class InstanceKlass: public Klass {
 ...

 protected:
  // Annotations for this class
  Annotations*       _annotations;
  // Array classes holding elements of this class.
  Klass*             _array_klasses;
  // Constant pool for this class.
  ConstantPool*     _constants;
  // The InnerClasses attribute and EnclosingMethod attribute. The
  // _inner_classes is an array of shorts. If the class has InnerClasses
  // attribute, then the _inner_classes array begins with 4-tuples of shorts
  // [inner_class_info_index, outer_class_info_index,
  // inner_name_index, inner_class_access_flags] for the InnerClasses
  // attribute. If the EnclosingMethod attribute exists, it occupies the
  // last two shorts [class_index, method_index] of the array. If only
  // the InnerClasses attribute exists, the _inner_classes array length is
  // number_of_inner_classes * 4. If the class has both InnerClasses
  // and EnclosingMethod attributes the _inner_classes array length is
  // number_of_inner_classes * 4 + enclosing_method_attribute_size.
  Array<jushort>*   _inner_classes;
 
  // Array name derived from this class which needs unreferencing
  // if this class is unloaded.
  Symbol*           _array_name;
 
  // Number of heapOopSize words used by non-static fields in this klass
  // (including inherited fields but after header_size()).
  int               _nonstatic_field_size;
  int               _static_field_size;    // number words used by static fields (oop and non-oop) in this klass
  // Constant pool index to the utf8 entry of the Generic signature,
  // or 0 if none.
  u2                _generic_signature_index;
  // Constant pool index to the utf8 entry for the name of source file
  // containing this klass, 0 if not specified.
  u2                _source_file_name_index;
  u2                _static_oop_field_count;// number of static oop fields in this klass
  u2                _java_fields_count;    // The number of declared Java fields
  int               _nonstatic_oop_map_size;// size in words of nonstatic oop map blocks
 

  u2                _minor_version;  // minor version number of class file
  u2                _major_version;  // major version number of class file
  Thread*           _init_thread;    // Pointer to current thread doing initialization (to handle recusive initialization)
  int               _vtable_len;     // length of Java vtable (in words)
  int               _itable_len;     // length of Java itable (in words)
  OopMapCache*      volatile _oop_map_cache;   // OopMapCache for all methods in the klass (allocated lazily)
  JNIid*            _jni_ids;              // First JNI identifier for static fields in this class
  jmethodID*        _methods_jmethod_ids;  // jmethodIDs corresponding to method_idnum, or NULL if none
  nmethodBucket*    _dependencies;         // list of dependent nmethods
  nmethod*          _osr_nmethods_head;    // Head of list of on-stack replacement nmethods for this class

 
  // Class states are defined as ClassState (see above).
  // Place the _init_state here to utilize the unused 2-byte after
  // _idnum_allocated_count.
  u1                _init_state;                    // state of class
  u1                _reference_type;                // reference type
 

  // Method array.
  Array<Method*>*   _methods;
  // Default Method Array, concrete methods inherited from interfaces
  Array<Method*>*   _default_methods;
  // Interface (Klass*s) this class declares locally to implement.
  Array<Klass*>*    _local_interfaces;
  // Interface (Klass*s) this class implements transitively.
  Array<Klass*>*    _transitive_interfaces;

  // Int array containing the vtable_indices for default_methods
  // offset matches _default_methods offset
  Array<int>*       _default_vtable_indices;
 
  // Instance and static variable information, starts with 6-tuples of shorts
  // [access, name index, sig index, initval index, low_offset, high_offset]
  // for all fields, followed by the generic signature data at the end of
  // the array. Only fields with generic signature attributes have the generic
  // signature data set in the array. The fields array looks like following:
  //
  // f1: [access, name index, sig index, initial value index, low_offset, high_offset]
  // f2: [access, name index, sig index, initial value index, low_offset, high_offset]
  //      ...
  // fn: [access, name index, sig index, initial value index, low_offset, high_offset]
  //     [generic signature index]
  //     [generic signature index]
  //     ...
  Array<u2>*        _fields;
 
  // embedded Java vtable follows here
  // embedded Java itables follows here
  // embedded static fields follows here
  // embedded nonstatic oop-map blocks follows here
  // embedded implementor of this interface follows here
  //   The embedded implementor only exists if the current klass is an
  //   iterface. The possible values of the implementor fall into following
  //   three cases:
  //     NULL: no implementor.
  //     A Klass* that's not itself: one implementor.
  //     Itsef: more than one implementors.
  // embedded host klass follows here
  //   The embedded host klass only exists in an anonymous class for
  //   dynamic language support (JSR 292 enabled). The host class grants
  //   its access privileges to this class also. The host class is either
  //   named, or a previously loaded anonymous class. A non-anonymous class
  //   or an anonymous class loaded through normal classloading does not
  //   have this embedded field.
  
  ...
}

int size = InstanceKlass::size(vtable_len,itable_len,nonstatic_oop_map_size,isinterf,is_anonymous);

static int size(
  int    vtable_length,
  int    itable_length,
  int    nonstatic_oop_map_size,
  bool   is_interface,
  bool   is_anonymous
){
return     align_object_size(header_size()    +  // InstanceKlass類本身佔用的內存大小
	   align_object_offset(vtable_length) +
	   align_object_offset(itable_length) +
	   //    [EMBEDDED nonstatic oop-map blocks] size in words = nonstatic_oop_map_size
	   //      The embedded nonstatic oop-map blocks are short pairs (offset, length)
	   //      indicating where oops are located in instances of this klass.
	   (
			  (is_interface || is_anonymous) ?
			  align_object_offset(nonstatic_oop_map_size) :
			  nonstatic_oop_map_size
	   ) +
	   //    [EMBEDDED implementor of the interface] only exist for interface
	   (
			   is_interface ? (int)sizeof(Klass*)/HeapWordSize : 0
	   ) +
	   //    [EMBEDDED host klass        ] only exist for an anonymous class (JSR 292 enabled)
	   (
			   is_anonymous ? (int)sizeof(Klass*)/HeapWordSize : 0)
	   );
}

// Sizing (in words) 
static int header_size(){ 
  return align_object_offset(sizeof(InstanceKlass)/HeapWordSize); // 以HeapWordSize為單位，64位一個字為8位元組，所以值為8 
}

// ReferenceType is used to distinguish between java/lang/ref/Reference subclasses

enum ReferenceType {
  REF_NONE,      // Regular class
  REF_OTHER,     // Subclass of java/lang/ref/Reference, but not subclass of one of the classes below
  REF_SOFT,      // Subclass of java/lang/ref/SoftReference
  REF_WEAK,      // Subclass of java/lang/ref/WeakReference
  REF_FINAL,     // Subclass of java/lang/ref/FinalReference
  REF_PHANTOM    // Subclass of java/lang/ref/PhantomReference
};

static int _offset_of_static_fields;

  static void init_offset_of_static_fields() {
    // Cache the offset of the static fields in the Class instance
    assert(_offset_of_static_fields == 0, "once");
    // java.lang.Class類使用InstanceMirrorKlass對象來表示，而java.lang.Class對象通過Oop對象來表示，那麼imk->size_helper()獲取的就是
    // Oop對象的大小，左移3位將字轉換為位元組。緊要着Oop對象後存儲靜態字段的值
    InstanceMirrorKlass* imk = InstanceMirrorKlass::cast(SystemDictionary::Class_klass());
    _offset_of_static_fields = imk->size_helper() << LogHeapWordSize; // LogHeapWordSize=3
  }
  
int size_helper() const {
    return layout_helper_to_size_helper(layout_helper());
  }
 
static int layout_helper_to_size_helper(jint lh) {
    assert(lh > (jint)_lh_neutral_value, "must be instance");
    return lh >> LogHeapWordSize;
}
 
int layout_helper() const{ return _layout_helper; }

java.lang.Class: field layout
  @ 12 --- instance fields start ---
  @ 12 "cachedConstructor" Ljava.lang.reflect.Constructor;
  @ 16 "newInstanceCallerCache" Ljava.lang.Class;
  @ 20 "name" Ljava.lang.String;
  @ 24 "reflectionData" Ljava.lang.ref.SoftReference;  
  @ 28 "genericInfo" Lsun.reflect.generics.repository.ClassRepository;
  @ 32 "enumConstants" [Ljava.lang.Object;
  @ 36 "enumConstantDirectory" Ljava.util.Map;
  @ 40 "annotationData" Ljava.lang.Class$AnnotationData;
  @ 44 "annotationType" Lsun.reflect.annotation.AnnotationType;
  @ 48 "classValueMap" Ljava.lang.ClassValue$ClassValueMap;
  @ 52 "protection_domain" Ljava.lang.Object;
  @ 56 "init_lock" Ljava.lang.Object;
  @ 60 "signers_name" Ljava.lang.Object;
  @ 64 "klass" J
  @ 72 "array_klass" J 
  @ 80 "classRedefinedCount" I
  @ 84 "oop_size" I
  @ 88 "static_oop_field_count" I
  @ 92 --- instance fields end ---
  @ 96 --- instance ends ---

InstanceKlass* InstanceKlass::allocate_instance_klass(
	ClassLoaderData*  loader_data,
	int               vtable_len,
	int               itable_len,
	int               static_field_size,
	int               nonstatic_oop_map_size,
	ReferenceType     rt,
	AccessFlags       access_flags,
	Symbol*           name,
	Klass*            super_klass,
	bool              is_anonymous,
	TRAPS
){
  bool  isinterf = access_flags.is_interface();
  int   size = InstanceKlass::size(
				 vtable_len,
				 itable_len,
				 nonstatic_oop_map_size,
				 isinterf,
				 is_anonymous
			   );

  // Allocation
  InstanceKlass* ik;
  ///////////////////////////////////////////////////////////////////////
  if (rt == REF_NONE) {
    if (name == vmSymbols::java_lang_Class()) { // 通過InstanceMirrorKlass對象表示java.lang.Class類
      ik = new (loader_data, size, THREAD) InstanceMirrorKlass(
                                           vtable_len,
					   itable_len,
					   static_field_size,
					   nonstatic_oop_map_size,
					   rt,
                                           access_flags,
					   is_anonymous);
    } else if (
    	  name == vmSymbols::java_lang_ClassLoader() ||
          (
             SystemDictionary::ClassLoader_klass_loaded() &&
             super_klass != NULL &&	 // ClassLoader_klass為java_lang_ClassLoader
             super_klass->is_subtype_of(SystemDictionary::ClassLoader_klass())
          )
    ){ //  通過InstanceClassLoaderKlass對象表示java.lang.ClassLoader或相關子類
      ik = new (loader_data, size, THREAD) InstanceClassLoaderKlass(
                                           vtable_len,
					   itable_len,
					   static_field_size,
					   nonstatic_oop_map_size,
					   rt,
                                           access_flags,
					   is_anonymous);
    } else { // 通過InstanceKlass對象表示普通類
      // normal class
      ik = new (loader_data, size, THREAD) InstanceKlass( 
				vtable_len, itable_len,
				static_field_size,
				nonstatic_oop_map_size,
				rt,
				access_flags,
				is_anonymous);
    }
  }
  ///////////////////////////////////////////////////////////////////////
  else { // 通過InstanceRefKlass對象表示引用
    // reference klass
    ik = new (loader_data, size, THREAD) InstanceRefKlass(
				vtable_len, itable_len,
				static_field_size,
				nonstatic_oop_map_size,
				rt,
				access_flags,
				is_anonymous);
  }
  ///////////////////////////////////////////////////////////////////////

  // 添加所有類型到我們內部類加載器列表中，包括在根加載器中的類
  // Add all classes to our internal class loader list here,
  // including classes in the bootstrap (NULL) class loader.
  // loader_data的類型為ClassLoaderData*,通過ClassLoaderData中的_klasses保持通過InstanceKlass._next_link屬性保持的列表
  loader_data->add_class(ik);
  Atomic::inc(&_total_instanceKlass_count);
  return ik;
}

void* Klass::operator new(size_t size, ClassLoaderData* loader_data, size_t word_size, TRAPS) throw() {
  void* x = Metaspace::allocate( // 在元數據區分配內存空間
				 loader_data,
				 word_size,
				 false,   /*read_only*/
				 MetaspaceObj::ClassType,
				 CHECK_NULL
			 );
  return x;
}