defineClassNative()是一个native方法，其承担了Class的加载。其native实现在dalvik_system_DexFile.cc这个类中。
static jclass DexFile_defineClassNative(JNIEnv* env,
jclass,
jstring javaName,
jobject javaLoader,
jobject cookie,
jobject dexFile) {
std::vector 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()));
//遍历dex文件
for (auto& dex_file : dex_files) {
//查询当前dex文件是否有需要被加载的类，如果有则返回类定义
const DexFile::ClassDef* dex_class_def = dex_file->FindClassDef(descriptor.c_str(), hash);
if (dex_class_def != nullptr) {
ScopedObjectAccess soa(env);
ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
StackHandleScope<1> hs(soa.Self());
Handle class_loader(
hs.NewHandle(soa.Decode(javaLoader)));
class_linker->RegisterDexFile(*dex_file, class_loader.Get());
//加载当前类
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(dexFile),
class_loader.Get());
if (result != nullptr) {
VLOG(class_linker) << "DexFile_defineClassNative returning " << result
<< " for " << class_name.c_str();
return soa.AddLocalReference(result);
}
}
}
VLOG(class_linker) << "Failed to find dex_class_def " << class_name.c_str();
return nullptr;
}
先来看看类查找的过程，DexFile的FindClassDef函数定义如下：
const DexFile::ClassDef* DexFile::FindClassDef(const char* descriptor, size_t hash) const {
DCHECK_EQ(ComputeModifiedUtf8Hash(descriptor), hash);
if (LIKELY(lookup_table_ != nullptr)) {
//先查找dex中是否有class的定义，如果有，则取出来
const uint32_t class_def_idx = lookup_table_->Lookup(descriptor, hash);
return (class_def_idx != DexFile::kDexNoIndex) ? &GetClassDef(class_def_idx) : nullptr;
}
// Fast path for rate no class defs case.
const uint32_t num_class_defs = NumClassDefs();
if (num_class_defs == 0) {
return nullptr;
}
const TypeId* type_id = FindTypeId(descriptor);
if (type_id != nullptr) {
uint16_t type_idx = GetIndexForTypeId(*type_id);
for (size_t i = 0; i < num_class_defs; ++i) {
const ClassDef& class_def = GetClassDef(i);
if (class_def.class_idx_ == type_idx) {
return &class_def;
}
}
}
return nullptr;
}
GetClassDef函数完成了从index查找Class的过程，其实就是一个数组
// Returns the ClassDef at the specified index.
const ClassDef& GetClassDef(uint16_t idx) const {
DCHECK_LT(idx, NumClassDefs()) << GetLocation();
return class_defs_[idx];
}
类查找到之后，就是初始化的过程了。
ClassLinker的类名为class_linker，其DefineClass方法定义如下：
mirror::Class* ClassLinker::DefineClass(Thread* self,
const char* descriptor,
size_t hash,
Handle class_loader,
const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def) {
StackHandleScope<3> hs(self);
auto klass = hs.NewHandle(nullptr);
// Load the class from the dex file.
if (UNLIKELY(!init_done_)) {
// finish up init of hand crafted class_roots_
if (strcmp(descriptor, "Ljava/lang/Object;") == 0) {
klass.Assign(GetClassRoot(kJavaLangObject));
} else if (strcmp(descriptor, "Ljava/lang/Class;") == 0) {
klass.Assign(GetClassRoot(kJavaLangClass));
} else if (strcmp(descriptor, "Ljava/lang/String;") == 0) {
klass.Assign(GetClassRoot(kJavaLangString));
} else if (strcmp(descriptor, "Ljava/lang/ref/Reference;") == 0) {
klass.Assign(GetClassRoot(kJavaLangRefReference));
} else if (strcmp(descriptor, "Ljava/lang/DexCache;") == 0) {
klass.Assign(GetClassRoot(kJavaLangDexCache));
}
}
if (klass.Get() == nullptr) {
// Allocate a class with the status of not ready.
// Interface object should get the right size here. Regular class will
// figure out the right size later and be replaced with one of the right
// size when the class becomes resolved.
klass.Assign(AllocClass(self, SizeOfClassWithoutEmbeddedTables(dex_file, dex_class_def)));
}
if (UNLIKELY(klass.Get() == nullptr)) {
self->AssertPendingOOMException();
return nullptr;
}
mirror::DexCache* dex_cache = RegisterDexFile(dex_file, class_loader.Get());
if (dex_cache == nullptr) {
self->AssertPendingOOMException();
return nullptr;
}
klass->SetDexCache(dex_cache);
SetupClass(dex_file, dex_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 lock(self, klass);
klass->SetClinitThreadId(self->GetTid());
// Add the newly loaded class to the loaded classes table.
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.
//加载class
LoadClass(self, dex_file, dex_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::kStatusError, self);
}
return nullptr;
}
// Finish loading (if necessary) by finding parents
CHECK(!klass->IsLoaded());
if (!LoadSuperAndInterfaces(klass, dex_file)) {
// Loading failed.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusError, self);
}
return nullptr;
}
CHECK(klass->IsLoaded());
// Link the class (if necessary)
CHECK(!klass->IsResolved());
// TODO: Use fast jobjects?
auto interfaces = hs.NewHandle>(nullptr);
MutableHandle h_new_class = hs.NewHandle(nullptr);
//主要是处理继承和接口
if (!LinkClass(self, descriptor, klass, interfaces, &h_new_class)) {
// Linking failed.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusError, self);
}
return nullptr;
}
self->AssertNoPendingException();
CHECK(h_new_class.Get() != nullptr) << descriptor;
CHECK(h_new_class->IsResolved()) << descriptor;
// Instrumentation may have updated entrypoints for all methods of all
// classes. However it could not update methods of this class while we
// were loading it. Now the class is resolved, we can update entrypoints
// as required by instrumentation.
if (Runtime::Current()->GetInstrumentation()->AreExitStubsInstalled()) {
// We must be in the kRunnable state to prevent instrumentation from
// suspending all threads to update entrypoints while we are doing it
// for this class.
DCHECK_EQ(self->GetState(), kRunnable);
Runtime::Current()->GetInstrumentation()->InstallStubsForClass(h_new_class.Get());
}
/*
* We send CLASS_PREPARE events to the debugger from here. The
* definition of "preparation" is creating the static fields for a
* class and initializing them to the standard default values, but not
* executing any code (that comes later, during "initialization").
*
* We did the static preparation in LinkClass.
*
* The class has been prepared and resolved but possibly not yet verified
* at this point.
*/
Dbg::PostClassPrepare(h_new_class.Get());
// Notify native debugger of the new class and its layout.
jit::Jit::NewTypeLoadedIfUsingJit(h_new_class.Get());
return h_new_class.Get();
}
LoadClass函数的定义如下：
void ClassLinker::LoadClass(Thread* self,
const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def,
Handle 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
}
bool has_oat_class = false;
if (Runtime::Current()->IsStarted() && !Runtime::Current()->IsAotCompiler()) {
OatFile::OatClass oat_class = FindOatClass(dex_file, klass->GetDexClassDefIndex(),
&has_oat_class);
if (has_oat_class) {
LoadClassMembers(self, dex_file, class_data, klass, &oat_class);
}
}
if (!has_oat_class) {
LoadClassMembers(self, dex_file, class_data, klass, nullptr);
}
}
无论是否找到oat文件，都会执行LoadClassMembers函数。其定义如下：
void ClassLinker::LoadClassMembers(Thread* self,
const DexFile& dex_file,
const uint8_t* class_data,
Handle klass,
const OatFile::OatClass* oat_class) {
{
// Note: We cannot have thread suspension until the field and method arrays are setup or else
// Class::VisitFieldRoots may miss some fields or methods.
ScopedAssertNoThreadSuspension nts(self, __FUNCTION__);
// Load static fields.
// We allow duplicate definitions of the same field in a class_data_item
// but ignore the repeated indexes here, b/21868015.
LinearAlloc* const allocator = GetAllocatorForClassLoader(klass->GetClassLoader());
ClassDataItemIterator it(dex_file, class_data);
LengthPrefixedArray* sfields = AllocArtFieldArray(self,
allocator,
it.NumStaticFields());
size_t num_sfields = 0;
uint32_t last_field_idx = 0u;
//加载static的Field
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* ifields = AllocArtFieldArray(self,
allocator,
it.NumInstanceFields());
size_t num_ifields = 0u;
last_field_idx = 0u;
//加载非static的Field
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;
}
}
if (UNLIKELY(num_sfields != it.NumStaticFields()) ||
UNLIKELY(num_ifields != it.NumInstanceFields())) {
LOG(WARNING) << "Duplicate fields in class " << PrettyDescriptor(klass.Get())
<< " (unique static fields: " << num_sfields << "/" << it.NumStaticFields()
<< ", unique instance fields: " << num_ifields << "/" << it.NumInstanceFields() << ")";
// NOTE: Not shrinking the over-allocated sfields/ifields, just setting size.
if (sfields != nullptr) {
sfields->SetSize(num_sfields);
}
if (ifields != nullptr) {
ifields->SetSize(num_ifields);
}
}
// Set the field arrays.
klass->SetSFieldsPtr(sfields);
DCHECK_EQ(klass->NumStaticFields(), num_sfields);
klass->SetIFieldsPtr(ifields);
DCHECK_EQ(klass->NumInstanceFields(), num_ifields);
// Load methods.
klass->SetMethodsPtr(
AllocArtMethodArray(self, allocator, it.NumDirectMethods() + it.NumVirtualMethods()),
it.NumDirectMethods(),
it.NumVirtualMethods());
size_t class_def_method_index = 0;
uint32_t last_dex_method_index = DexFile::kDexNoIndex;
size_t last_class_def_method_index = 0;
// TODO These should really use the iterators.
//加载DirectMethod
for (size_t i = 0; it.HasNextDirectMethod(); i++, it.Next()) {
ArtMethod* method = klass->GetDirectMethodUnchecked(i, image_pointer_size_);
LoadMethod(self, dex_file, it, klass, method);
LinkCode(method, oat_class, 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++;
}
//加载VirtualMethod
for (size_t i = 0; it.HasNextVirtualMethod(); i++, it.Next()) {
ArtMethod* method = klass->GetVirtualMethodUnchecked(i, image_pointer_size_);
LoadMethod(self, dex_file, it, klass, method);
DCHECK_EQ(class_def_method_index, it.NumDirectMethods() + i);
LinkCode(method, oat_class, class_def_method_index);
class_def_method_index++;
}
DCHECK(!it.HasNext());
}
// Ensure that the card is marked so that remembered sets pick up native roots.
Runtime::Current()->GetHeap()->WriteBarrierEveryFieldOf(klass.Get());
self->AllowThreadSuspension();
}
如上对Field和Method的加载，分别使用了LoadField和LoadMethod，先来看一下LoadField方法。
void ClassLinker::LoadField(const ClassDataItemIterator& it,
Handle klass,
ArtField* dst) {
const uint32_t field_idx = it.GetMemberIndex();//获取索引
dst->SetDexFieldIndex(field_idx);//设置索引
dst->SetDeclaringClass(klass.Get());//设置类型
dst->SetAccessFlags(it.GetFieldAccessFlags());//设置访问限制
}
Field的加载相对比较简单，指定类型，设置访问限制，并设置索引。Method的加载相对麻烦，如下：
void ClassLinker::LoadMethod(Thread* self,
const DexFile& dex_file,
const ClassDataItemIterator& it,
Handle 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(self, "LoadMethod");
dst->SetDexMethodIndex(dex_method_idx);
dst->SetDeclaringClass(klass.Get());
dst->SetCodeItemOffset(it.GetMethodCodeItemOffset());
dst->SetDexCacheResolvedMethods(klass->GetDexCache()->GetResolvedMethods(), image_pointer_size_);
dst->SetDexCacheResolvedTypes(klass->GetDexCache()->GetResolvedTypes(), image_pointer_size_);
uint32_t access_flags = it.GetMethodAccessFlags();
if (UNLIKELY(strcmp("finalize", method_name) == 0)) {
// Set finalizable flag on declaring class.
if (strcmp("V", dex_file.GetShorty(method_id.proto_idx_)) == 0) {
// Void return type.
if (klass->GetClassLoader() != nullptr) { // All non-boot finalizer methods are flagged.
klass->SetFinalizable();
} else {
std::string temp;
const char* klass_descriptor = klass->GetDescriptor(&temp);
// The Enum class declares a "final" finalize() method to prevent subclasses from
// introducing a finalizer. We don't want to set the finalizable flag for Enum or its
// subclasses, so we exclude it here.
// We also want to avoid setting the flag on Object, where we know that finalize() is
// empty.
if (strcmp(klass_descriptor, "Ljava/lang/Object;") != 0 &&
strcmp(klass_descriptor, "Ljava/lang/Enum;") != 0) {
klass->SetFinalizable();
}
}
}
} else if (method_name[0] == '<') {
// Fix broken access flags for initializers. Bug 11157540.
bool is_init = (strcmp("", method_name) == 0);
bool is_clinit = !is_init && (strcmp("", method_name) == 0);
if (UNLIKELY(!is_init && !is_clinit)) {
LOG(WARNING) << "Unexpected '<' at start of method name " << method_name;
} else {
if (UNLIKELY((access_flags & kAccConstructor) == 0)) {
LOG(WARNING) << method_name << " didn't have expected constructor access flag in class "
<< PrettyDescriptor(klass.Get()) << " in dex file " << dex_file.GetLocation();
access_flags |= kAccConstructor;
}
}
}
dst->SetAccessFlags(access_flags);
}
LoadMethod完成了ArtMethod的初始化的工作。这样一个完成的类就加载好了。这里需要注意的是ArtMethod非常不稳定，基本google每发布一个android大版本，其结构都会有所调整，这也造成了开发者使用ArtMethod的不稳定性。