应用程序加载（一） -- dyld流程分析
应用程序加载（二） -- dyld&objc关联以及类的加载初探
应用程序加载（三）-- 类的加载
应用程序加载（四）-- 分类的加载
应用程序加载（五）-- 类扩展和关联对象
前一篇文章通过一个小例子了解到应用程序加载的逻辑。这篇文章紧接着去研究类的加载。
切入点 - _objc_init
在上篇文章中对dyld的分析，最后回到objc源码中是通过_dyld_objc_notify_register这个函数，该函数是被_objc_init函数中调用的。
从dyld到objc的简要流程图：
_objc_init分析
看看_objc_init的函数实现：
/***********************************************************************
* _objc_init
* Bootstrap initialization. Registers our image notifier with dyld.
* Called by libSystem BEFORE library initialization time
**********************************************************************/
void _objc_init(void)
{
static bool initialized = false;
if (initialized) return;
initialized = true;
// fixme defer initialization until an objc-using image is found?
environ_init();
tls_init();
static_init();
runtime_init();
exception_init();
cache_init();
_imp_implementationWithBlock_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
#if __OBJC2__
didCallDyldNotifyRegister = true;
#endif
}
- 前三行代码：定义一个静态变量作为开关，目的是下面的操作只进行一次
- 然后调用7个
xxx_init()函数- environ_init：环境变量相关内容
- tls_init：线程相关内容
- static_init：c++静态构造函数相关内容
- runtime_init：运行时相关内容
- exception_init：异常相关初始化处理
- cache_init：缓存相关初始化处理
- _imp_implementationWithBlock_init：系统层级的一些额外处理
- 调用_dyld_objc_notify_register函数，这个函数是在
dyld中实现的，并且提供三个回调函数：- map_images：映射镜像
- load_images：加载镜像
- unmap_image：不需要映射的镜像
此时结合上篇的内容，应用启动后，从dyld开始，到objc，然后又回到dyld中。因为程序启动的入口是main函数，而main函数是在dyld中调用的。完整的流程图如下：
类加载map_images
从dyld到objc再到dyld。整个启动过程已经了解。但是还是没有看到类加载的位置。但是我们遗漏了objc给dyld中“预留”的几个回调函数。看看map_images回调函数的的实现：
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);
}
- 核心代码
map_images_nolock调用，接下来看看它的实现
void
map_images_nolock(unsigned mhCount, const char * const mhPaths[],
const struct mach_header * const mhdrs[])
{
//省略部分代码
// Find all images with Objective-C metadata.
hCount = 0;
//省略hCount计算代码
if (hCount > 0) {
_read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
}
//省略部分代码
}
- 此处核心代码是
_read_images函数调用，进入的条件是变量hCount。但是没必要细究这些，直接看它的实现，代码很长，可以直接跳过，下面有对代码的解释
void _read_images(header_info **hList, uint32_t hCount, int totalClasses, int unoptimizedTotalClasses)
{
header_info *hi;
uint32_t hIndex;
size_t count;
size_t i;
Class *resolvedFutureClasses = nil;
size_t resolvedFutureClassCount = 0;
static bool doneOnce;
bool launchTime = NO;
TimeLogger ts(PrintImageTimes);
runtimeLock.assertLocked();
#define EACH_HEADER \
hIndex = 0; \
hIndex < hCount && (hi = hList[hIndex]); \
hIndex++
if (!doneOnce) {
doneOnce = YES;
launchTime = YES;
#if SUPPORT_NONPOINTER_ISA
// Disable non-pointer isa under some conditions.
# if SUPPORT_INDEXED_ISA
// Disable nonpointer isa if any image contains old Swift code
for (EACH_HEADER) {
if (hi->info()->containsSwift() &&
hi->info()->swiftUnstableVersion() < objc_image_info::SwiftVersion3)
{
DisableNonpointerIsa = true;
if (PrintRawIsa) {
_objc_inform("RAW ISA: disabling non-pointer isa because "
"the app or a framework contains Swift code "
"older than Swift 3.0");
}
break;
}
}
# endif
# if TARGET_OS_OSX
// Disable non-pointer isa if the app is too old
// (linked before OS X 10.11)
if (dyld_get_program_sdk_version() < DYLD_MACOSX_VERSION_10_11) {
DisableNonpointerIsa = true;
if (PrintRawIsa) {
_objc_inform("RAW ISA: disabling non-pointer isa because "
"the app is too old (SDK version " SDK_FORMAT ")",
FORMAT_SDK(dyld_get_program_sdk_version()));
}
}
// Disable non-pointer isa if the app has a __DATA,__objc_rawisa section
// New apps that load old extensions may need this.
for (EACH_HEADER) {
if (hi->mhdr()->filetype != MH_EXECUTE) continue;
unsigned long size;
if (getsectiondata(hi->mhdr(), "__DATA", "__objc_rawisa", &size)) {
DisableNonpointerIsa = true;
if (PrintRawIsa) {
_objc_inform("RAW ISA: disabling non-pointer isa because "
"the app has a __DATA,__objc_rawisa section");
}
}
break; // assume only one MH_EXECUTE image
}
# endif
#endif
if (DisableTaggedPointers) {
disableTaggedPointers();
}
initializeTaggedPointerObfuscator();
if (PrintConnecting) {
_objc_inform("CLASS: found %d classes during launch", totalClasses);
}
// namedClasses
// Preoptimized classes don't go in this table.
// 4/3 is NXMapTable's load factor
int namedClassesSize =
(isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
gdb_objc_realized_classes =
NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);
ts.log("IMAGE TIMES: first time tasks");
}
// Fix up @selector references
static size_t UnfixedSelectors;
{
mutex_locker_t lock(selLock);
for (EACH_HEADER) {
if (hi->hasPreoptimizedSelectors()) continue;
bool isBundle = hi->isBundle();
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) {
sels[i] = sel;
}
}
}
}
ts.log("IMAGE TIMES: fix up selector references");
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
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;
}
classref_t const *classlist = _getObjc2ClassList(hi, &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);
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;
}
}
}
ts.log("IMAGE TIMES: discover classes");
// Fix up remapped classes
// Class list and nonlazy class list remain unremapped.
// Class refs and super refs are remapped for message dispatching.
if (!noClassesRemapped()) {
for (EACH_HEADER) {
Class *classrefs = _getObjc2ClassRefs(hi, &count);
for (i = 0; i < count; i++) {
remapClassRef(&classrefs[i]);
}
// fixme why doesn't test future1 catch the absence of this?
classrefs = _getObjc2SuperRefs(hi, &count);
for (i = 0; i < count; i++) {
remapClassRef(&classrefs[i]);
}
}
}
ts.log("IMAGE TIMES: remap classes");
#if SUPPORT_FIXUP
// Fix up old objc_msgSend_fixup call sites
for (EACH_HEADER) {
message_ref_t *refs = _getObjc2MessageRefs(hi, &count);
if (count == 0) continue;
if (PrintVtables) {
_objc_inform("VTABLES: repairing %zu unsupported vtable dispatch "
"call sites in %s", count, hi->fname());
}
for (i = 0; i < count; i++) {
fixupMessageRef(refs+i);
}
}
ts.log("IMAGE TIMES: fix up objc_msgSend_fixup");
#endif
bool cacheSupportsProtocolRoots = sharedCacheSupportsProtocolRoots();
// Discover protocols. Fix up protocol refs.
for (EACH_HEADER) {
extern objc_class OBJC_CLASS_$_Protocol;
Class cls = (Class)&OBJC_CLASS_$_Protocol;
ASSERT(cls);
NXMapTable *protocol_map = protocols();
bool isPreoptimized = hi->hasPreoptimizedProtocols();
// Skip reading protocols if this is an image from the shared cache
// and we support roots
// Note, after launch we do need to walk the protocol as the protocol
// in the shared cache is marked with isCanonical() and that may not
// be true if some non-shared cache binary was chosen as the canonical
// definition
if (launchTime && isPreoptimized && cacheSupportsProtocolRoots) {
if (PrintProtocols) {
_objc_inform("PROTOCOLS: Skipping reading protocols in image: %s",
hi->fname());
}
continue;
}
bool isBundle = hi->isBundle();
protocol_t * const *protolist = _getObjc2ProtocolList(hi, &count);
for (i = 0; i < count; i++) {
readProtocol(protolist[i], cls, protocol_map,
isPreoptimized, isBundle);
}
}
ts.log("IMAGE TIMES: discover protocols");
// Fix up @protocol references
// Preoptimized images may have the right
// answer already but we don't know for sure.
for (EACH_HEADER) {
// At launch time, we know preoptimized image refs are pointing at the
// shared cache definition of a protocol. We can skip the check on
// launch, but have to visit @protocol refs for shared cache images
// loaded later.
if (launchTime && cacheSupportsProtocolRoots && hi->isPreoptimized())
continue;
protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
for (i = 0; i < count; i++) {
remapProtocolRef(&protolist[i]);
}
}
ts.log("IMAGE TIMES: fix up @protocol references");
// Discover categories. Only do this after the initial category
// attachment has been done. For categories present at startup,
// discovery is deferred until the first load_images call after
// the call to _dyld_objc_notify_register completes. rdar://problem/53119145
if (didInitialAttachCategories) {
for (EACH_HEADER) {
load_categories_nolock(hi);
}
}
ts.log("IMAGE TIMES: discover categories");
// Category discovery MUST BE Late to avoid potential races
// when other threads call the new category code before
// this thread finishes its fixups.
// +load handled by prepare_load_methods()
// Realize non-lazy classes (for +load methods and static instances)
for (EACH_HEADER) {
classref_t const *classlist =
_getObjc2NonlazyClassList(hi, &count);
for (i = 0; i < count; i++) {
Class cls = remapClass(classlist[i]);
if (!cls) continue;
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);
}
}
ts.log("IMAGE TIMES: realize non-lazy classes");
// Realize newly-resolved future classes, in case CF manipulates them
if (resolvedFutureClasses) {
for (i = 0; i < resolvedFutureClassCount; i++) {
Class cls = resolvedFutureClasses[i];
if (cls->isSwiftStable()) {
_objc_fatal("Swift class is not allowed to be future");
}
realizeClassWithoutSwift(cls, nil);
cls->setInstancesRequireRawIsaRecursively(false/*inherited*/);
}
free(resolvedFutureClasses);
}
ts.log("IMAGE TIMES: realize future classes");
if (DebugNonFragileIvars) {
realizeAllClasses();
}
// Print preoptimization statistics
if (PrintPreopt) {
static unsigned int PreoptTotalMethodLists;
static unsigned int PreoptOptimizedMethodLists;
static unsigned int PreoptTotalClasses;
static unsigned int PreoptOptimizedClasses;
for (EACH_HEADER) {
if (hi->hasPreoptimizedSelectors()) {
_objc_inform("PREOPTIMIZATION: honoring preoptimized selectors "
"in %s", hi->fname());
}
else if (hi->info()->optimizedByDyld()) {
_objc_inform("PREOPTIMIZATION: IGNORING preoptimized selectors "
"in %s", hi->fname());
}
classref_t const *classlist = _getObjc2ClassList(hi, &count);
for (i = 0; i < count; i++) {
Class cls = remapClass(classlist[i]);
if (!cls) continue;
PreoptTotalClasses++;
if (hi->hasPreoptimizedClasses()) {
PreoptOptimizedClasses++;
}
const method_list_t *mlist;
if ((mlist = ((class_ro_t *)cls->data())->baseMethods())) {
PreoptTotalMethodLists++;
if (mlist->isFixedUp()) {
PreoptOptimizedMethodLists++;
}
}
if ((mlist=((class_ro_t *)cls->ISA()->data())->baseMethods())) {
PreoptTotalMethodLists++;
if (mlist->isFixedUp()) {
PreoptOptimizedMethodLists++;
}
}
}
}
_objc_inform("PREOPTIMIZATION: %zu selector references not "
"pre-optimized", UnfixedSelectors);
_objc_inform("PREOPTIMIZATION: %u/%u (%.3g%%) method lists pre-sorted",
PreoptOptimizedMethodLists, PreoptTotalMethodLists,
PreoptTotalMethodLists
? 100.0*PreoptOptimizedMethodLists/PreoptTotalMethodLists
: 0.0);
_objc_inform("PREOPTIMIZATION: %u/%u (%.3g%%) classes pre-registered",
PreoptOptimizedClasses, PreoptTotalClasses,
PreoptTotalClasses
? 100.0*PreoptOptimizedClasses/PreoptTotalClasses
: 0.0);
_objc_inform("PREOPTIMIZATION: %zu protocol references not "
"pre-optimized", UnfixedProtocolReferences);
}
#undef EACH_HEADER
}
通过代码中的注释，可以了解到：
- 条件控制进⾏⼀次的加载
- 修复预编译阶段的
@selector的混乱问题 - 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!
- 当我们类⾥⾯有协议的时候 : readProtocol
- 修复没有被加载的协议
- 分类处理
- 类的加载处理
- 没有被处理的类，优化那些被侵犯的类
在其中，发现了readClass函数的调用，通过函数名，其实就可以了解，此处就是加载类的地方，如图
此文先了解到这，下篇文章再仔细了解一下类的加载逻辑。
扩展 - 环境变量
在_objc_init中调用的environ_init函数的实现：
void environ_init(void)
{
//省略代码
...
bool PrintHelp = false;
bool PrintOptions = false;
//省略代码
...
// Print OBJC_HELP and OBJC_PRINT_OPTIONS output.
if (PrintHelp || PrintOptions) {
//省略代码
...
for (size_t i = 0; i < sizeof(Settings)/sizeof(Settings[0]); i++) {
const option_t *opt = &Settings[i];
if (PrintHelp) _objc_inform("%s: %s", opt->env, opt->help);
if (PrintOptions && *opt->var) _objc_inform("%s is set", opt->env);
}
}
}
通过产看最后for循环中的源码，可以打印所有的环境变量。但是被两个bool类型的变量控制。
- PrintHelp：环境变量和帮助说明。
- PrintOptions：当前环境变量的状态。
objc源码是可调式的，我们可以修改PrintHelp=true，就可以打印看到所有的环境变量。
bool PrintHelp = false; 修改成=》 bool PrintHelp = true;
运行打印如下：
objc[47614]: Objective-C runtime debugging. Set variable=YES to enable.
objc[47614]: OBJC_HELP: describe available environment variables
objc[47614]: OBJC_HELP is set
objc[47614]: OBJC_PRINT_OPTIONS: list which options are set
objc[47614]: OBJC_PRINT_OPTIONS is set
objc[47614]: OBJC_PRINT_IMAGES: log image and library names as they are loaded
objc[47614]: OBJC_PRINT_IMAGE_TIMES: measure duration of image loading steps
objc[47614]: OBJC_PRINT_LOAD_METHODS: log calls to class and category +load methods
objc[47614]: OBJC_PRINT_INITIALIZE_METHODS: log calls to class +initialize methods
objc[47614]: OBJC_PRINT_RESOLVED_METHODS: log methods created by +resolveClassMethod: and +resolveInstanceMethod:
objc[47614]: OBJC_PRINT_CLASS_SETUP: log progress of class and category setup
objc[47614]: OBJC_PRINT_PROTOCOL_SETUP: log progress of protocol setup
objc[47614]: OBJC_PRINT_IVAR_SETUP: log processing of non-fragile ivars
objc[47614]: OBJC_PRINT_VTABLE_SETUP: log processing of class vtables
objc[47614]: OBJC_PRINT_VTABLE_IMAGES: print vtable images showing overridden methods
objc[47614]: OBJC_PRINT_CACHE_SETUP: log processing of method caches
objc[47614]: OBJC_PRINT_FUTURE_CLASSES: log use of future classes for toll-free bridging
objc[47614]: OBJC_PRINT_PREOPTIMIZATION: log preoptimization courtesy of dyld shared cache
objc[47614]: OBJC_PRINT_CXX_CTORS: log calls to C++ ctors and dtors for instance variables
objc[47614]: OBJC_PRINT_EXCEPTIONS: log exception handling
objc[47614]: OBJC_PRINT_EXCEPTION_THROW: log backtrace of every objc_exception_throw()
objc[47614]: OBJC_PRINT_ALT_HANDLERS: log processing of exception alt handlers
objc[47614]: OBJC_PRINT_REPLACED_METHODS: log methods replaced by category implementations
objc[47614]: OBJC_PRINT_DEPRECATION_WARNINGS: warn about calls to deprecated runtime functions
objc[47614]: OBJC_PRINT_POOL_HIGHWATER: log high-water marks for autorelease pools
objc[47614]: OBJC_PRINT_CUSTOM_CORE: log classes with custom core methods
objc[47614]: OBJC_PRINT_CUSTOM_RR: log classes with custom retain/release methods
objc[47614]: OBJC_PRINT_CUSTOM_AWZ: log classes with custom allocWithZone methods
objc[47614]: OBJC_PRINT_RAW_ISA: log classes that require raw pointer isa fields
objc[47614]: OBJC_DEBUG_UNLOAD: warn about poorly-behaving bundles when unloaded
objc[47614]: OBJC_DEBUG_FRAGILE_SUPERCLASSES: warn about subclasses that may have been broken by subsequent changes to superclasses
objc[47614]: OBJC_DEBUG_NIL_SYNC: warn about @synchronized(nil), which does no synchronization
objc[47614]: OBJC_DEBUG_NONFRAGILE_IVARS: capriciously rearrange non-fragile ivars
objc[47614]: OBJC_DEBUG_ALT_HANDLERS: record more info about bad alt handler use
objc[47614]: OBJC_DEBUG_MISSING_POOLS: warn about autorelease with no pool in place, which may be a leak
objc[47614]: OBJC_DEBUG_POOL_ALLOCATION: halt when autorelease pools are popped out of order, and allow heap debuggers to track autorelease pools
objc[47614]: OBJC_DEBUG_DUPLICATE_CLASSES: halt when multiple classes with the same name are present
objc[47614]: OBJC_DEBUG_DONT_CRASH: halt the process by exiting instead of crashing
objc[47614]: OBJC_DISABLE_VTABLES: disable vtable dispatch
objc[47614]: OBJC_DISABLE_PREOPTIMIZATION: disable preoptimization courtesy of dyld shared cache
objc[47614]: OBJC_DISABLE_TAGGED_POINTERS: disable tagged pointer optimization of NSNumber et al.
objc[47614]: OBJC_DISABLE_TAG_OBFUSCATION: disable obfuscation of tagged pointers
objc[47614]: OBJC_DISABLE_NONPOINTER_ISA: disable non-pointer isa fields
objc[47614]: OBJC_DISABLE_INITIALIZE_FORK_SAFETY: disable safety checks for +initialize after fork
OBJC_PRINT_LOAD_METHODS环境变量
OBJC_PRINT_LOAD_METHODS环境变量可以帮助我们打印所有的load方法的调用，添加环境变量的方式如图：
该工程是一个“空工程”，创建后仅仅在AppDelegate和ViewController中添加了+load方法。
运行后能看到工程中调用load方法的类。
使用场景：load方法中如果有耗时的操作，就会影响应用的启动速度，所以通过这种方式，可以很方便的查找。
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