RootType

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

RootInfo

class RootInfo {
 public:
  // Thread id 0 is for non thread roots.
  explicit RootInfo(RootType type, uint32_t thread_id = 0)
     : type_(type), thread_id_(thread_id) {
  }
  RootInfo(const RootInfo&) = default;
  virtual ~RootInfo() {
  }
 
 private:
  const RootType type_;
  const uint32_t thread_id_;
};

VisitRootFlags

enum VisitRootFlags : uint8_t {
  kVisitRootFlagAllRoots = (1 << 0),
  kVisitRootFlagNewRoots = (1 << 1),
  kVisitRootFlagStartLoggingNewRoots = (1 << 2),
  kVisitRootFlagStopLoggingNewRoots = (1 << 3),
  kVisitRootFlagClearRootLog = (1 << 4),
  kVisitRootFlagClassLoader = (1 << 5),
  // There is no (1 << 6).
  kVisitRootFlagPrecise = (1 << 7),
};

Runtime::VisitRoots是访问虚拟机进程中所有root对象的入口

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

  VisitThreadRoots(visitor, flags);
  VisitNonThreadRoots(visitor);
}

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

void Runtime::VisitNonThreadRoots(RootVisitor* visitor) {
  java_vm_->VisitRoots(visitor);
  sentinel_.VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  pre_allocated_OutOfMemoryError_when_throwing_exception_
      .VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  pre_allocated_OutOfMemoryError_when_throwing_oome_
      .VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  pre_allocated_OutOfMemoryError_when_handling_stack_overflow_
      .VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  pre_allocated_NoClassDefFoundError_.VisitRootIfNonNull(visitor, RootInfo(kRootVMInternal));
  VisitImageRoots(visitor);
  verifier::ClassVerifier::VisitStaticRoots(visitor);
  VisitTransactionRoots(visitor);
}

Thread

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

(1) jni函数调用时，引用型参数会借助一个HandleScope保存在栈上。HandleScopeVisitRoots函数将遍历tlsPtr_top_handle_scope链表，然后访问其中的引用型对象。

template <bool kPrecise>
void Thread::VisitRoots(RootVisitor* visitor) {
  const uint32_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));
//(1)
  HandleScopeVisitRoots(visitor, thread_id);
  // Visit roots for deoptimization.
  if (tlsPtr_.stacked_shadow_frame_record != nullptr) {
    RootCallbackVisitor visitor_to_callback(visitor, thread_id);
    ReferenceMapVisitor<RootCallbackVisitor, kPrecise> mapper(this, nullptr, visitor_to_callback);
    for (StackedShadowFrameRecord* record = tlsPtr_.stacked_shadow_frame_record;
         record != nullptr;
         record = record->GetLink()) {
      for (ShadowFrame* shadow_frame = record->GetShadowFrame();
           shadow_frame != nullptr;
           shadow_frame = shadow_frame->GetLink()) {
        mapper.VisitShadowFrame(shadow_frame);
      }
    }
  }
  for (DeoptimizationContextRecord* record = tlsPtr_.deoptimization_context_stack;
       record != nullptr;
       record = record->GetLink()) {
    if (record->IsReference()) {
      visitor->VisitRootIfNonNull(record->GetReturnValueAsGCRoot(),
                                  RootInfo(kRootThreadObject, thread_id));
    }
    visitor->VisitRootIfNonNull(record->GetPendingExceptionAsGCRoot(),
                                RootInfo(kRootThreadObject, thread_id));
  }
  if (tlsPtr_.frame_id_to_shadow_frame != nullptr) {
    RootCallbackVisitor visitor_to_callback(visitor, thread_id);
    ReferenceMapVisitor<RootCallbackVisitor, kPrecise> mapper(this, nullptr, visitor_to_callback);
    for (FrameIdToShadowFrame* record = tlsPtr_.frame_id_to_shadow_frame;
         record != nullptr;
         record = record->GetNext()) {
      mapper.VisitShadowFrame(record->GetShadowFrame());
    }
  }
  for (auto* verifier = tlsPtr_.method_verifier; verifier != nullptr; verifier = verifier->link_) {
    verifier->VisitRoots(visitor, RootInfo(kRootNativeStack, 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 (auto& entry : *GetInstrumentationStack()) {
    visitor->VisitRootIfNonNull(&entry.second.this_object_, RootInfo(kRootVMInternal, thread_id));
  }
}