背景
由於前前前陣子寫了個殼，得去了解類的加載流程，當時記了一些潦草的筆記。這幾天把這些東西簡單梳理了一下，本文分析的代碼基於Android8.1.0源碼。
流程分析
從loadClass開始，我們來看下Android中類加載的流程
/libcore/ojluni/src/main/java/java/lang/ClassLoader.java::loadClass
loadClass流程如下：
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
try {
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
c = findClass(name);
}
}
return c;
}
/libcore/ojluni/src/main/java/java/lang/ClassLoader.java::findClass
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
ClassLoader類的findClass是沒有實際查找代碼的，所以調用findClass其實是調用其實現類的findClass函數，例如：BaseDexClassLoader
/libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java::findClass
每個BaseDexClassLoader都持有一個DexPathList，BaseDexClassLoader的findClass類調用了DexPathList的findClass。
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
Class c = pathList.findClass(name, suppressedExceptions);
if (c == null) {
ClassNotFoundException cnfe = new ClassNotFoundException(
"Didn't find class \"" + name + "\" on path: " + pathList);
for (Throwable t : suppressedExceptions) {
cnfe.addSuppressed(t);
}
throw cnfe;
}
return c;
}
/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java::findClass
遍歷所有dexElements，並調用Element類的findClass。
public Class<?> findClass(String name, List<Throwable> suppressed) {
for (Element element : dexElements) {
Class<?> clazz = element.findClass(name, definingContext, suppressed);
if (clazz != null) {
return clazz;
}
}
if (dexElementsSuppressedExceptions != null) {
suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
}
return null;
}
題外話，dexElements對象其實是DexPathList$Element類的數組，用於存儲已加載的dex或者jar的信息。
/libcore/dalvik/src/main/java/dalvik/system/DexPathList$Element::findClass
Element的findClass，又去調用DexFile類的loadClassBinaryName，可以理解爲在單獨的dex或者jar對象中加載類
public Class<?> findClass(String name, ClassLoader definingContext,
List<Throwable> suppressed) {
return dexFile != null ? dexFile.loadClassBinaryName(name, definingContext, suppressed)
: null;
}
libcore\dalvik\src\main\java\dalvik\system\DexFile.java::loadClassBinaryName
去調用defineClass函數
public Class loadClassBinaryName(String name, ClassLoader loader, List<Throwable> suppressed) {
return defineClass(name, loader, mCookie, this, suppressed);
}
libcore\dalvik\src\main\java\dalvik\system\DexFile.java::defineClass
調用defineClassNative，準備進入Native層
private static Class defineClass(String name, ClassLoader loader, Object cookie,
DexFile dexFile, List<Throwable> suppressed) {
Class result = null;
try {
result = defineClassNative(name, loader, cookie, dexFile);
} catch (NoClassDefFoundError e) {
if (suppressed != null) {
suppressed.add(e);
}
} catch (ClassNotFoundException e) {
if (suppressed != null) {
suppressed.add(e);
}
}
return result;
}
art\runtime\native\dalvik_system_DexFile.cc::DexFile_defineClassNative
檢查dex是否加載，類名是否合理，並遍歷DexFile對象，查找Dex文件中的ClassDef部分，找到就去調用ClassLinker::DefineClass函數。
static jclass DexFile_defineClassNative(JNIEnv* env,
jclass,
jstring javaName,
jobject javaLoader,
jobject cookie,
jobject dexFile) {
std::vector<const DexFile*> dex_files;
const OatFile* oat_file;
if (!ConvertJavaArrayToDexFiles(env, cookie, /*out*/ dex_files, /*out*/ oat_file)) {
VLOG(class_linker) << "Failed to find dex_file";
DCHECK(env->ExceptionCheck());
return nullptr;
}
ScopedUtfChars class_name(env, javaName);
if (class_name.c_str() == nullptr) {
VLOG(class_linker) << "Failed to find class_name";
return nullptr;
}
const std::string descriptor(DotToDescriptor(class_name.c_str()));
const size_t hash(ComputeModifiedUtf8Hash(descriptor.c_str()));
for (auto& dex_file : dex_files) {
const DexFile::ClassDef* dex_class_def =
OatDexFile::FindClassDef(*dex_file, descriptor.c_str(), hash);
if (dex_class_def != nullptr) {
ScopedObjectAccess soa(env);
ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
StackHandleScope<1> hs(soa.Self());
Handle<mirror::ClassLoader> class_loader(
hs.NewHandle(soa.Decode<mirror::ClassLoader>(javaLoader)));
ObjPtr<mirror::DexCache> dex_cache =
class_linker->RegisterDexFile(*dex_file, class_loader.Get());
if (dex_cache == nullptr) {
// OOME or InternalError (dexFile already registered with a different class loader).
soa.Self()->AssertPendingException();
return nullptr;
}
ObjPtr<mirror::Class> result = class_linker->DefineClass(soa.Self(),
descriptor.c_str(),
hash,
class_loader,
*dex_file,
*dex_class_def);
// Add the used dex file. This only required for the DexFile.loadClass API since normal
// class loaders already keep their dex files live.
class_linker->InsertDexFileInToClassLoader(soa.Decode<mirror::Object>(dexFile),
class_loader.Get());
if (result != nullptr) {
VLOG(class_linker) << "DexFile_defineClassNative returning " << result
<< " for " << class_name.c_str();
return soa.AddLocalReference<jclass>(result);
}
}
}
VLOG(class_linker) << "Failed to find dex_class_def " << class_name.c_str();
return nullptr;
}
art\runtime\class_linker.cc::DefineClass
DefineClass這個函數做了許多工作，相當於底層類加載邏輯的分發器，整體邏輯如下圖：
mirror::Class* ClassLinker::DefineClass(Thread* self,
const char* descriptor,
size_t hash,
Handle<mirror::ClassLoader> class_loader,
const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def) {
StackHandleScope<3> hs(self);
auto klass = hs.NewHandle<mirror::Class>(nullptr);
......
// Get the real dex file. This will return the input if there aren't any callbacks or they do
// nothing.
DexFile const* new_dex_file = nullptr;
DexFile::ClassDef const* new_class_def = nullptr;
// TODO We should ideally figure out some way to move this after we get a lock on the klass so it
// will only be called once.
Runtime::Current()->GetRuntimeCallbacks()->ClassPreDefine(descriptor,
klass,
class_loader,
dex_file,
dex_class_def,
&new_dex_file,
&new_class_def);
// Check to see if an exception happened during runtime callbacks. Return if so.
if (self->IsExceptionPending()) {
return nullptr;
}
ObjPtr<mirror::DexCache> dex_cache = RegisterDexFile(*new_dex_file, class_loader.Get());
if (dex_cache == nullptr) {
self->AssertPendingException();
return nullptr;
}
klass->SetDexCache(dex_cache);
SetupClass(*new_dex_file, *new_class_def, klass, class_loader.Get());
// Mark the string class by setting its access flag.
if (UNLIKELY(!init_done_)) {
if (strcmp(descriptor, "Ljava/lang/String;") == 0) {
klass->SetStringClass();
}
}
ObjectLock<mirror::Class> lock(self, klass);
klass->SetClinitThreadId(self->GetTid());
// Make sure we have a valid empty iftable even if there are errors.
klass->SetIfTable(GetClassRoot(kJavaLangObject)->GetIfTable());
// Add the newly loaded class to the loaded classes table.
ObjPtr<mirror::Class> existing = InsertClass(descriptor, klass.Get(), hash);
if (existing != nullptr) {
// We failed to insert because we raced with another thread. Calling EnsureResolved may cause
// this thread to block.
return EnsureResolved(self, descriptor, existing);
}
// Load the fields and other things after we are inserted in the table. This is so that we don't
// end up allocating unfree-able linear alloc resources and then lose the race condition. The
// other reason is that the field roots are only visited from the class table. So we need to be
// inserted before we allocate / fill in these fields.
LoadClass(self, *new_dex_file, *new_class_def, klass);
if (self->IsExceptionPending()) {
VLOG(class_linker) << self->GetException()->Dump();
// An exception occured during load, set status to erroneous while holding klass' lock in case
// notification is necessary.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
}
return nullptr;
}
// Finish loading (if necessary) by finding parents
CHECK(!klass->IsLoaded());
if (!LoadSuperAndInterfaces(klass, *new_dex_file)) {
// Loading failed.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
}
return nullptr;
}
CHECK(klass->IsLoaded());
// At this point the class is loaded. Publish a ClassLoad event.
// Note: this may be a temporary class. It is a listener's responsibility to handle this.
Runtime::Current()->GetRuntimeCallbacks()->ClassLoad(klass);
// Link the class (if necessary)
CHECK(!klass->IsResolved());
// TODO: Use fast jobjects?
auto interfaces = hs.NewHandle<mirror::ObjectArray<mirror::Class>>(nullptr);
MutableHandle<mirror::Class> h_new_class = hs.NewHandle<mirror::Class>(nullptr);
if (!LinkClass(self, descriptor, klass, interfaces, &h_new_class)) {
// Linking failed.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
}
return nullptr;
}
self->AssertNoPendingException();
CHECK(h_new_class != nullptr) << descriptor;
CHECK(h_new_class->IsResolved() && !h_new_class->IsErroneousResolved()) << descriptor;
if (Runtime::Current()->GetInstrumentation()->AreExitStubsInstalled()) {
DCHECK_EQ(self->GetState(), kRunnable);
Runtime::Current()->GetInstrumentation()->InstallStubsForClass(h_new_class.Get());
}
Runtime::Current()->GetRuntimeCallbacks()->ClassPrepare(klass, h_new_class);
// Notify native debugger of the new class and its layout.
jit::Jit::NewTypeLoadedIfUsingJit(h_new_class.Get());
return h_new_class.Get();
}
在DefineClass裏面也能看到許多mirror::Class::SetStatus()函數的調用，用來設置Class的加載狀態。它可以有以下幾種狀態：
enum Status {
kStatusRetired = -3, // Retired, should not be used. Use the newly cloned one instead.
kStatusErrorResolved = -2,
kStatusErrorUnresolved = -1,
kStatusNotReady = 0,
kStatusIdx = 1, // Loaded, DEX idx in super_class_type_idx_ and interfaces_type_idx_.
kStatusLoaded = 2, // DEX idx values resolved.
kStatusResolving = 3, // Just cloned from temporary class object.
kStatusResolved = 4, // Part of linking.
kStatusVerifying = 5, // In the process of being verified.
kStatusRetryVerificationAtRuntime = 6, // Compile time verification failed, retry at runtime.
kStatusVerifyingAtRuntime = 7, // Retrying verification at runtime.
kStatusVerified = 8, // Logically part of linking; done pre-init.
kStatusSuperclassValidated = 9, // Superclass validation part of init done.
kStatusInitializing = 10, // Class init in progress.
kStatusInitialized = 11, // Ready to go.
kStatusMax = 12,
};
延申：mirror命名空間下的類是底層對Java層類的映射，比如上面的mirror::Class類就是對java.lang.Class類的映射，SetAccessFlags就是對Class類的accessFlags字段賦值。
art\runtime\class_linker.cc::SetupClass
SetupClass設置類的一些基本字段信息。並把Class加載狀態設置爲kStatusIdx
void ClassLinker::SetupClass(const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def,
Handle<mirror::Class> klass,
ObjPtr<mirror::ClassLoader> class_loader) {
CHECK(klass != nullptr);
CHECK(klass->GetDexCache() != nullptr);
CHECK_EQ(mirror::Class::kStatusNotReady, klass->GetStatus());
const char* descriptor = dex_file.GetClassDescriptor(dex_class_def);
CHECK(descriptor != nullptr);
klass->SetClass(GetClassRoot(kJavaLangClass));
uint32_t access_flags = dex_class_def.GetJavaAccessFlags();
CHECK_EQ(access_flags & ~kAccJavaFlagsMask, 0U);
klass->SetAccessFlags(access_flags);
klass->SetClassLoader(class_loader);
DCHECK_EQ(klass->GetPrimitiveType(), Primitive::kPrimNot);
mirror::Class::SetStatus(klass, mirror::Class::kStatusIdx, nullptr);
klass->SetDexClassDefIndex(dex_file.GetIndexForClassDef(dex_class_def));
klass->SetDexTypeIndex(dex_class_def.class_idx_);
}
art\runtime\class_linker.cc::InsertClass
InsertClass函數判斷類是否在列表中：
- 如果在列表中，則直接返回
- 如果沒有，則添加到列表
mirror::Class* ClassLinker::InsertClass(const char* descriptor, ObjPtr<mirror::Class> klass, size_t hash) {
if (VLOG_IS_ON(class_linker)) {
ObjPtr<mirror::DexCache> dex_cache = klass->GetDexCache();
std::string source;
if (dex_cache != nullptr) {
source += " from ";
source += dex_cache->GetLocation()->ToModifiedUtf8();
}
LOG(INFO) << "Loaded class " << descriptor << source;
}
{
WriterMutexLock mu(Thread::Current(), *Locks::classlinker_classes_lock_);
ObjPtr<mirror::ClassLoader> const class_loader = klass->GetClassLoader();
ClassTable* const class_table = InsertClassTableForClassLoader(class_loader);
ObjPtr<mirror::Class> existing = class_table->Lookup(descriptor, hash);
if (existing != nullptr) {
return existing.Ptr();
}
VerifyObject(klass);
class_table->InsertWithHash(klass, hash);
if (class_loader != nullptr) {
// This is necessary because we need to have the card dirtied for remembered sets.
Runtime::Current()->GetHeap()->WriteBarrierEveryFieldOf(class_loader);
}
if (log_new_roots_) {
new_class_roots_.push_back(GcRoot<mirror::Class>(klass));
}
}
if (kIsDebugBuild) {
// Test that copied methods correctly can find their holder.
for (ArtMethod& method : klass->GetCopiedMethods(image_pointer_size_)) {
CHECK_EQ(GetHoldingClassOfCopiedMethod(&method), klass);
}
}
return nullptr;
}
對於InsertClass結果不爲空的情況，表示類已經在加載表中了。
這種情況下，調用EnsureResolved來等待類過期並且重新加載完成
mirror::Class* ClassLinker::EnsureResolved(Thread* self,
const char* descriptor,
ObjPtr<mirror::Class> klass) {
......
// For temporary classes we must wait for them to be retired.
if (init_done_ && klass->IsTemp()) {
CHECK(!klass->IsResolved());
if (klass->IsErroneousUnresolved()) {
ThrowEarlierClassFailure(klass);
return nullptr;
}
StackHandleScope<1> hs(self);
Handle<mirror::Class> h_class(hs.NewHandle(klass));
ObjectLock<mirror::Class> lock(self, h_class);
// Loop and wait for the resolving thread to retire this class.
while (!h_class->IsRetired() && !h_class->IsErroneousUnresolved()) {
lock.WaitIgnoringInterrupts();
}
if (h_class->IsErroneousUnresolved()) {
ThrowEarlierClassFailure(h_class.Get());
return nullptr;
}
CHECK(h_class->IsRetired());
// Get the updated class from class table.
klass = LookupClass(self, descriptor, h_class.Get()->GetClassLoader());
}
// Wait for the class if it has not already been linked.
size_t index = 0;
// Maximum number of yield iterations until we start sleeping.
static const size_t kNumYieldIterations = 1000;
// How long each sleep is in us.
static const size_t kSleepDurationUS = 1000; // 1 ms.
while (!klass->IsResolved() && !klass->IsErroneousUnresolved()) {
StackHandleScope<1> hs(self);
HandleWrapperObjPtr<mirror::Class> h_class(hs.NewHandleWrapper(&klass));
{
ObjectTryLock<mirror::Class> lock(self, h_class);
// Can not use a monitor wait here since it may block when returning and deadlock if another
// thread has locked klass.
if (lock.Acquired()) {
// Check for circular dependencies between classes, the lock is required for SetStatus.
if (!h_class->IsResolved() && h_class->GetClinitThreadId() == self->GetTid()) {
ThrowClassCircularityError(h_class.Get());
mirror::Class::SetStatus(h_class, mirror::Class::kStatusErrorUnresolved, self);
return nullptr;
}
}
}
{
// Handle wrapper deals with klass moving.
ScopedThreadSuspension sts(self, kSuspended);
if (index < kNumYieldIterations) {
sched_yield();
} else {
usleep(kSleepDurationUS);
}
}
++index;
}
if (klass->IsErroneousUnresolved()) {
ThrowEarlierClassFailure(klass);
return nullptr;
}
// Return the loaded class. No exceptions should be pending.
CHECK(klass->IsResolved()) << klass->PrettyClass();
self->AssertNoPendingException();
return klass.Ptr();
}
然後將EnsureResolved函數的返回的Class作爲DefineClass函數的返回值
mirror::Class* ClassLinker::DefineClass(Thread* self,
const char* descriptor,
size_t hash,
Handle<mirror::ClassLoader> class_loader,
const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def) {
......
ObjPtr<mirror::Class> existing = InsertClass(descriptor, klass.Get(), hash);
if (existing != nullptr) {
return EnsureResolved(self, descriptor, existing);
}
......
}
art\runtime\class_linker.cc::LoadClass
LoadClass函數獲取了dex文件中的classData部分，然後去調用LoadClassMembers
void ClassLinker::LoadClass(Thread* self,
const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def,
Handle<mirror::Class> klass) {
const uint8_t* class_data = dex_file.GetClassData(dex_class_def);
if (class_data == nullptr) {
return; // no fields or methods - for example a marker interface
}
LoadClassMembers(self, dex_file, class_data, klass);
}
art\runtime\class_linker.cc::LoadClassMembers
LoadClassMembers函數主要邏輯是遍歷類中的所有字段和函數，然後分別調用LoadField，LoadMethod和LinkCode
void ClassLinker::LoadClassMembers(Thread* self,
const DexFile& dex_file,
const uint8_t* class_data,
Handle<mirror::Class> klass){
......
LinearAlloc* const allocator = GetAllocatorForClassLoader(klass->GetClassLoader());
ClassDataItemIterator it(dex_file, class_data);
LengthPrefixedArray<ArtField>* sfields = AllocArtFieldArray(self,
allocator,
it.NumStaticFields());
size_t num_sfields = 0;
uint32_t last_field_idx = 0u;
for (; it.HasNextStaticField(); it.Next()) {
uint32_t field_idx = it.GetMemberIndex();
DCHECK_GE(field_idx, last_field_idx); // Ordering enforced by DexFileVerifier.
if (num_sfields == 0 || LIKELY(field_idx > last_field_idx)) {
DCHECK_LT(num_sfields, it.NumStaticFields());
LoadField(it, klass, &sfields->At(num_sfields));
++num_sfields;
last_field_idx = field_idx;
}
}
// Load instance fields.
LengthPrefixedArray<ArtField>* ifields = AllocArtFieldArray(self,
allocator,
it.NumInstanceFields());
size_t num_ifields = 0u;
last_field_idx = 0u;
for (; it.HasNextInstanceField(); it.Next()) {
uint32_t field_idx = it.GetMemberIndex();
DCHECK_GE(field_idx, last_field_idx); // Ordering enforced by DexFileVerifier.
if (num_ifields == 0 || LIKELY(field_idx > last_field_idx)) {
DCHECK_LT(num_ifields, it.NumInstanceFields());
LoadField(it, klass, &ifields->At(num_ifields));
++num_ifields;
last_field_idx = field_idx;
}
}
......
size_t class_def_method_index = 0;
uint32_t last_dex_method_index = DexFile::kDexNoIndex;
size_t last_class_def_method_index = 0;
for (size_t i = 0; it.HasNextDirectMethod(); i++, it.Next()) {
ArtMethod* method = klass->GetDirectMethodUnchecked(i, image_pointer_size_);
LoadMethod(dex_file, it, klass, method);
LinkCode(this, method, oat_class_ptr, class_def_method_index);
uint32_t it_method_index = it.GetMemberIndex();
if (last_dex_method_index == it_method_index) {
// duplicate case
method->SetMethodIndex(last_class_def_method_index);
} else {
method->SetMethodIndex(class_def_method_index);
last_dex_method_index = it_method_index;
last_class_def_method_index = class_def_method_index;
}
class_def_method_index++;
}
for (size_t i = 0; it.HasNextVirtualMethod(); i++, it.Next()) {
ArtMethod* method = klass->GetVirtualMethodUnchecked(i, image_pointer_size_);
LoadMethod(dex_file, it, klass, method);
DCHECK_EQ(class_def_method_index, it.NumDirectMethods() + i);
LinkCode(this, method, oat_class_ptr, class_def_method_index);
class_def_method_index++;
}
......
}
art\runtime\class_linker.cc::LoadField
LoadField設置ArtField結構中字段的一些值
void ClassLinker::LoadField(const ClassDataItemIterator& it,
Handle<mirror::Class> klass,
ArtField* dst) {
const uint32_t field_idx = it.GetMemberIndex();
dst->SetDexFieldIndex(field_idx);
dst->SetDeclaringClass(klass.Get());
dst->SetAccessFlags(it.GetFieldAccessFlags());
}
art\runtime\class_linker.cc::LoadMethod
LoadMethod函數主要做設置ArtMethod結構的一些屬性，比如函數的MethodIdx，CodeItem在dex文件中的偏移，函數的AccessFlag等。
void ClassLinker::LoadMethod(const DexFile& dex_file,
const ClassDataItemIterator& it,
Handle<mirror::Class> klass,
ArtMethod* dst){
uint32_t dex_method_idx = it.GetMemberIndex();
const DexFile::MethodId& method_id = dex_file.GetMethodId(dex_method_idx);
const char* method_name = dex_file.StringDataByIdx(method_id.name_idx_);
ScopedAssertNoThreadSuspension ants("LoadMethod");
dst->SetDexMethodIndex(dex_method_idx);
dst->SetDeclaringClass(klass.Get());
dst->SetCodeItemOffset(it.GetMethodCodeItemOffset());
dst->SetDexCacheResolvedMethods(klass->GetDexCache()->GetResolvedMethods(), image_pointer_size_);
uint32_t access_flags = it.GetMethodAccessFlags();
......
dst->SetAccessFlags(access_flags);
}
延申：ArtMethod是存儲Java函數在虛擬機內相關信息的結構，它不同於mirror命名空間下的Method類，ArtMethod在Java層沒有類與之直接映射。
art\runtime\class_linker.cc::LinkCode
LinkCode函數主要功能是判斷代碼是否編譯，以及函數的類型來爲函數設置入口代碼。
static void LinkCode(ClassLinker* class_linker,
ArtMethod* method,
const OatFile::OatClass* oat_class,
uint32_t class_def_method_index){
Runtime* const runtime = Runtime::Current();
if (runtime->IsAotCompiler()) {
// The following code only applies to a non-compiler runtime.
return;
}
// Method shouldn't have already been linked.
DCHECK(method->GetEntryPointFromQuickCompiledCode() == nullptr);
if (oat_class != nullptr) {
// Every kind of method should at least get an invoke stub from the oat_method.
// non-abstract methods also get their code pointers.
const OatFile::OatMethod oat_method = oat_class->GetOatMethod(class_def_method_index);
oat_method.LinkMethod(method);
}
// Install entry point from interpreter.
const void* quick_code = method->GetEntryPointFromQuickCompiledCode();
bool enter_interpreter = class_linker->ShouldUseInterpreterEntrypoint(method, quick_code);
if (!method->IsInvokable()) {
EnsureThrowsInvocationError(class_linker, method);
return;
}
if (method->IsStatic() && !method->IsConstructor()) {
// For static methods excluding the class initializer, install the trampoline.
// It will be replaced by the proper entry point by ClassLinker::FixupStaticTrampolines
// after initializing class (see ClassLinker::InitializeClass method).
method->SetEntryPointFromQuickCompiledCode(GetQuickResolutionStub());
} else if (quick_code == nullptr && method->IsNative()) {
method->SetEntryPointFromQuickCompiledCode(GetQuickGenericJniStub());
} else if (enter_interpreter) {
// Set entry point from compiled code if there's no code or in interpreter only mode.
method->SetEntryPointFromQuickCompiledCode(GetQuickToInterpreterBridge());
}
if (method->IsNative()) {
// Unregistering restores the dlsym lookup stub.
method->UnregisterNative();
if (enter_interpreter || quick_code == nullptr) {
// We have a native method here without code. Then it should have either the generic JNI
// trampoline as entrypoint (non-static), or the resolution trampoline (static).
// TODO: this doesn't handle all the cases where trampolines may be installed.
const void* entry_point = method->GetEntryPointFromQuickCompiledCode();
DCHECK(class_linker->IsQuickGenericJniStub(entry_point) ||
class_linker->IsQuickResolutionStub(entry_point));
}
}
}
跟到這裏，DefineClass裏面的流程已經過了一半了，接下來繼續來跟LinkClass那一分支。
從LinkClass函數定義來看，LinkClass有一個參數是h_new_class_out，它是一個指針，並且根據變量名猜測，它是會被LinkClass修改和賦值的，
bool ClassLinker::LinkClass(Thread* self,
const char* descriptor,
Handle<mirror::Class> klass,
Handle<mirror::ObjectArray<mirror::Class>> interfaces,
MutableHandle<mirror::Class>* h_new_class_out);
看DefineClass裏調用LinkClass的代碼，它傳了一個爲nullptr的h_new_class進去，也印證了上面的猜想：
......
MutableHandle<mirror::Class> h_new_class = hs.NewHandle<mirror::Class>(nullptr);
if (!LinkClass(self, descriptor, klass, interfaces, &h_new_class)) {
// Linking failed.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
}
return nullptr;
}
......
art\runtime\class_linker.cc::LinkClass
LinkClass中調用LinkSuperClass、LinkMethods、LinkInstanceFields和LinkStaticFields這些函數。接着給h_new_class_out分配一個值。
bool ClassLinker::LinkClass(Thread* self,
const char* descriptor,
Handle<mirror::Class> klass,
Handle<mirror::ObjectArray<mirror::Class>> interfaces,
MutableHandle<mirror::Class>* h_new_class_out) {
CHECK_EQ(mirror::Class::kStatusLoaded, klass->GetStatus());
if (!LinkSuperClass(klass)) {
return false;
}
ArtMethod* imt_data[ImTable::kSize];
// If there are any new conflicts compared to super class.
bool new_conflict = false;
std::fill_n(imt_data, arraysize(imt_data), Runtime::Current()->GetImtUnimplementedMethod());
if (!LinkMethods(self, klass, interfaces, &new_conflict, imt_data)) {
return false;
}
if (!LinkInstanceFields(self, klass)) {
return false;
}
size_t class_size;
if (!LinkStaticFields(self, klass, &class_size)) {
return false;
}
CreateReferenceInstanceOffsets(klass);
CHECK_EQ(mirror::Class::kStatusLoaded, klass->GetStatus());
ImTable* imt = nullptr;
if (klass->ShouldHaveImt()) {
if (!new_conflict) {
ImTable* super_imt = FindSuperImt(klass.Get(), image_pointer_size_);
if (super_imt != nullptr) {
bool imt_equals = true;
for (size_t i = 0; i < ImTable::kSize && imt_equals; ++i) {
imt_equals = imt_equals && (super_imt->Get(i, image_pointer_size_) == imt_data[i]);
}
if (imt_equals) {
imt = super_imt;
}
}
}
if (imt == nullptr) {
LinearAlloc* allocator = GetAllocatorForClassLoader(klass->GetClassLoader());
imt = reinterpret_cast<ImTable*>(
allocator->Alloc(self, ImTable::SizeInBytes(image_pointer_size_)));
if (imt == nullptr) {
return false;
}
imt->Populate(imt_data, image_pointer_size_);
}
}
if (!klass->IsTemp() || (!init_done_ && klass->GetClassSize() == class_size)) {
// We don't need to retire this class as it has no embedded tables or it was created the
// correct size during class linker initialization.
CHECK_EQ(klass->GetClassSize(), class_size) << klass->PrettyDescriptor();
if (klass->ShouldHaveEmbeddedVTable()) {
klass->PopulateEmbeddedVTable(image_pointer_size_);
}
if (klass->ShouldHaveImt()) {
klass->SetImt(imt, image_pointer_size_);
}
if (cha_ != nullptr) {
cha_->UpdateAfterLoadingOf(klass);
}
mirror::Class::SetStatus(klass, mirror::Class::kStatusResolved, self);
h_new_class_out->Assign(klass.Get());
} else {
CHECK(!klass->IsResolved());
StackHandleScope<1> hs(self);
auto h_new_class = hs.NewHandle(klass->CopyOf(self, class_size, imt, image_pointer_size_));
klass->SetMethodsPtrUnchecked(nullptr, 0, 0);
klass->SetSFieldsPtrUnchecked(nullptr);
klass->SetIFieldsPtrUnchecked(nullptr);
if (UNLIKELY(h_new_class == nullptr)) {
self->AssertPendingOOMException();
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
return false;
}
CHECK_EQ(h_new_class->GetClassSize(), class_size);
ObjectLock<mirror::Class> lock(self, h_new_class);
FixupTemporaryDeclaringClass(klass.Get(), h_new_class.Get());
{
WriterMutexLock mu(self, *Locks::classlinker_classes_lock_);
ObjPtr<mirror::ClassLoader> const class_loader = h_new_class.Get()->GetClassLoader();
ClassTable* const table = InsertClassTableForClassLoader(class_loader);
ObjPtr<mirror::Class> existing = table->UpdateClass(descriptor, h_new_class.Get(),
ComputeModifiedUtf8Hash(descriptor));
if (class_loader != nullptr) {
// We updated the class in the class table, perform the write barrier so that the GC knows
// about the change.
Runtime::Current()->GetHeap()->WriteBarrierEveryFieldOf(class_loader);
}
CHECK_EQ(existing, klass.Get());
if (log_new_roots_) {
new_class_roots_.push_back(GcRoot<mirror::Class>(h_new_class.Get()));
}
}
if (cha_ != nullptr) {
cha_->UpdateAfterLoadingOf(h_new_class);
}
mirror::Class::SetStatus(klass, mirror::Class::kStatusRetired, self);
CHECK_EQ(h_new_class->GetStatus(), mirror::Class::kStatusResolving);
mirror::Class::SetStatus(h_new_class, mirror::Class::kStatusResolved, self);
h_new_class_out->Assign(h_new_class.Get());
}
return true;
}