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+---
+layout: post
+title:  "JVM 对象遍历"
+date:   2024-09-05 11:00:00 +0200
+tags: [GC, G1]
+---
+
+GC 中，对于存活的对象需要遍历其全引用类型的属性，前面的文章并没有解释对象是如何被遍历的，本文参考 [以 ZGC 为例，谈一谈 JVM 是如何实现 Reference 语义的](https://mp.weixin.qq.com/s?__biz=Mzg2MzU3Mjc3Ng==&mid=2247489586&idx=1&sn=4306549c480f668458ab4df0d4b2ea47&chksm=ce77de75f9005763016605e0d268e1a4393a83bfe2a281c915bbf55de99d25cda529195c2843&scene=178&cur_album_id=2291913023118213124#rd) （后文中称其为引文） 中 "JVM 如何遍历对象的引用关系图" 小节简单介绍对象的遍历。 读者一定要先读引文，再读本文就很简单。
+
+## OopMapBlock
+
+OopMapBlock 是对象中引用类型属性的结构，加快对象的遍历。由于引用类型的字段不都是连续的，所以 OopMapBlock 对于某一个 class 会有多个。如图，图片来自引文。
+
+<image src="/assets/ygc-obj-iter/omb.png" width="60%">
+
+## 遍历逻辑
+
+在回收阶段，方法 `G1ParScanThreadState::do_copy_to_survivor_space` 拷贝存活对象的时候需要遍历其引用的属性。
+
+对于普通对象：
+
+```cpp
+obj->oop_iterate_backwards(&_scanner, klass);
+```
+
+数组类型的对象：
+
+```cpp
+to_array->oop_iterate_range(&_scanner, 0, checked_cast<int>(step._index));
+```
+
+对于 Java 对象，对象头中存储对象的类型。
+
+```cpp
+class oopDesc {
+  volatile markWord _mark;
+  union _metadata {
+    Klass*      _klass;
+    narrowKlass _compressed_klass;
+  } _metadata
+}
+```
+
+首先根据 `Klass` 类型拿到对应的函数，然后调用函数。
+
+```cpp
+//->oopDesc::oop_iterate_backwards
+//->OopIteratorClosureDispatch::oop_oop_iterate_backwards
+void OopIteratorClosureDispatch::oop_oop_iterate_backwards(OopClosureType* cl, oop obj, Klass* klass) {
+  OopOopIterateBackwardsDispatch<OopClosureType>::function(klass)(cl, obj, klass);
+}
+```
+
+`_function` 对于每个 class 类型，都有对应的遍历函数。
+
+```cpp
+static FunctionType function(Klass* klass) {
+  return _table._function[klass->kind()];
+}
+
+
+void set_init_function() {
+  _function[KlassType::Kind] = &init<KlassType>;
+}
+```
+
+调用链是：
+
+```cpp
+// &init<KlassType> -> 
+// _table.set_resolve_function_and_execute
+
+//set_resolve_function 绑定函数
+//_function[KlassType::Kind](cl, obj, k); // 函数调用
+//oop_oop_iterate_backwards 
+
+static void oop_oop_iterate_backwards(OopClosureType* cl, oop obj, Klass* k) {
+  ((KlassType*)k)->KlassType::template oop_oop_iterate_reverse<T>(obj, cl);
+}
+```
+
+`oop_oop_iterate_reverse` 根据 `KlassType` 的类型遍历对象。
+
+以 `InstanceKlass` 为例，获取 `nonstatic_oop_maps` 的地址，根据 `OopMapBlock` 的信息遍历对象。
+
+```cpp
+void InstanceKlass::oop_oop_iterate_reverse(oop obj, OopClosureType* closure) {
+  oop_oop_iterate_oop_maps_reverse<T>(obj, closure);
+}
+
+ALWAYSINLINE void InstanceKlass::oop_oop_iterate_oop_maps_reverse(oop obj, OopClosureType* closure) {
+  OopMapBlock* const start_map = start_of_nonstatic_oop_maps();
+  OopMapBlock* map             = start_map + nonstatic_oop_map_count();
+  while (start_map < map) {
+    --map;
+    oop_oop_iterate_oop_map_reverse<T>(map, obj, closure);
+  }
+}
+
+void InstanceKlass::oop_oop_iterate_oop_map_reverse(OopMapBlock* map, oop obj, OopClosureType* closure) {
+  T* const start = obj->field_addr<T>(map->offset());
+  T*       p     = start + map->count();
+
+  while (start < p) {
+    --p;
+    Devirtualizer::do_oop(closure, p);
+  }
+}
+```
+
+## 总结
+
+本文参考 [以 ZGC 为例，谈一谈 JVM 是如何实现 Reference 语义的](https://mp.weixin.qq.com/s?__biz=Mzg2MzU3Mjc3Ng==&mid=2247489586&idx=1&sn=4306549c480f668458ab4df0d4b2ea47&chksm=ce77de75f9005763016605e0d268e1a4393a83bfe2a281c915bbf55de99d25cda529195c2843&scene=178&cur_album_id=2291913023118213124#rd) 简单的介绍了对象的遍历。
+
+
diff --git a/_posts/2024-09-10-g1-gc-young-gc-iter-obj.md b/_posts/2024-09-10-g1-gc-young-gc-iter-obj.md
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-layout: post
-title:  "JVM 对象遍历"
-date:   2024-09-05 11:00:00 +0200
-tags: [GC, G1]
----
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