protobuf中的反射解析
2014-07-17
本篇讲述了Reflection以及GeneratedMessageReflection的实现。
reflection的那些GetXXX，方法，都是怎么实现的，可以从Message中根据Descriptor取出东西？
GeneratedMessageReflection 实现
构造函数&fields
GeneratedMessageReflection(const Descriptor* descriptor,
const Message* default_instance,
const int offsets[],
int has_bits_offset,
int unknown_fields_offset,
int extensions_offset,
const DescriptorPool* pool,
MessageFactory* factory,
int object_size);
主要fields：
- descriptor_:Descriptor*
- default_instance_:Message*
- offsets_ int*
- has_bits_offset_:int
- unknown_fields_offset_:int
- object_size_:int
- descriptor_pool_:DescriptorPool*
- message_factory_:MessageFactory*
看一下一个典型的函数GetString：
string GeneratedMessageReflection::GetString(
const Message& message, const FieldDescriptor* field) const {
USAGE_CHECK_ALL(GetString, SINGULAR, STRING);
if (field->is_extension()) {
return GetExtensionSet(message).GetString(field->number(),
field->default_value_string());
} else {
switch (field->options().ctype()) {
default: // TODO(kenton): Support other string reps.
case FieldOptions::STRING:
//字符串是以string*的方式存储的。
return *GetField<const string*>(message, field);
}
GOOGLE_LOG(FATAL) << "Can't get here.";
return kEmptyString; // Make compiler happy.
}
}
这个类有一些核心的辅助函数
// These simple template accessors obtain pointers (or references) to
// the given field.
template <typename Type>
inline const Type& GeneratedMessageReflection::GetRaw(
const Message& message, const FieldDescriptor* field) const {
const void* ptr = reinterpret_cast<const uint8*>(&message) +
offsets_[field->index()];
return *reinterpret_cast<const Type*>(ptr);
}
template <typename Type>
inline Type* GeneratedMessageReflection::MutableRaw(
Message* message, const FieldDescriptor* field) const {
void* ptr = reinterpret_cast<uint8*>(message) + offsets_[field->index()];
return reinterpret_cast<Type*>(ptr);
}
template <typename Type>
inline const Type& GeneratedMessageReflection::DefaultRaw(
const FieldDescriptor* field) const {
const void* ptr = reinterpret_cast<const uint8*>(default_instance_) +
offsets_[field->index()];
return *reinterpret_cast<const Type*>(ptr);
}
inline const uint32* GeneratedMessageReflection::GetHasBits(
const Message& message) const {
const void* ptr = reinterpret_cast<const uint8*>(&message) + has_bits_offset_;
return reinterpret_cast<const uint32*>(ptr);
}
inline uint32* GeneratedMessageReflection::MutableHasBits(
Message* message) const {
void* ptr = reinterpret_cast<uint8*>(message) + has_bits_offset_;
return reinterpret_cast<uint32*>(ptr);
}
inline const ExtensionSet& GeneratedMessageReflection::GetExtensionSet(
const Message& message) const {
GOOGLE_DCHECK_NE(extensions_offset_, -1);
const void* ptr = reinterpret_cast<const uint8*>(&message) +
extensions_offset_;
return *reinterpret_cast<const ExtensionSet*>(ptr);
}
inline ExtensionSet* GeneratedMessageReflection::MutableExtensionSet(
Message* message) const {
GOOGLE_DCHECK_NE(extensions_offset_, -1);
void* ptr = reinterpret_cast<uint8*>(message) + extensions_offset_;
return reinterpret_cast<ExtensionSet*>(ptr);
}
// Simple accessors for manipulating has_bits_.
inline bool GeneratedMessageReflection::HasBit(
const Message& message, const FieldDescriptor* field) const {
return GetHasBits(message)[field->index() / 32] &
(1 << (field->index() % 32));
}
inline void GeneratedMessageReflection::SetBit(
Message* message, const FieldDescriptor* field) const {
MutableHasBits(message)[field->index() / 32] |= (1 << (field->index() % 32));
}
inline void GeneratedMessageReflection::ClearBit(
Message* message, const FieldDescriptor* field) const {
MutableHasBits(message)[field->index() / 32] &= ~(1 << (field->index() % 32));
}
// Template implementations of basic accessors. Inline because each
// template instance is only called from one location. These are
// used for all types except messages.
template <typename Type>
inline const Type& GeneratedMessageReflection::GetField(
const Message& message, const FieldDescriptor* field) const {
return GetRaw<Type>(message, field);
}
template <typename Type>
inline void GeneratedMessageReflection::SetField(
Message* message, const FieldDescriptor* field, const Type& value) const {
*MutableRaw<Type>(message, field) = value;
SetBit(message, field);
}
template <typename Type>
inline Type* GeneratedMessageReflection::MutableField(
Message* message, const FieldDescriptor* field) const {
SetBit(message, field);
return MutableRaw<Type>(message, field);
}
template <typename Type>
inline const Type& GeneratedMessageReflection::GetRepeatedField(
const Message& message, const FieldDescriptor* field, int index) const {
return GetRaw<RepeatedField<Type> >(message, field).Get(index);
}
template <typename Type>
inline const Type& GeneratedMessageReflection::GetRepeatedPtrField(
const Message& message, const FieldDescriptor* field, int index) const {
return GetRaw<RepeatedPtrField<Type> >(message, field).Get(index);
}
template <typename Type>
inline void GeneratedMessageReflection::SetRepeatedField(
Message* message, const FieldDescriptor* field,
int index, Type value) const {
MutableRaw<RepeatedField<Type> >(message, field)->Set(index, value);
}
template <typename Type>
inline Type* GeneratedMessageReflection::MutableRepeatedField(
Message* message, const FieldDescriptor* field, int index) const {
RepeatedPtrField<Type>* repeated =
MutableRaw<RepeatedPtrField<Type> >(message, field);
return repeated->Mutable(index);
}
template <typename Type>
inline void GeneratedMessageReflection::AddField(
Message* message, const FieldDescriptor* field, const Type& value) const {
MutableRaw<RepeatedField<Type> >(message, field)->Add(value);
}
template <typename Type>
inline Type* GeneratedMessageReflection::AddField(
Message* message, const FieldDescriptor* field) const {
RepeatedPtrField<Type>* repeated =
MutableRaw<RepeatedPtrField<Type> >(message, field);
return repeated->Add();
}
GMR和动态生成的Message的关系
反射的Reflection离不开下面几个类：
- Reflection
- DynamicMessageFactory
- Message
- Descriptor
常用的使用场景是：
- 创建一个DynamicMessageFactory d_factory
- const Message * msg = d_factory->GetPrototype(descriptor);
- return msg->New();
DynamicMessageFactory::GetPrototype
- 首先构建TypeInfo对象
- 使用TypeInfo对象构建DynamicMessage对象 prototype
- 构建prototype的reflection对象（其中default_intance 是prototype）
- return prototype。
GMR和静态Message的关系
我们来看一段addressbook.pb.h和addressbook.pb.cc的代码。
addressbook.pb.cc中有下面三个函数：
// Internal implementation detail -- do not call these.
void protobuf_AddDesc_addressbook_2eproto();
void protobuf_AssignDesc_addressbook_2eproto();
void protobuf_ShutdownFile_addressbook_2eproto();
AddDesc
// Force AddDescriptors() to be called at static initialization time.
struct StaticDescriptorInitializer_addressbook_2eproto {
StaticDescriptorInitializer_addressbook_2eproto() {
protobuf_AddDesc_addressbook_2eproto();
}
} static_descriptor_initializer_addressbook_2eproto_;
void protobuf_AddDesc_addressbook_2eproto() {
static bool already_here = false;
if (already_here) return;
already_here = true;
GOOGLE_PROTOBUF_VERIFY_VERSION;
::google::protobuf::DescriptorPool::InternalAddGeneratedFile(
"\n\021addressbook.proto\022\010tutorial\"\'\n\010FullNam"
"e\022\r\n\005first\030\001 \002(\t\022\014\n\004last\030\002 \001(\t\"\200\002\n\006Perso"
"n\022\014\n\004name\030\001 \002(\t\022\n\n\002id\030\002 \002(\005\022\r\n\005email\030\003 \001"
"(\t\022+\n\005phone\030\004 \003(\0132\034.tutorial.Person.Phon"
"eNumber\022$\n\010fullname\030\005 \002(\0132\022.tutorial.Ful"
"lName\032M\n\013PhoneNumber\022\016\n\006number\030\001 \002(\t\022.\n\004"
"type\030\002 \001(\0162\032.tutorial.Person.PhoneType:\004"
"HOME\"+\n\tPhoneType\022\n\n\006MOBILE\020\000\022\010\n\004HOME\020\001\022"
"\010\n\004WORK\020\002\"/\n\013AddressBook\022 \n\006person\030\001 \003(\013"
"2\020.tutorial.PersonB)\n\024com.example.tutori"
"alB\021AddressBookProtos", 421);
::google::protobuf::MessageFactory::InternalRegisterGeneratedFile(
"addressbook.proto", &protobuf_RegisterTypes);
FullName::default_instance_ = new FullName();
Person::default_instance_ = new Person();
Person_PhoneNumber::default_instance_ = new Person_PhoneNumber();
AddressBook::default_instance_ = new AddressBook();
FullName::default_instance_->InitAsDefaultInstance();
Person::default_instance_->InitAsDefaultInstance();
Person_PhoneNumber::default_instance_->InitAsDefaultInstance();
AddressBook::default_instance_->InitAsDefaultInstance();
::google::protobuf::internal::OnShutdown(&protobuf_ShutdownFile_addressbook_2eproto);
}
AssignDesc
这个操作只执行一遍。构造default_instance,offsets等用于构造反射对象。
GOOGLE_PROTOBUF_DECLARE_ONCE(protobuf_AssignDescriptors_once_);
inline void protobuf_AssignDescriptorsOnce() {
::google::protobuf::GoogleOnceInit(&protobuf_AssignDescriptors_once_,
&protobuf_AssignDesc_addressbook_2eproto);
}
void protobuf_AssignDesc_addressbook_2eproto() {
protobuf_AddDesc_addressbook_2eproto();
const ::google::protobuf::FileDescriptor* file =
::google::protobuf::DescriptorPool::generated_pool()->FindFileByName(
"addressbook.proto");
GOOGLE_CHECK(file != NULL);
FullName_descriptor_ = file->message_type(0);
static const int FullName_offsets_[2] = {
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(FullName, first_),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(FullName, last_),
};
FullName_reflection_ =
new ::google::protobuf::internal::GeneratedMessageReflection(
FullName_descriptor_,
FullName::default_instance_,
FullName_offsets_,
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(FullName, _has_bits_[0]),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(FullName, _unknown_fields_),
-1,
::google::protobuf::DescriptorPool::generated_pool(),
::google::protobuf::MessageFactory::generated_factory(),
sizeof(FullName));
Person_descriptor_ = file->message_type(1);
static const int Person_offsets_[5] = {
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person, name_),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person, id_),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person, email_),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person, phone_),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person, fullname_),
};
Person_reflection_ =
new ::google::protobuf::internal::GeneratedMessageReflection(
Person_descriptor_,
Person::default_instance_,
Person_offsets_,
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person, _has_bits_[0]),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person, _unknown_fields_),
-1,
::google::protobuf::DescriptorPool::generated_pool(),
::google::protobuf::MessageFactory::generated_factory(),
sizeof(Person));
Person_PhoneNumber_descriptor_ = Person_descriptor_->nested_type(0);
static const int Person_PhoneNumber_offsets_[2] = {
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person_PhoneNumber, number_),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person_PhoneNumber, type_),
};
Person_PhoneNumber_reflection_ =
new ::google::protobuf::internal::GeneratedMessageReflection(
Person_PhoneNumber_descriptor_,
Person_PhoneNumber::default_instance_,
Person_PhoneNumber_offsets_,
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person_PhoneNumber, _has_bits_[0]),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(Person_PhoneNumber, _unknown_fields_),
-1,
::google::protobuf::DescriptorPool::generated_pool(),
::google::protobuf::MessageFactory::generated_factory(),
sizeof(Person_PhoneNumber));
Person_PhoneType_descriptor_ = Person_descriptor_->enum_type(0);
AddressBook_descriptor_ = file->message_type(2);
static const int AddressBook_offsets_[1] = {
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(AddressBook, person_),
};
AddressBook_reflection_ =
new ::google::protobuf::internal::GeneratedMessageReflection(
AddressBook_descriptor_,
AddressBook::default_instance_,
AddressBook_offsets_,
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(AddressBook, _has_bits_[0]),
GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(AddressBook, _unknown_fields_),
-1,
::google::protobuf::DescriptorPool::generated_pool(),
::google::protobuf::MessageFactory::generated_factory(),
sizeof(AddressBook));
}
ShutdownFile
这个函数被注册到google::protobuf::internal::OnShutdown(); 在AddDesc中注册的。
void protobuf_ShutdownFile_addressbook_2eproto() {
delete FullName::default_instance_;
delete FullName_reflection_;
delete Person::default_instance_;
delete Person_reflection_;
delete Person_PhoneNumber::default_instance_;
delete Person_PhoneNumber_reflection_;
delete AddressBook::default_instance_;
delete AddressBook_reflection_;
}
静态代码的序列化/反序列化
静态代码继承自Message，自然有一系列来自父类的Parse/Serialize方法。
不过静态类有且仅有一个自己的Serailize方法：
void Person::SerializeWithCachedSizes(::google::protobuf::io::CodedOutputStream* output) const;
::google::protobuf::uint8* Person::SerializeWithCachedSizesToArray(::google::protobuf::uint8* target) const;
而其他的方法，都是在父类中，使用反射实现的，参见pb的codec