两个方法的区别

1.两个方法的调用方式

load是拿到函数地址直接进行调用
initialize是通过objc_msgSend()进行调用的

2.两个方法的调用时机

load是runtime加载类，分类的时候调用的(只调用一次)
initialize是类第一次接收到消息时调用的，而且每个类只能被初始化一次(父类initialize方法可能被调用多次)

3.两个方法的调用顺序

• load

  1. 先调用类的load方法
     先编译的类优先调用
     调用子类的load的之前，会先调用父类的load方法
  2. 再调用分类的load方法
     先编译的分类优先先调用(只看编译顺序，不区分是父类的分类还是子类的分类)

• initialize

  先初始化父类
  再初始化子类(可能最终调用的是父类的initialize方法)

load源码分析步骤:

objc4源码解读过程：objc-os.mm
_objc_init
load_images
prepare_load_methods
schedule_class_load
add_class_to_loadable_list
add_category_to_loadable_list
call_load_methods
call_class_loads
call_category_loads
(*load_method)(cls, SEL_load)

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);
}

从上面看到+load方法是根据方法地址直接调用，并不是经过objc_msgSend函数调用，loadable_classes里存放着很多下面这种结构体：
cls也就是哪个类，method就是load方法
struct loadable_class {
Class cls;
IMP method;
};

遍历loadable_classes数组，拿到load方法地址直接调用,那么也就是数组的顺序就是调用load方法的顺序，那就接着看看这个数组是怎么添加的，看prepare_load_methods这个函数

 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);
    }
}

看到这个函数schedule_class_load,再接着深入看

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); 
}

可以看到schedule_class_load(cls->superclass);这行代码会递归将父类填到到这个列表，然后再添加当前类，
if (cls->data()->flags & RW_LOADED) return;
这行代码是判断是否添加过，如果添加过直接return，保证每个类只调用一次load方法
cls->setInfo(RW_LOADED); 这行代码是将类添加到数组里时设置的标识位用来判断是否添加过

而分类就是正常遍历直接添加，所以就是按照编译顺序调用的，先编译先调用
综上大家应该了解了+load方法调用的底层结构了，如果手动调用[self load]方法，调用的还是objc_msgSend(),则按照正常方法调用步骤，找isa，superclass，一步步寻找方法进行调用，load方法可能会被分类覆盖，但是一般不需要手动调用，

+initialize源码分析步骤：

objc4源码解读过程
objc-msg-arm64.s
objc_msgSend
objc-runtime-new.mm
class_getInstanceMethod
lookUpImpOrNil
lookUpImpOrForward
_class_initialize
callInitialize
objc_msgSend(cls, SEL_initialize)

IMP lookUpImpOrForward(Class cls, SEL sel, id inst, 
                       bool initialize, bool cache, bool resolver)
{
    IMP imp = nil;
    bool triedResolver = NO;
    runtimeLock.assertUnlocked();
    // Optimistic cache lookup
    if (cache) {
        imp = cache_getImp(cls, sel);
        if (imp) return imp;
    }
    // runtimeLock is held during isRealized and isInitialized checking
    // to prevent races against concurrent realization.
    // runtimeLock is held during method search to make
    // method-lookup + cache-fill atomic with respect to method addition.
    // Otherwise, a category could be added but ignored indefinitely because
    // the cache was re-filled with the old value after the cache flush on
    // behalf of the category.
    runtimeLock.read();
    if (!cls->isRealized()) {
        // Drop the read-lock and acquire the write-lock.
        // realizeClass() checks isRealized() again to prevent
        // a race while the lock is down.
        runtimeLock.unlockRead();
        runtimeLock.write();
        realizeClass(cls);
        runtimeLock.unlockWrite();
        runtimeLock.read();
    }
    if (initialize  &&  !cls->isInitialized()) {
        runtimeLock.unlockRead();
        _class_initialize (_class_getNonMetaClass(cls, inst));
        runtimeLock.read();
        // If sel == initialize, _class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }
 retry:    
    runtimeLock.assertReading();
    // Try this class's cache.
    imp = cache_getImp(cls, sel);
    if (imp) goto done;
    // Try this class's method lists.
    {
        Method meth = getMethodNoSuper_nolock(cls, sel);
        if (meth) {
            log_and_fill_cache(cls, meth->imp, sel, inst, cls);
            imp = meth->imp;
            goto done;
        }
    }
    // Try superclass caches and method lists.
    {
        unsigned attempts = unreasonableClassCount();
        for (Class curClass = cls->superclass;
             curClass != nil;
             curClass = curClass->superclass)
        {
            // Halt if there is a cycle in the superclass chain.
            if (--attempts == 0) {
                _objc_fatal("Memory corruption in class list.");
            }
            
            // Superclass cache.
            imp = cache_getImp(curClass, sel);
            if (imp) {
                if (imp != (IMP)_objc_msgForward_impcache) {
                    // Found the method in a superclass. Cache it in this class.
                    log_and_fill_cache(cls, imp, sel, inst, curClass);
                    goto done;
                }
                else {
                    // Found a forward:: entry in a superclass.
                    // Stop searching, but don't cache yet; call method 
                    // resolver for this class first.
                    break;
                }
            }
            
            // Superclass method list.
            Method meth = getMethodNoSuper_nolock(curClass, sel);
            if (meth) {
                log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
                imp = meth->imp;
                goto done;
            }
        }
    }

    // No implementation found. Try method resolver once.

    if (resolver  &&  !triedResolver) {
        runtimeLock.unlockRead();
        _class_resolveMethod(cls, sel, inst);
        runtimeLock.read();
        // Don't cache the result; we don't hold the lock so it may have 
        // changed already. Re-do the search from scratch instead.
        triedResolver = YES;
        goto retry;
    }

    // No implementation found, and method resolver didn't help. 
    // Use forwarding.

    imp = (IMP)_objc_msgForward_impcache;
    cache_fill(cls, sel, imp, inst);

 done:
    runtimeLock.unlockRead();

    return imp;
}

• 看上面的源码可以看到下面片段代码，会先判断当前类是否初始化过，initialize参数系统传的是YES,所以当前类没初始化过就调用_class_initialize()

 if (initialize  &&  !cls->isInitialized()) {
        runtimeLock.unlockRead();
        _class_initialize (_class_getNonMetaClass(cls, inst));
        runtimeLock.read();
        // If sel == initialize, _class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }

_class_initialize源码如下

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!");
    }
}

下面代码是一部分可以看到，初始化当前类时会先判断父类是否为真，父类是否初始化，如果未初始化去初始化父类，递归调用。

supercls = cls->superclass;
    if (supercls  &&  !supercls->isInitialized()) {
        _class_initialize(supercls);
    }

• 父类初始化完成在初始化当前类调用callInitialize，如下面代码所示,初始化完成调用lockAndFinishInitializing

callInitialize源码

• 如下，可以看到是掉用的objc_msgSend()

void callInitialize(Class cls)
{
    ((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
    asm("");
}

#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;
    }

lockAndFinishInitializing源码,将类标识为已初始化将flags= RW_INITIALIZED，然后判断是否初始化代码：

bool isInitialized() {
return getMeta()->data()->flags & RW_INITIALIZED;
}

static void lockAndFinishInitializing(Class cls, Class supercls)
{
    monitor_locker_t lock(classInitLock);
    if (!supercls  ||  supercls->isInitialized()) {
        _finishInitializing(cls, supercls);
    } else {
        _finishInitializingAfter(cls, supercls);
    }
}

如果你看明白了上面的原理，看看下面这种情况会打印什么，如果你还能猜到打印什么，那证明你真的看懂了原理，

@interface LSPerson : NSObject
@end
@implementation LSPerson
+ (void)initialize{
    NSLog(@"LSPerson +initialize");
}
@end



@interface LSStudent : LSPerson
@end
@implementation LSStudent
+ (void)initialize{
    NSLog(@"LSStudent +initialize");
}
@end



@interface LSTeacher : LSPerson
@end
@implementation LSTeacher
+ (void)initialize{
    NSLog(@"LSTeacher +initialize");
}
@end

• 1.第一种情况 正常运行，结果在最下面

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [LSStudent alloc];
        [LSTeacher alloc];
    }
    return 0;
}

• 2.第二种情况 将 LSStudent里的load方法注释掉，然后运行

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [LSStudent alloc];
        [LSTeacher alloc];
    }
    return 0;
}

• 3.第三种情况 将 LSStudent里的load方法注释掉，将 LSTeacher里的load方法也注释掉，然后运行

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [LSStudent alloc];
        [LSTeacher alloc];
    }
    return 0;
}

• 4.第四种情况 将 LSStudent里的load方法注释掉，将 LSTeacher里的load方法也注释掉，然后运行

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [LSStudent initialize];
        [LSTeacher initialize];
    }
    return 0;
}

• 第一种情况打印结果:都不注释

LSPerson +initialize
LSStudent +initialize
LSTeacher +initialize

• 第二种情况打印结果:注释LSStudent的initialize方法

LSPerson +initialize
LSPerson +initialize
LSTeacher +initialize

• 第三种情况打印结果:注释LSStudent的initialize方法，注释LSTeacher的initialize方法

LSPerson +initialize
LSPerson +initialize
LSPerson +initialize

• 第四种情况打印结果:

LSPerson +initialize
LSPerson +initialize
LSPerson +initialize
LSPerson +initialize
LSPerson +initialize

以下是调用
[LSStudent alloc];
[LSTeacher alloc];
的伪代码，如果是objc_msgSend([LSStudent class],@selector(initialize)),而student类没有就会从父类找，所以掉了父类的initialize方法，并不是初始化了多次，teacher也是类似，而第四种情况是，类第一次收到消息触发initialize方法，然后所有逻辑都走完之后再去调用你真正想调用的initialize方法，所以比第三种情况多2打印了遍

        BOOL sutdentInitialized = NO;
        BOOL personInitialized = NO;
        BOOL teacherInitialized = NO;
        
        if (!sutdentInitialized) {
            if (!personInitialized) {
                objc_msgSend(LSStudent.superclass, @selector(initialize));
                personInitialized = YES;
            }
            objc_msgSend([LSStudent class], @selector(initialize));
            sutdentInitialized = YES;
        }
        
        
        if (!teacherInitialized) {
            if (!personInitialized) {
                objc_msgSend(LSTeacher.superclass, @selector(initialize));
                personInitialized = YES;
            }

            objc_msgSend([LSTeacher class], @selector(initialize));
            teacherInitialized = YES;
        }

最后附上两个方法的分析结论和步骤图

load方法

屏幕快照 2018-11-21 下午2.19.21.png

initialize方法 屏幕快照 2018-11-21 下午2.19.27.png