Runtime_VisitRoots

void Runtime::VisitRoots(RootVisitor* visitor, VisitRootFlags flags) {
  VisitNonConcurrentRoots(visitor, flags);
  VisitConcurrentRoots(visitor, flags);
}

void Runtime::VisitNonConcurrentRoots(RootVisitor* visitor, VisitRootFlags flags) {
  VisitThreadRoots(visitor, flags);
  VisitNonThreadRoots(visitor);
}

void Runtime::VisitThreadRoots(RootVisitor* visitor, VisitRootFlags flags) {
  thread_list_->VisitRoots(visitor, flags);
}

void ThreadList::VisitRoots(RootVisitor* visitor, VisitRootFlags flags) const {
  MutexLock mu(Thread::Current(), *Locks::thread_list_lock_);
  for (const auto& thread : list_) {
    thread->VisitRoots(visitor, flags);
  }
}

void Thread::VisitRoots(RootVisitor* visitor, VisitRootFlags flags) {
  if ((flags & VisitRootFlags::kVisitRootFlagPrecise) != 0) {
    VisitRoots</* kPrecise */ true>(visitor);
  } else {
    VisitRoots</* kPrecise */ false>(visitor);
  }
}

//define the meaning of enum RootType
template <bool kPrecise>
void Thread::VisitRoots(RootVisitor* visitor) {
  const pid_t thread_id = GetThreadId();
  visitor->VisitRootIfNonNull(&tlsPtr_.opeer, RootInfo(kRootThreadObject, thread_id));
  if (tlsPtr_.exception != nullptr && tlsPtr_.exception != GetDeoptimizationException()) {
    visitor->VisitRoot(reinterpret_cast<mirror::Object**>(&tlsPtr_.exception),
                       RootInfo(kRootNativeStack, thread_id));
  }
  if (tlsPtr_.async_exception != nullptr) {
    visitor->VisitRoot(reinterpret_cast<mirror::Object**>(&tlsPtr_.async_exception),
                       RootInfo(kRootNativeStack, thread_id));
  }
  visitor->VisitRootIfNonNull(&tlsPtr_.monitor_enter_object, RootInfo(kRootNativeStack, thread_id));
  tlsPtr_.jni_env->VisitJniLocalRoots(visitor, RootInfo(kRootJNILocal, thread_id));
  tlsPtr_.jni_env->VisitMonitorRoots(visitor, RootInfo(kRootJNIMonitor, thread_id));
  HandleScopeVisitRoots(visitor, thread_id);
  if (tlsPtr_.debug_invoke_req != nullptr) {
    tlsPtr_.debug_invoke_req->VisitRoots(visitor, RootInfo(kRootDebugger, thread_id));
  }
  ......
  // Visit roots on this thread's stack
  RuntimeContextType context;
  RootCallbackVisitor visitor_to_callback(visitor, thread_id);
  ReferenceMapVisitor<RootCallbackVisitor, kPrecise> mapper(this, &context, visitor_to_callback);
  mapper.template WalkStack<StackVisitor::CountTransitions::kNo>(false);
  for (instrumentation::InstrumentationStackFrame& frame : *GetInstrumentationStack()) {
    visitor->VisitRootIfNonNull(&frame.this_object_, RootInfo(kRootVMInternal, thread_id));
  }
}

void Runtime::VisitNonThreadRoots(RootVisitor* visitor) {
  java_vm_->VisitRoots(visitor);
  sentinel_.VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  pre_allocated_OutOfMemoryError_.VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  pre_allocated_NoClassDefFoundError_.VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  verifier::MethodVerifier::VisitStaticRoots(visitor);
  VisitTransactionRoots(visitor);
}

void Runtime::VisitConcurrentRoots(RootVisitor* visitor, VisitRootFlags flags) {
  intern_table_->VisitRoots(visitor, flags);
  class_linker_->VisitRoots(visitor, flags);
  heap_->VisitAllocationRecords(visitor);
  if ((flags & kVisitRootFlagNewRoots) == 0) {
    // Guaranteed to have no new roots in the constant roots.
    VisitConstantRoots(visitor);
  }
  Dbg::VisitRoots(visitor);
}

enum RootType {
  kRootUnknown = 0,
  kRootJNIGlobal,
  kRootJNILocal,
  kRootJavaFrame,
  kRootNativeStack,
  kRootStickyClass, //contains mirror class
  kRootThreadBlock,
  kRootMonitorUsed,
  kRootThreadObject,
  kRootInternedString,
  kRootFinalizing,  // used for HPROF's conversion to HprofHeapTag
  kRootDebugger,
  kRootReferenceCleanup,  // used for HPROF's conversion to HprofHeapTag
  kRootVMInternal,
  kRootJNIMonitor,
};

// Single root version, not overridable.
ALWAYS_INLINE void VisitRootIfNonNull(mirror::Object** root, const RootInfo& info)
    REQUIRES_SHARED(Locks::mutator_lock_) {
  if (*root != nullptr) {
    VisitRoot(root, info);
  }
}

// Single root version, not overridable.
ALWAYS_INLINE void VisitRoot(mirror::Object** root, const RootInfo& info)
    REQUIRES_SHARED(Locks::mutator_lock_) {
  VisitRoots(&root, 1, info);
}

// Only visits roots one at a time, doesn't handle updating roots. Used when performance isn't critical.
class SingleRootVisitor : public RootVisitor {

void VisitRoots(mirror::Object*** roots, size_t count, const RootInfo& info) OVERRIDE
    REQUIRES_SHARED(Locks::mutator_lock_) {
  for (size_t i = 0; i < count; ++i) {
    VisitRoot(*roots[i], info);
  }
}

void VisitJniLocalRoots(RootVisitor* visitor, const RootInfo& root_info)
    REQUIRES_SHARED(Locks::mutator_lock_) {
  locals_.VisitRoots(visitor, root_info);
}

void JavaVMExt::VisitRoots(RootVisitor* visitor) {
  Thread* self = Thread::Current();
  ReaderMutexLock mu(self, *Locks::jni_globals_lock_);
  globals_.VisitRoots(visitor, RootInfo(kRootJNIGlobal));
  // The weak_globals table is visited by the GC itself (because it mutates the table).
}

template <typename Visitor>
inline void Heap::VisitObjectsPaused(Visitor&& visitor) {
  Thread* self = Thread::Current();
  Locks::mutator_lock_->AssertExclusiveHeld(self);
  VisitObjectsInternalRegionSpace(visitor);
  VisitObjectsInternal(visitor);
}

// Visit objects in the region spaces.
template <typename Visitor>
inline void Heap::VisitObjectsInternalRegionSpace(Visitor&& visitor) {
     region_space_->Walk(visitor);
}

// Visit objects in the other spaces.
template <typename Visitor>
inline void Heap::VisitObjectsInternal(Visitor&& visitor) {
   if (bump_pointer_space_ != nullptr) {
    // Visit objects in bump pointer space.
    bump_pointer_space_->Walk(visitor);
  }
  
  for (auto* it = allocation_stack_->Begin(), *end = allocation_stack_->End(); it < end; ++it) {
    mirror::Object* const obj = it->AsMirrorPtr();
    visitor(obj);
  }
  {
    ReaderMutexLock mu(Thread::Current(), *Locks::heap_bitmap_lock_);
    GetLiveBitmap()->Visit<Visitor>(visitor);
  }

template <typename Visitor>
inline void HeapBitmap::Visit(Visitor&& visitor) {
  for (const auto& bitmap : continuous_space_bitmaps_) {
    bitmap->VisitMarkedRange(bitmap->HeapBegin(), bitmap->HeapLimit(), visitor);
  }
  for (const auto& bitmap : large_object_bitmaps_) {
    bitmap->VisitMarkedRange(bitmap->HeapBegin(), bitmap->HeapLimit(), visitor);
  }
}

template <bool kVisitNativeRoots,
          VerifyObjectFlags kVerifyFlags,
          ReadBarrierOption kReadBarrierOption,
          typename Visitor,
          typename JavaLangRefVisitor>
inline void Object::VisitReferences(const Visitor& visitor,
                                    const JavaLangRefVisitor& ref_visitor) {
  ObjPtr<Class> klass = GetClass<kVerifyFlags, kReadBarrierOption>();
  visitor(this, ClassOffset(), false);
  const uint32_t class_flags = klass->GetClassFlags<kVerifyNone>();
  if (LIKELY(class_flags == kClassFlagNormal)) {
    DCHECK((!klass->IsVariableSize<kVerifyFlags, kReadBarrierOption>()));
    VisitInstanceFieldsReferences<kVerifyFlags, kReadBarrierOption>(klass, visitor);