
                
            } /* @catch begin */ else {
                
                /**
                 * objc_exception_extract函數從_stack->pointers[0]中取得上面拋出的異常對象。
                 */
                id _caught = objc_exception_extract(&_stack);
                
                /**
                 * 這裏爲什麼再次調用objc_exception_try_enter對_stack壓棧？先保留這個疑問，繼續看下面的代碼。
                 */
                objc_exception_try_enter (&_stack);
                
                /**
                 * 在@catch中設置一個跳轉位置
                 */
                if (_setjmp(_stack.buf))
                    
                    /**
                     * 若是@catch{}中再次拋出異常，在這裏捕獲。
                     */
                    _rethrow = objc_exception_extract(&_stack);
                
                else { /* @catch continue */
                    
                    /**
                     * objc_exception_match函數判斷_caught對象是不是須要捕獲的目標對象。對應代碼：
                     * 
                     * @catch (NSException *exception) {
                     */
                    if (objc_exception_match((struct objc_class *)objc_getClass("NSException"), (struct objc_object *)_caught)) {
                        NSException *exception = _caught;
                        
                        /**
                         * 比較捕獲的異常是否是NSInvalidArgumentException類型。對應代碼：
                         *
                         * 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 {
                            
                            /**
                             * 拋出異常對象，而後跳轉到前面@catch中的if語句中的_setjmp()位置。
                             * 這就解釋了前面爲何要在@catch中再次將_stack壓棧和調用_setjmp()的緣由。
                             * 在當前@catch中，若是不設置一個跳轉點來捕獲@catch中拋出的異常，那麼程序就直接跳轉到全局棧的下一個@catch中，而下面的@finally{}代碼就沒法執行。
                             * 在@catch中設置跳轉點就是爲了最後總能執行@finally中的代碼。
                             */
                            objc_exception_throw( exception);
                        }
                        
                    } /* last catch end */ else {
                        
                        /**
                         * 若是異常對象沒被處理，先將其保存到_rethrow變量。
                         * objc_exception_try_exit函數將_stack從全局棧中彈出。
                         */
                        _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中沒有被處理的或被捕獲的異常對象，
                 * 最後，_rethrow異常對象被拋到全局棧的下一個@catch中。
                 */
                if (_rethrow) objc_exception_throw(_rethrow);
            }
            
        } /* @try scope end */

    }
    return 0;
}

 

　　以上代碼中還涉及了objc_exception_try_enter、objc_exception_extract、objc_exception_throw、objc_exception_try_exit等函數，這些函數能夠在蘋果開源的objc4的objc-exception.mm文件中找到，objc4源碼可在這裏下載。下面代碼只顯示部分方便閱讀：

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__