iOS底层原理18：类的加载

在上一篇我们分析了_objc_init方法，程序运行时，dyld将使用包含objc_image_info的镜像文件数组，回调 mapped 函数，最后会执行libObjc的map_images方法

map_images的主要流程

map_images方法

• map_images方法源码如下，从注释可以看出，这个方法主要用来处理映射到内存中的镜像文件

/***********************************************************************
* map_images
* Process the given images which are being mapped in by dyld.
* Calls ABI-agnostic code after taking ABI-specific locks.
*
* Locking: write-locks runtimeLock
**********************************************************************/
void
map_images(unsigned count, const char * const paths[],
           const struct mach_header * const mhdrs[])
{
    mutex_locker_t lock(runtimeLock);
    return map_images_nolock(count, paths, mhdrs);
}

map_images_nolock方法

• 查看map_images_nolock方法，里面代码比较多，直接看重点

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_read_images方法

• 查看_read_images方法

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_read_images内容比较多，根据苹果注释信息，主要有以下几步：

• 条件控制进行一次加载
• 修复预编译阶段的@selector的混乱的问题
• 错误混乱的类处理
• 修复重映射一些没有被镜像文件加载进来的类
• 修复一些消息
• 当类中有协议时：readProtocol
• 修复没有被加载的协议
• 分类的处理
• 类的加载处理
• 没有被处理的类，优化那些被侵犯的类

条件控制进行一次加载

doneOnce是全局静态变量，加载一次后doneOnce=YES，下次就不会在进入判断。第一次进来主要创建表gdb_objc_realized_classes，表里存放的是不在dyld共享缓存中的命名类，无论是否实现

static bool doneOnce;
if (!doneOnce) {
    doneOnce = YES; // doneOnce:全局静态变量，只加载一次
    launchTime = YES;
    
    // ...此处省略代码
    // namedClasses 是不在dyld共享缓存中的命名类，无论是否实现。
    // Preoptimized classes don't go in this table.
    // 4/3 is NXMapTable's load factor 4/3是NXMapTable的负载系数
    int namedClassesSize = 
        (isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
    // 创建哈希表 存放所有的类
    gdb_objc_realized_classes =
        NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);

    ts.log("IMAGE TIMES: first time tasks");
}

修复@selector的混乱

• 修复@selector的混乱，从macho文件中获取对象方法列表，方法列表存放在 DATA段 的 __objc_selrefs

// Fix up @selector references
static size_t UnfixedSelectors;
{
    mutex_locker_t lock(selLock);
    for (EACH_HEADER) {
        if (hi->hasPreoptimizedSelectors()) continue;

        bool isBundle = hi->isBundle();
        // 从macho文件中获取方法名列表，方法列表存放在 DATA段 的 __objc_selrefs
        SEL *sels = _getObjc2SelectorRefs(hi, &count);
        UnfixedSelectors += count;
        for (i = 0; i < count; i++) {
            const char *name = sel_cname(sels[i]);
            SEL sel = sel_registerNameNoLock(name, isBundle);
            if (sels[i] != sel) {
                // TODO:自定义打印
                _objc_inform("=======修复@selector的混乱：%p = %p", (SEL)sels[i], sel);
                sels[i] = sel;
            }
        }
    }
}

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错误混乱的类处理

// Discover classes. Fix up unresolved future classes. Mark bundle classes.
// 发现类。修复未解析的未来类。标记捆绑类
bool hasDyldRoots = dyld_shared_cache_some_image_overridden();

for (EACH_HEADER) {
    if (! mustReadClasses(hi, hasDyldRoots)) {
        // Image is sufficiently optimized that we need not call readClass()
        continue;
    }
    
    // 从Mach-O中读取类列表信息，类总数count
    classref_t const *classlist = _getObjc2ClassList(hi, &count);
    if (hIndex == hCount - 1) {
        _objc_inform("=======发现类。修复未解析的未来类。标记捆绑类。：HTTest = %zu", count);
    }
    bool headerIsBundle = hi->isBundle();
    bool headerIsPreoptimized = hi->hasPreoptimizedClasses();

    for (i = 0; i < count; i++) {
        Class cls = (Class)classlist[I];
        Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
        
        // 类信息发生混乱，类运行时可能发生移动，但是没有被删除，相当于常说的野指针
        if (newCls != cls  &&  newCls) {
            // Class was moved but not deleted. Currently this occurs 
            // only when the new class resolved a future class.
            // Non-lazily realize the class below.
            resolvedFutureClasses = (Class *)
                realloc(resolvedFutureClasses, 
                        (resolvedFutureClassCount+1) * sizeof(Class));
            resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
        }
    }
}

• 自定义一个类HTPerson，设置相应的断点

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• 执行readClass方法后，通过lldb查看cls的值，发现已经跟类名关联上了

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我们来探究readClass方法，看看他做了什么操作

readClass方法

这个方法主要是用来更新两张表：类名称表gdb_objc_realized_classes， 所有类的表allocatedClasses

image

通过断点调试，我们发现readClass主要有三步操作：

• 获取类名mangledName
• 将类名和地址关联起来
• 添加【类和元类】到 所有类的表（allocatedClasses表）中，就是runtime_init中开辟的那个表

获取类名

• 查看nonlazyMangledName方法，内部是通过bites属性找到ro数据，类名存放在class_ro_t结构中

// Get the class's mangled name, or NULL if the class has a lazy
// name that hasn't been created yet.
const char *nonlazyMangledName() const {
    return bits.safe_ro()->getName();
}

• 查看safe_ro方法，内部就是获取ro结构体数据，因为class_ro_t结构体中存储了类名

const class_ro_t *safe_ro() const {
    class_rw_t *maybe_rw = data();
    if (maybe_rw->flags & RW_REALIZED) {
        // maybe_rw is rw
        // 已实现的类通过 bits -> rw -> ro来获取
        return maybe_rw->ro();
    } else {
        // maybe_rw is actually ro
        // 未加载过的类bits属性 存储的就是 ro结构体数据
        return (class_ro_t *)maybe_rw;
    }
}

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addNamedClass将类名和地址关联绑定起来

• 查看addNamedClass方法，查看类名和地址是如何关联起来

static void addNamedClass(Class cls, const char *name, Class replacing = nil)
{
    runtimeLock.assertLocked();
    Class old;
    if ((old = getClassExceptSomeSwift(name))  &&  old != replacing) {
        inform_duplicate(name, old, cls);

        // getMaybeUnrealizedNonMetaClass uses name lookups.
        // Classes not found by name lookup must be in the
        // secondary meta->nonmeta table.
        addNonMetaClass(cls);
    } else {
        // 更新gdb_objc_realized_classes表，将key设置为 name value 设置为cls
        NXMapInsert(gdb_objc_realized_classes, name, cls);
    }
    ASSERT(!(cls->data()->flags & RO_META));

    // wrong: constructed classes are already realized when they get here
    // ASSERT(!cls->isRealized());
}

• 更新gdb_objc_realized_classes哈希表，key是name，value是cls

添加【类和元类】到 所有类的表中

• 查看addClassTableEntry方法，源码如下，这一步就是将类和元类加入到allocatedClasses表中

static void
addClassTableEntry(Class cls, bool addMeta = true)
{
    runtimeLock.assertLocked();

    // This class is allowed to be a known class via the shared cache or via
    // data segments, but it is not allowed to be in the dynamic table already.
    // _objc_init -> runtime_init 中初始化的表：所有类的表
    auto &set = objc::allocatedClasses.get();

    ASSERT(set.find(cls) == set.end());

    if (!isKnownClass(cls))
        set.insert(cls);
    if (addMeta)
        // 将元类插入哈希表中
        addClassTableEntry(cls->ISA(), false);
}

非懒加载类的加载

// +load方法的调用，是在 load_images方法中
// +load handled by prepare_load_methods()

// 加载非懒加载类
// Realize non-lazy classes (for +load methods and static instances)
for (EACH_HEADER) {
    // 从Mach-O中读取非懒加载类列表信息，非懒加载类总数count
    classref_t const *classlist = hi->nlclslist(&count);
    if (hIndex == hCount - 1) {
        _objc_inform("=======加载非懒加载类：HTTest = %zu", count);
    }
    
    for (i = 0; i < count; i++) {
        Class cls = remapClass(classlist[i]);
        if (!cls) continue;
        // 添加【非懒加载类和他的元类】到 所有类的表中
        addClassTableEntry(cls);

        if (cls->isSwiftStable()) {
            if (cls->swiftMetadataInitializer()) {
                _objc_fatal("Swift class %s with a metadata initializer "
                            "is not allowed to be non-lazy",
                            cls->nameForLogging());
            }
            // fixme also disallow relocatable classes
            // We can't disallow all Swift classes because of
            // classes like Swift.__EmptyArrayStorage
        }
        
        realizeClassWithoutSwift(cls, nil);
    }
}

• 非懒加载类：实现了+ (void)load方法的类，程序启动时会加载
• 懒加载类：没有实现+ (void)load方法的类，在类首次使用时才会加载
• 可以查看可执行文件的Mach-O，在DATA段的__objc_nlclslist中存放的是非懒加载类

现在有两个类：HTPerson和HTTeacher，其中HTPerson类实现了+ (void)load方法，运行程序，通过MachOView查看可执行文件

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• 设置相应的断点，通过lldb打印cls，发现此时只有一个非懒加载类，即HTPerson类，地址为0x00000001000082e8，与Mach-O的非懒加载类表相对应

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• 接下来我们继续分析最重要的一个方法，realizeClassWithoutSwift
  
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类的加载

realizeClassWithoutSwift方法分析

realizeClassWithoutSwift对类cls执行首次初始化，包括分配其读写数据（即 rw数据，用于运行时记录类的信息），返回类的实际类结构。

• 非懒加载类在程序启动时，就会执行realizeClassWithoutSwift方法
• 懒加载类在使用时才会去加载，我们在方法慢速查找时有看到过，执行流程：lookUpImpOrForward --> realizeAndInitializeIfNeeded_locked --> realizeClassMaybeSwiftAndLeaveLocked --> realizeClassMaybeSwiftMaybeRelock --> realizeClassWithoutSwift

image

• realizeClassWithoutSwift方法源码如下，代码比较多，源码如下

static Class realizeClassWithoutSwift(Class cls, Class previously)
{
    runtimeLock.assertLocked();

    class_rw_t *rw;
    Class supercls;
    Class metacls;

    if (!cls) return nil;
    // 如果类已经实现，直接返回
    if (cls->isRealized()) {
        validateAlreadyRealizedClass(cls);
        return cls;
    }
    ASSERT(cls == remapClass(cls));

    // fixme verify class is not in an un-dlopened part of the shared cache?
    // 获取ro数据，类未初始化时，类结构objc_class的 bits属性存储的其实是 ro数据
    auto ro = (const class_ro_t *)cls->data();
    auto isMeta = ro->flags & RO_META;
    if (ro->flags & RO_FUTURE) {
        // This was a future class. rw data is already allocated.
        rw = cls->data();
        ro = cls->data()->ro();
        ASSERT(!isMeta);
        cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
    } else {
        // Normal class. Allocate writeable class data.
        // 开辟可读写数据，即rw，bits属性此时存储的是 rw数据
        rw = objc::zalloc();
        rw->set_ro(ro);
        rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
        cls->setData(rw);
    }

    cls->cache.initializeToEmptyOrPreoptimizedInDisguise();

#if FAST_CACHE_META
    if (isMeta) cls->cache.setBit(FAST_CACHE_META);
#endif

    // Choose an index for this class.
    // Sets cls->instancesRequireRawIsa if indexes no more indexes are available
    cls->chooseClassArrayIndex();

    if (PrintConnecting) {
        _objc_inform("CLASS: realizing class '%s'%s %p %p #%u %s%s",
                     cls->nameForLogging(), isMeta ? " (meta)" : "", 
                     (void*)cls, ro, cls->classArrayIndex(),
                     cls->isSwiftStable() ? "(swift)" : "",
                     cls->isSwiftLegacy() ? "(pre-stable swift)" : "");
    }

    // Realize superclass and metaclass, if they aren't already.
    // This needs to be done after RW_REALIZED is set above, for root classes.
    // This needs to be done after class index is chosen, for root metaclasses.
    // This assumes that none of those classes have Swift contents,
    //   or that Swift's initializers have already been called.
    //   fixme that assumption will be wrong if we add support
    //   for ObjC subclasses of Swift classes.
    // 递归实现 父类和元类
    supercls = realizeClassWithoutSwift(remapClass(cls->getSuperclass()), nil);
    metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);

#if SUPPORT_NONPOINTER_ISA
    if (isMeta) {
        // Metaclasses do not need any features from non pointer ISA
        // This allows for a faspath for classes in objc_retain/objc_release.
        cls->setInstancesRequireRawIsa();
    } else {
        // Disable non-pointer isa for some classes and/or platforms.
        // Set instancesRequireRawIsa.
        bool instancesRequireRawIsa = cls->instancesRequireRawIsa();
        bool rawIsaIsInherited = false;
        static bool hackedDispatch = false;

        if (DisableNonpointerIsa) {
            // Non-pointer isa disabled by environment or app SDK version
            instancesRequireRawIsa = true;
        }
        else if (!hackedDispatch  &&  0 == strcmp(ro->getName(), "OS_object"))
        {
            // hack for libdispatch et al - isa also acts as vtable pointer
            hackedDispatch = true;
            instancesRequireRawIsa = true;
        }
        else if (supercls  &&  supercls->getSuperclass()  &&
                 supercls->instancesRequireRawIsa())
        {
            // This is also propagated by addSubclass()
            // but nonpointer isa setup needs it earlier.
            // Special case: instancesRequireRawIsa does not propagate
            // from root class to root metaclass
            instancesRequireRawIsa = true;
            rawIsaIsInherited = true;
        }

        if (instancesRequireRawIsa) {
            cls->setInstancesRequireRawIsaRecursively(rawIsaIsInherited);
        }
    }
// SUPPORT_NONPOINTER_ISA
#endif

    // Update superclass and metaclass in case of remapping
    // 设置superclass 和 isa
    cls->setSuperclass(supercls);
    cls->initClassIsa(metacls);

    // Reconcile instance variable offsets / layout.
    // This may reallocate class_ro_t, updating our ro variable.
    if (supercls  &&  !isMeta) reconcileInstanceVariables(cls, supercls, ro);

    // Set fastInstanceSize if it wasn't set already.
    // 设置fastInstanceSize，编译器快速计算对象内存大小
    cls->setInstanceSize(ro->instanceSize);

    // Copy some flags from ro to rw
    if (ro->flags & RO_HAS_CXX_STRUCTORS) {
        cls->setHasCxxDtor();
        if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
            cls->setHasCxxCtor();
        }
    }
    
    // Propagate the associated objects forbidden flag from ro or from
    // the superclass.
    if ((ro->flags & RO_FORBIDS_ASSOCIATED_OBJECTS) ||
        (supercls && supercls->forbidsAssociatedObjects()))
    {
        rw->flags |= RW_FORBIDS_ASSOCIATED_OBJECTS;
    }

    // Connect this class to its superclass's subclass lists
    // 建立类 子类的双向链表关系
    if (supercls) {
        addSubclass(supercls, cls);
    } else {
        addRootClass(cls);
    }

    // Attach categories
    // 附加类别 - 方法化当前的类，方法排序，对类进行扩展
    methodizeClass(cls, previously);

    return cls;
}

realizeClassWithoutSwift主要进行了下列几步操作：
1、rw初始化，这里涉及到干净内存clean memory和脏内存dirty memory的概念。

• ro属于clean memory，在编译时即确定的内存空间，只读，加载后不会发生改变的内存空间，包括类名称、方法、协议和实例变量的信息；
• rw的数据空间属于dirty memory，rw是运行时的结构，可读可写，由于其动态性，可以往类中添加属性、方法、协议。在运行时会发生变更的内存。
• rwe类的额外信息。在WWDC2020中也提到，只有不到10%的类真正的更改了他们的方法，并不是每一个类都需要插入数据，进行修改的类很少，避免资源的消耗，所以就有了rwe。

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2、递归处理，进行父类和元类的实现。

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3、isa处理，在前面学习isa的时候，对于NONPOINTER_ISA进行了位域处理，指针优化，isa的末尾位是1，isa不单单代表一个指针。而对于元类以及特殊情况下的场景的一些类，无需开启指针优化的类，使用Raw Isa，isa的末尾位是0。

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4、设置superclass 和 isa属性，用来获取父类和元类

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5、设置fastInstanceSize，编译器快速计算对象内存大小

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6、c++析构函数的相关设置，以及关联对象的相关设置。

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7、建立子类与父类的双定链表关系，保证子类能找到父类，父类也可以找到子类。

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8、方法化当前的类，向类中添加方法，协议、属性，同时对方法列表进行排序等操作。

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methodizeClass方法分析

methodizeClass方法源码如下：

static void methodizeClass(Class cls, Class previously)
{
    runtimeLock.assertLocked();

    bool isMeta = cls->isMetaClass();
    auto rw = cls->data();
    auto ro = rw->ro();
    auto rwe = rw->ext();

    // Methodizing for the first time
    if (PrintConnecting) {
        _objc_inform("CLASS: methodizing class '%s' %s", 
                     cls->nameForLogging(), isMeta ? "(meta)" : "");
    }

    // Install methods and properties that the class implements itself.
    method_list_t *list = ro->baseMethods();
    if (list) {
        prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls), nullptr);
        if (rwe) rwe->methods.attachLists(&list, 1);
    }

    property_list_t *proplist = ro->baseProperties;
    if (rwe && proplist) {
        rwe->properties.attachLists(&proplist, 1);
    }

    protocol_list_t *protolist = ro->baseProtocols;
    if (rwe && protolist) {
        rwe->protocols.attachLists(&protolist, 1);
    }

    // Root classes get bonus method implementations if they don't have 
    // them already. These apply before category replacements.
    if (cls->isRootMetaclass()) {
        // root metaclass
        addMethod(cls, @selector(initialize), (IMP)&objc_noop_imp, "", NO);
    }

    // Attach categories.
    if (previously) {
        if (isMeta) {
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_METACLASS);
        } else {
            // When a class relocates, categories with class methods
            // may be registered on the class itself rather than on
            // the metaclass. Tell attachToClass to look for those.
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_CLASS_AND_METACLASS);
        }
    }
    objc::unattachedCategories.attachToClass(cls, cls,
                                             isMeta ? ATTACH_METACLASS : ATTACH_CLASS);

#if DEBUG
    // Debug: sanity-check all SELs; log method list contents
    for (const auto& meth : rw->methods()) {
        if (PrintConnecting) {
            _objc_inform("METHOD %c[%s %s]", isMeta ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(meth.name()));
        }
        ASSERT(sel_registerName(sel_getName(meth.name())) == meth.name());
    }
#endif
}

• 1、对方法、属性、协议进行处理。见下图：
  

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  从上图可以发现：rwe为空，很显然，此时还没有对类进行相关的扩展操作，所以rwe还没有被创建初始化。此时针对方法、属性、协议的添加操作时无效的！

• 2、方法列表的处理中有些不同，调用了prepareMethodLists方法。那么该方法做了哪些操作呢？见下图：

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核心流程，fixupMethodList，根据注释：根据需要对selector进行修复。进入fixupMethodList方法，查看实现流程。见下图：

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• 继续methodizeClass源码的解读。找到了类初始化过程中非常关键的步骤，向类中添加分类方法、协议等，rwe的初始化也在其中。

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