上一篇 HotSpot的類模型（2） 介紹了類模型的基礎類Klass的重要屬性及方法，這一篇介紹一下InstanceKlass及InstanceKlass的子類。

2、InstanceKlass類

每個InstanceKlass物件表示一個具體的Java類（這裡的Java類不包括Java陣列）。InstanceKlass類及重要屬性的定義如下：

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.
  
  ...
}

重要屬性的介紹如下表所示。

有了InstanceKlass與Klass中定義的這些屬性足夠用來儲存Java類元資訊。在後續的類解析中會看到對相關變數的屬性填充操作。除了儲存類元資訊外，此類還有另外一個重要的功能，即支援方法分派，主要是通過Java虛方法表和Java介面函式表來完成的，不過C++並不像Java一樣，儲存資訊時非要在類中定義出相關屬性，C++只是在分配記憶體時為要儲存的資訊分配好特定的記憶體，然後直接通過記憶體偏移來操作即可。

接下來幾個屬性是沒有對應的屬性名，只能通過指標和偏移量的方式訪問：

• Java vtable：Java虛擬函式表，大小等於_vtable_len；
• Java itables：Java介面函式表，大小等於 _itable_len；
• 非靜態oop-map blocks ，大小等於_nonstatic_oop_map_size；
• 介面的實現類，只有當前類表示一個介面時存在。如果介面沒有任何實現類則為NULL；如果只有一個實現類則為該實現類的Klass指標；如果有多個實現類，為當前類本身；
• host klass，只在匿名類中存在，為了支援JSR 292中的動態語言特性，會給匿名類生成一個host klass。 

HotSpot在解析一個類時會呼叫InstanceKlass::allocate_instance_klass()方法分配記憶體，而分配多大的記憶體則是通過呼叫InstanceKlass::size()計算出來的，呼叫語句如下： 

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

呼叫的size()方法的實現如下：

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()    +  // header_size()為55
           align_object_offset(vtable_length) +
           align_object_offset(itable_length) +
           (
                  (is_interface || is_anonymous) ?
                  align_object_offset(nonstatic_oop_map_size) :
                  nonstatic_oop_map_size
		   ) +
           (
        		   is_interface ? (int)sizeof(Klass*)/HeapWordSize : 0
           ) +
           (
        		   is_anonymous ? (int)sizeof(Klass*)/HeapWordSize : 0)
		   );
  }

可以看到除了會為類中本身的屬性分配記憶體，也會為vtable與itable等分配記憶體。呼叫的header_size()方法就是計算此類的物件所佔用的記憶體大小，實現如下：

// 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 
}

呼叫的align_object_offset()方法是進行記憶體對齊，這是一塊非常重要的C++知識點，後面會專門進行講解。 

3、InstanceKlass類的子類

InstanceKlass共有3個直接子類，這3個子類用來表示一些特殊的類，下面簡單介紹一下這3個子類：

（1）InstanceRefKlass

java/lang/ref/Reference的子類需要使用InstanceRefKlass類來表示，因為這些類需要垃圾回收器特殊處理 ，在後續講解強引用、弱引用、虛引用以及幽靈引用時在詳細介紹。

（2）InstanceMirrorKlass類 

用於表示特殊的java.lang.Class類，我們需要分清相關類的表示方法，如下圖所示。

java.lang.Class物件是通過對應的Oop物件來儲存類的靜態屬性，因此他們的例項大小不同，需要特殊的方式來計算他們的大小以及屬性遍歷。

Klass的屬性_java_mirror就指向儲存該類靜態欄位的Oop物件，可通過該屬性訪問類的靜態欄位。 Oop是HotSpot的物件表示模型，在後面會詳細介紹。

 （3）InstanceClassLoaderKlass類　

沒有新增新的欄位，增加了新的oop遍歷方法，主要用於類載入器依賴遍歷使用。  

建立InstanceKlass例項會呼叫InstanceKlass::allocate_instance_klass()方法。在建立時，會涉及到C++new運算子的過載，通過過載new運算子來分配物件的記憶體空間，也就是呼叫InstanceKlass::size()方法得到的大小，然後再呼叫對應類的建構函式初始化相應的屬性。方法的實現如下：

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()) {
      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 &&
             super_klass->is_subtype_of(SystemDictionary::ClassLoader_klass()) // ClassLoader_klass為java_lang_ClassLoader
		  )
    ){
      ik = new (loader_data, size, THREAD) InstanceClassLoaderKlass(
                       vtable_len,
					   itable_len,
					   static_field_size,
					   nonstatic_oop_map_size,
					   rt,
                       access_flags, is_anonymous);
    } else {
      // 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 {
    // 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);
  }
  ///////////////////////////////////////////////////////////////////////

  // 新增所有型別到我們內部類載入器列表中，包括在根載入器中的類
  // Add all classes to our internal class loader list here,
  // including classes in the bootstrap (NULL) class loader.
  // loader_data的型別為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);

  return ik;
}

方法的實現比較簡單，當rt等於REF_NONE時，也就是為非Reference型別時，會根據類名建立對應C++類的物件。Class類建立InstanceMirrorKlass、ClassLoader類或ClassLoader的子類建立InstanceClassLoaderKlass類、普通類通過InstanceKlass來表示。當rt不為REF_NONE時，會建立InstanceRefKlass物件。REF_NONE列舉常量的定義如下： 

// 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
};

可以看到，所有的Reference及子類都會用InstanceRefKlass來表示。當無法判斷到底是哪個子類時，會將Reference設定為REF_OTHER。　　

相關文章的連結如下：

1、在Ubuntu 16.04上編譯OpenJDK8的原始碼 

2、除錯HotSpot原始碼

3、HotSpot專案結構　

4、HotSpot的啟動過程 

5、HotSpot二分模型 （1）

6、HotSpot的類模型（2）  

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