https://cloud.tencent.com/developer/article/1984092?areaSource=105001.9&traceId=ySuPzDMCMO5dVSJSYsUT9
Klass: https://www.sczyh30.com/posts/Java/jvm-klass-oop/
// Klass layout:
// [C++ vtbl ptr ] (contained in Metadata)
// [layout_helper ]
// [super_check_offset ] for fast subtype checks
// [name ]
// [secondary_super_cache] for fast subtype checks
// [secondary_supers ] array of 2ndary supertypes
// [primary_supers 0]
// [primary_supers 1]
// [primary_supers 2]
// ...
// [primary_supers 7]
// [java_mirror ]
// [super ]
// [subklass ] first subclass
// [next_sibling ] link to chain additional subklasses
// [next_link ]
// [class_loader_data]
// [modifier_flags]
// [access_flags ]
// [last_biased_lock_bulk_revocation_time] (64 bits)
// [prototype_header]
// [biased_lock_revocation_count]
// [_modified_oops]
// [_accumulated_modified_oops]
// [trace_id]
/research/jdk/hotspot/src/share/vm/oops/klass.hpp
class Klass : public Metadata {
friend class VMStructs;
protected:
// note: put frequently-used fields together at start of klass structure
// for better cache behavior (may not make much of a difference but sure won't hurt)
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;
// 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;
// Class name. 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
oop _java_mirror;
// Superclass
Klass* _super;
// First subclass (NULL if none); _subklass->next_sibling() is next one
Klass* _subklass;
// Sibling link (or NULL); links all subklasses of a klass
Klass* _next_sibling;
// All klasses loaded by a class loader are chained through these links
Klass* _next_link;
// The VM's representation of the ClassLoader used to load this class.
// Provide access the corresponding instance java.lang.ClassLoader.
ClassLoaderData* _class_loader_data;
jint _modifier_flags; // Processed access flags, for use by Class.getModifiers.
AccessFlags _access_flags; // Access flags. The class/interface distinction is stored here.
// Biased locking implementation and statistics
// (the 64-bit chunk goes first, to avoid some fragmentation)
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;
TRACE_DEFINE_KLASS_TRACE_ID;
// Remembered sets support for the oops in the klasses.
jbyte _modified_oops; // Card Table Equivalent (YC/CMS support)
jbyte _accumulated_modified_oops; // Mod Union Equivalent (CMS support)
// Constructor
Klass();
void* operator new(size_t size, ClassLoaderData* loader_data, size_t word_size, TRAPS);
public:
bool is_klass() const volatile { return true; }
// super
Klass* super() const { return _super; }
void set_super(Klass* k) { _super = k; }
// initializes _super link, _primary_supers & _secondary_supers arrays
void initialize_supers(Klass* k, TRAPS);
void initialize_supers_impl1(Klass* k);
void initialize_supers_impl2(Klass* k);
// klass-specific helper for initializing _secondary_supers
virtual GrowableArray<Klass*>* compute_secondary_supers(int num_extra_slots);
// java_super is the Java-level super type as specified by Class.getSuperClass.
virtual Klass* java_super() const { return NULL; }
juint super_check_offset() const { return _super_check_offset; }
void set_super_check_offset(juint o) { _super_check_offset = o; }
Klass* secondary_super_cache() const { return _secondary_super_cache; }
void set_secondary_super_cache(Klass* k) { _secondary_super_cache = k; }
Array<Klass*>* secondary_supers() const { return _secondary_supers; }
void