1 OOP-Klass(Ordinary Object Pointer)模型
OOP-Klass模型用来描述class的属性和行为
设计为OOP和Klass两部分是因为不希望每个对象都有一个C ++ vtbl指针， 因此，普通的oops没有任何虚拟功能。 相反，他们将所有“虚拟”函数转发到它们的klass，它具有vtbl并根据对象的实际类型执行C ++调度。
1.1 OOP
oopDesc是对象类的最高父类。 {name}Desc类描述了Java对象的格式，可从C++访问这些字段
- 路径:
/hotspot/share/oops/oop.hpp
完整的类层次结构，请阅读src/hotspot/share/oops/oopsHierarchy.hpp
-
OOP体系
1.2 Klass
-
Klass体系
Klass对象提供
- 语言级别的类对象（方法字典等）
- 为对象提供虚拟机调度行为
class Klass : public Metadata {
friend class VMStructs;
friend class JVMCIVMStructs;
protected:
// 如果添加指向任何元数据对象的新字段，则必须将此字段添加到Klass :: metaspace_pointers_do()
// 注意：在klass结构的起始处将常用字段放在一起，以获得更好的缓存行为（虽然可能不会有太大的区别，但可以肯定不会造成伤害）
enum { _primary_super_limit = 8 };
// The "layout helper" is a combined descriptor of object layout.
// For klasses which are neither instance nor array, the value is zero.
//
// For instances, layout helper is a positive number, the instance size.
// This size is already passed through align_object_size and scaled to bytes.
// The low order bit is set if instances of this class cannot be
// allocated using the fastpath.
//
// For arrays, layout helper is a negative number, containing four
// distinct bytes, as follows:
// MSB:[tag, hsz, ebt, log2(esz)]:LSB
// where:
// tag is 0x80 if the elements are oops, 0xC0 if non-oops
// hsz is array header size in bytes (i.e., offset of first element)
// ebt is the BasicType of the elements
// esz is the element size in bytes
// This packed word is arranged so as to be quickly unpacked by the
// various fast paths that use the various subfields.
//
// The esz bits can be used directly by a SLL instruction, without masking.
//
// Note that the array-kind tag looks like 0x00 for instance klasses,
// since their length in bytes is always less than 24Mb.
//
// Final note: This comes first, immediately after C++ vtable,
// because it is frequently queried.
jint _layout_helper;
// Klass identifier used to implement devirtualized oop closure dispatching.
const KlassID _id;
// The fields _super_check_offset, _secondary_super_cache, _secondary_supers
// and _primary_supers all help make fast subtype checks. See big discussion
// in doc/server_compiler/checktype.txt
//
// Where to look to observe a supertype (it is &_secondary_super_cache for
// secondary supers, else is &_primary_supers[depth()].
juint _super_check_offset;
// 类名. Instance classes: java/lang/String, etc. Array classes: [I,
// [Ljava/lang/String;, etc. Set to zero for all other kinds of classes.
Symbol* _name;
// Cache of last observed secondary supertype
Klass* _secondary_super_cache;
// Array of all secondary supertypes
Array<Klass*>* _secondary_supers;
// Ordered list of all primary supertypes
Klass* _primary_supers[_primary_super_limit];
// java/lang/Class instance mirroring this class
OopHandle _java_mirror;
// Superclass
Klass* _super;
// First subclass (NULL if none); _subklass->next_sibling() is next one
Klass* volatile _subklass;
// Sibling link (or NULL); links all subklasses of a klass
Klass* volatile _next_sibling;
// All klasses loaded by a class loader are chained through these links
Klass* _next_link;
// 用于加载此类的VM对类加载器的表示。
//提供访问相应的java.lang.ClassLoader实例
ClassLoaderData* _class_loader_data;
jint _modifier_flags; // 处理的访问标志，由Class.getModifiers使用
AccessFlags _access_flags; // 访问标志。 类/接口的区别就存储在这里
JFR_ONLY(DEFINE_TRACE_ID_FIELD;)
// 偏向锁定实现和统计
// 64位块优先，以避免碎片
jlong _last_biased_lock_bulk_revocation_time;
markOop _prototype_header; // Used when biased locking is both enabled and disabled for this type
jint _biased_lock_revocation_count;
// 虚表长度
int _vtable_len;
...
