Load/Initialize

+ load

runtime源码中load_images的实现

/***********************************************************************
* load_images
* Process +load in the given images which are being mapped in by dyld.
* Calls ABI-agnostic code after taking ABI-specific locks.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
__private_extern__ const char *
load_images(enum dyld_image_states state, uint32_t infoCount,
            const struct dyld_image_info infoList[])
{
    BOOL found;

    recursive_mutex_lock(&loadMethodLock);

    // Discover load methods
    rwlock_write(&runtimeLock);
    found = load_images_nolock(state, infoCount, infoList);
    rwlock_unlock_write(&runtimeLock);

    // Call +load methods (without runtimeLock - re-entrant)
    if (found) {
        call_load_methods();
    }

    recursive_mutex_unlock(&loadMethodLock);

    return NULL;
}

load_images_nolock

/***********************************************************************
* load_images_nolock
* Prepares +load in the given images which are being mapped in by dyld.
* Returns YES if there are now +load methods to be called by call_load_methods.
*
* Locking: loadMethodLock(both) and runtimeLock(new) acquired by load_images
**********************************************************************/
__private_extern__ BOOL 
load_images_nolock(enum dyld_image_states state,uint32_t infoCount,
                   const struct dyld_image_info infoList[])
{
    BOOL found = NO;
    uint32_t i;

    i = infoCount;
    while (i--) {
        header_info *hi;
        for (hi = FirstHeader; hi != NULL; hi = hi->next) {
            const headerType *mhdr = (headerType*)infoList[i].imageLoadAddress;
            if (hi->mhdr == mhdr) {
                prepare_load_methods(hi);
                found = YES;
            }
        }
    }

    return found;
}

prepare_load_methods

__private_extern__ void prepare_load_methods(header_info *hi)
{
    size_t count, i;

    rwlock_assert_writing(&runtimeLock);

    class_t **classlist = 
        _getObjc2NonlazyClassList(hi, &count);
    for (i = 0; i < count; i++) {
        class_t *cls = remapClass(classlist[i]);
        schedule_class_load(cls);
    }

    category_t **categorylist = _getObjc2NonlazyCategoryList(hi, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = categorylist[i];
        // Do NOT use cat->cls! It may have been remapped.
        class_t *cls = remapClass(cat->cls);
        realizeClass(cls);
        assert(isRealized(cls->isa));
        add_category_to_loadable_list((Category)cat);
    }
}

class 和 category 被分开处理，先通过 schedule_class_load 将需要执行 load 的 class 添加到一个全局列表里，之后再通过 add_category_to_loadable_list 将需要执行 load 的 category 添加到另一个全局列表里

// List of classes that need +load called (pending superclass +load)
// This list always has superclasses first because of the way it is constructed
static struct loadable_class *loadable_classes NOBSS = NULL;

// List of categories that need +load called (pending parent class +load)
static struct loadable_category *loadable_categories NOBSS = NULL;

schedule_class_load

/***********************************************************************
* prepare_load_methods
* Schedule +load for classes in this image, any un-+load-ed 
* superclasses in other images, and any categories in this image.
**********************************************************************/
// Recursively schedule +load for cls and any un-+load-ed superclasses.
// cls must already be connected.
static void schedule_class_load(class_t *cls)
{
    assert(isRealized(cls));  // _read_images should realize

    if (cls->data->flags & RW_LOADED) return;

    class_t *supercls = getSuperclass(cls);
    if (supercls) schedule_class_load(supercls);

    add_class_to_loadable_list((Class)cls);
    changeInfo(cls, RW_LOADED, 0); 
}

class 的处理是递归处理父类，确保父类先被添加到 loadable_classes

call_load_methods

__private_extern__ void call_load_methods(void)
{
    static BOOL loading = NO;
    BOOL more_categories;

    recursive_mutex_assert_locked(&loadMethodLock);

    // Re-entrant calls do nothing; the outermost call will finish the job.
    if (loading) return;
    loading = YES;

    do {
        // 1. Repeatedly call class +loads until there aren't any more
        while (loadable_classes_used > 0) {
            call_class_loads();
        }

        // 2. Call category +loads ONCE
        more_categories = call_category_loads();

        // 3. Run more +loads if there are classes OR more untried categories
    } while (loadable_classes_used > 0  ||  more_categories);

    loading = NO;
}

1. Repeatedly call class +loads until there aren't any more

call_class_loads

/***********************************************************************
* call_class_loads
* Call all pending class +load methods.
* If new classes become loadable, +load is NOT called for them.
*
* Called only by call_load_methods().
**********************************************************************/
static void call_class_loads(void)
{
    int i;

    // Detach current loadable list.
    struct loadable_class *classes = loadable_classes;
    int used = loadable_classes_used;
    loadable_classes = NULL;
    loadable_classes_allocated = 0;
    loadable_classes_used = 0;

    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Class cls = classes[i].cls;
        IMP load_method = classes[i].method;
        if (!cls) continue; 

        if (PrintLoading) {
            _objc_inform("LOAD: +[%s load]\n", _class_getName(cls));
        }
        (*load_method) ((id) cls, SEL_load);
    }

    // Destroy the detached list.
    if (classes) _free_internal(classes);
}

load 方法的本质:是直接执行函数指针，因此 load 方法不会执行 objc_msgSend 的那一整套流程,call_category_loads 的最终实现也和 call_class_loads 一样都是直接获取函数指针来执行

load 方法总结

load 是在被添加到 runtime 时开始执行，父类最先执行，然后是子类，最后是 Category。又因为是直接获取函数指针来执行，不会像 objc_msgSend 一样会有方法查找的过程

+initialize

runtime源码_class_initialize

/***********************************************************************
* class_initialize.  Send the '+initialize' message on demand to any
* uninitialized class. Force initialization of superclasses first.
*
* Called only from _class_lookupMethodAndLoadCache (or itself).
**********************************************************************/
__private_extern__ void _class_initialize(Class cls)
{
    Class supercls;
    BOOL reallyInitialize = NO;

    // Get the real class from the metaclass. The superclass chain 
    // hangs off the real class only.
    cls = _class_getNonMetaClass(cls);

    // Make sure super is done initializing BEFORE beginning to initialize cls.
    // See note about deadlock above.
    supercls = _class_getSuperclass(cls);
    if (supercls  &&  !_class_isInitialized(supercls)) {
        _class_initialize(supercls);
    }

    // Try to atomically set CLS_INITIALIZING.
    monitor_enter(&classInitLock);
    if (!_class_isInitialized(cls) && !_class_isInitializing(cls)) {
        _class_setInitializing(cls);
        reallyInitialize = YES;
    }
    monitor_exit(&classInitLock);

    if (reallyInitialize) {
        // We successfully set the CLS_INITIALIZING bit. Initialize the class.

        // Record that we're initializing this class so we can message it.
        _setThisThreadIsInitializingClass(cls);

        // Send the +initialize message.
        // Note that +initialize is sent to the superclass (again) if 
        // this class doesn't implement +initialize. 2157218
        if (PrintInitializing) {
            _objc_inform("INITIALIZE: calling +[%s initialize]",
                         _class_getName(cls));
        }

        ((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);

        if (PrintInitializing) {
            _objc_inform("INITIALIZE: finished +[%s initialize]",
                         _class_getName(cls));
        }        

        // Done initializing. 
        ......
}

initialize 也是取出父类递归执行，确保父类的方法先被执行到,可以发现 initialize 最终是通过 objc_msgSend 来执行的，即 initialize 是会经过一系列方法查找来执行的

initialize 方法总结

initialize 最终是通过 objc_msgSend 来执行的，objc_msgSend 会执行一系列方法查找，并且 Category 的方法会覆盖类中的方法

objc4-723 Runtime源码 load/initialize相关实现

load

① load_images

/***********************************************************************
* load_images
* Process +load in the given images which are being mapped in by dyld.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
extern bool hasLoadMethods(const headerType *mhdr);
extern void prepare_load_methods(const headerType *mhdr);

void
load_images(const char *path __unused, const struct mach_header *mh)
{
    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;

    recursive_mutex_locker_t lock(loadMethodLock);

    // Discover load methods
    {
        rwlock_writer_t lock2(runtimeLock);
        prepare_load_methods((const headerType *)mh);
    }

    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}

② prepare_load_methods

void prepare_load_methods(const headerType *mhdr)
{
    size_t count, i;

    runtimeLock.assertWriting();

    classref_t *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);
    for (i = 0; i < count; i++) {
        schedule_class_load(remapClass(classlist[i]));
    }

    category_t **categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = categorylist[i];
        Class cls = remapClass(cat->cls);
        if (!cls) continue;  // category for ignored weak-linked class
        realizeClass(cls);
        assert(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }
}

2.1 schedule_class_load

/***********************************************************************
* prepare_load_methods
* Schedule +load for classes in this image, any un-+load-ed 
* superclasses in other images, and any categories in this image.
**********************************************************************/
// Recursively schedule +load for cls and any un-+load-ed superclasses.
// cls must already be connected.
static void schedule_class_load(Class cls)
{
    if (!cls) return;
    assert(cls->isRealized());  // _read_images should realize

    if (cls->data()->flags & RW_LOADED) return;

    // Ensure superclass-first ordering
    schedule_class_load(cls->superclass);

    add_class_to_loadable_list(cls);
    cls->setInfo(RW_LOADED); 
}

2.2 add_category_to_loadable_list

/***********************************************************************
* add_category_to_loadable_list
* Category cat's parent class exists and the category has been attached
* to its class. Schedule this category for +load after its parent class
* becomes connected and has its own +load method called.
**********************************************************************/
void add_category_to_loadable_list(Category cat)
{
    IMP method;

    loadMethodLock.assertLocked();

    method = _category_getLoadMethod(cat);

    // Don't bother if cat has no +load method
    if (!method) return;

    if (PrintLoading) {
        _objc_inform("LOAD: category '%s(%s)' scheduled for +load", 
                     _category_getClassName(cat), _category_getName(cat));
    }

    if (loadable_categories_used == loadable_categories_allocated) {
        loadable_categories_allocated = loadable_categories_allocated*2 + 16;
        loadable_categories = (struct loadable_category *)
            realloc(loadable_categories,
                              loadable_categories_allocated *
                              sizeof(struct loadable_category));
    }

    loadable_categories[loadable_categories_used].cat = cat;
    loadable_categories[loadable_categories_used].method = method;
    loadable_categories_used++;
}

③ call_load_methods

/***********************************************************************
* call_class_loads
* Call all pending class +load methods.
* If new classes become loadable, +load is NOT called for them.
*
* Called only by call_load_methods().
**********************************************************************/
static void call_class_loads(void)
{
    int i;

    // Detach current loadable list.
    struct loadable_class *classes = loadable_classes;
    int used = loadable_classes_used;
    loadable_classes = nil;
    loadable_classes_allocated = 0;
    loadable_classes_used = 0;

    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Class cls = classes[i].cls;
        load_method_t load_method = (load_method_t)classes[i].method;
        if (!cls) continue; 

        if (PrintLoading) {
            _objc_inform("LOAD: +[%s load]\n", cls->nameForLogging());
        }
        (*load_method)(cls, SEL_load);
    }

    // Destroy the detached list.
    if (classes) free(classes);
}

initialize

_class_initialize

/***********************************************************************
* class_initialize.  Send the '+initialize' message on demand to any
* uninitialized class. Force initialization of superclasses first.
**********************************************************************/
void _class_initialize(Class cls)
{
    assert(!cls->isMetaClass());

    Class supercls;
    bool reallyInitialize = NO;

    // Make sure super is done initializing BEFORE beginning to initialize cls.
    // See note about deadlock above.
    supercls = cls->superclass;
    if (supercls  &&  !supercls->isInitialized()) {
        _class_initialize(supercls);
    }

    // Try to atomically set CLS_INITIALIZING.
    {
        monitor_locker_t lock(classInitLock);
        if (!cls->isInitialized() && !cls->isInitializing()) {
            cls->setInitializing();
            reallyInitialize = YES;
        }
    }

    if (reallyInitialize) {
        // We successfully set the CLS_INITIALIZING bit. Initialize the class.

        // Record that we're initializing this class so we can message it.
        _setThisThreadIsInitializingClass(cls);

        if (MultithreadedForkChild) {
            // LOL JK we don't really call +initialize methods after fork().
            performForkChildInitialize(cls, supercls);
            return;
        }

        // Send the +initialize message.
        // Note that +initialize is sent to the superclass (again) if 
        // this class doesn't implement +initialize. 2157218
        if (PrintInitializing) {
            _objc_inform("INITIALIZE: thread %p: calling +[%s initialize]",
                         pthread_self(), cls->nameForLogging());
        }

        // Exceptions: A +initialize call that throws an exception 
        // is deemed to be a complete and successful +initialize.
        //
        // Only __OBJC2__ adds these handlers. !__OBJC2__ has a
        // bootstrapping problem of this versus CF's call to
        // objc_exception_set_functions().
#if __OBJC2__
        @try
#endif
        {
            callInitialize(cls);

            if (PrintInitializing) {
                _objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
                             pthread_self(), cls->nameForLogging());
            }
        }
#if __OBJC2__
        @catch (...) {
            if (PrintInitializing) {
                _objc_inform("INITIALIZE: thread %p: +[%s initialize] "
                             "threw an exception",
                             pthread_self(), cls->nameForLogging());
            }
            @throw;
        }
        @finally
#endif
        {
            // Done initializing.
            lockAndFinishInitializing(cls, supercls);
        }
        return;
    }

    else if (cls->isInitializing()) {
        // We couldn't set INITIALIZING because INITIALIZING was already set.
        // If this thread set it earlier, continue normally.
        // If some other thread set it, block until initialize is done.
        // It's ok if INITIALIZING changes to INITIALIZED while we're here, 
        //   because we safely check for INITIALIZED inside the lock 
        //   before blocking.
        if (_thisThreadIsInitializingClass(cls)) {
            return;
        } else if (!MultithreadedForkChild) {
            waitForInitializeToComplete(cls);
            return;
        } else {
            // We're on the child side of fork(), facing a class that
            // was initializing by some other thread when fork() was called.
            _setThisThreadIsInitializingClass(cls);
            performForkChildInitialize(cls, supercls);
        }
    }

    else if (cls->isInitialized()) {
        // Set CLS_INITIALIZING failed because someone else already 
        //   initialized the class. Continue normally.
        // NOTE this check must come AFTER the ISINITIALIZING case.
        // Otherwise: Another thread is initializing this class. ISINITIALIZED 
        //   is false. Skip this clause. Then the other thread finishes 
        //   initialization and sets INITIALIZING=no and INITIALIZED=yes. 
        //   Skip the ISINITIALIZING clause. Die horribly.
        return;
    }

    else {
        // We shouldn't be here. 
        _objc_fatal("thread-safe class init in objc runtime is buggy!");
    }
}