load方法和initialize方法类似点

1. 都只会调用一次    2. 父类在子类之前加载复制代码

不同点在于：

1. 加载时间不同，load方法在main()函数前进行调用，initialize在第一次调用类的所属方法时在调用
    <可能永远不调用>
     。    2. load方法不会被Category覆盖。复制代码
    

initialize源码

//向对象发送消息时，lookUpImpOrForward函数判断对象是否初始化，没有初始化则先初始化在调用类的方法IMP lookUpImpOrForward(Class cls, SEL sel, id inst, bool initialize, bool cache, bo ol resolver);//第一次调用类if (initialize && !cls->isInitialized()) {      _class_initialize (_class_getNonMetaClass(cls, inst));  }  // 第一次调用类的方法，初始化对象void _class_initialize(Class cls) {    Class supercls;    bool reallyInitialize = NO;    // 递归初始化父类    supercls = cls->superclass;    if (supercls && !supercls->isInitialized()) {          _class_initialize(supercls);    }    {        monitor_locker_t lock(classInitLock);        if (!cls->isInitialized() && !cls->isInitializing()) {              cls->setInitializing();              reallyInitialize = YES;        }    }    if (reallyInitialize) {         _setThisThreadIsInitializingClass(cls);        if (MultithreadedForkChild) { performForkChildInitialize(cls, supercls);            return;    }        @try {            // 通过objc_msgSend函数调用initialize方法                callInitialize(cls);          }        @catch (...) {             @throw;        }        @finally {            // 执行initialize方法后，进行系统的initialize过程            lockAndFinishInitializing(cls, supercls);        }        return;    }    else if (cls->isInitializing()) {        if (_thisThreadIsInitializingClass(cls)) {            return;        } else if (!MultithreadedForkChild) {            waitForInitializeToComplete(cls);            return;         } else {        _setThisThreadIsInitializingClass(cls);        performForkChildInitialize(cls, supercls);        }    }}复制代码