HotSpot的类模型（3）

上一篇HotSpot的类模型（2）介绍了类模型的基础类Klass的重要属性及方法，这一篇介绍一下InstanceKlass及InstanceKlass的子类。

2、InstanceKlass类

每个InstanceKlass对象表示一个具体的Java类（这里的Java类不包括Java数组）。InstanceKlass类及重要属性的定义如下：

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.
  
  ...
}

重要属性的介绍如下表所示。

有了InstanceKlass与Klass中定义的这些属性足够用来保存Java类元信息。在后续的类解析中会看到对相关变量的属性填充操作。除了保存类元信息外，此类还有另外一个重要的功能，即支持方法分派，主要是通过Java虚方法表和Java接口函数表来完成的，不过C++并不像Java一样，保存信息时非要在类中定义出相关属性，C++只是在分配内存时为要存储的信息分配好特定的内存，然后直接通过内存偏移来操作即可。

接下来几个属性是没有对应的属性名，只能通过指针和偏移量的方式访问：

• Java vtable：Java虚函数表，大小等于_vtable_len；
• Java itables：Java接口函数表，大小等于 _itable_len；
• 非静态oop-map blocks ，大小等于_nonstatic_oop_map_size；
• 接口的实现类，只有当前类表示一个接口时存在。如果接口没有任何实现类则为NULL；如果只有一个实现类则为该实现类的Klass指针；如果有多个实现类，为当前类本身；
• host klass，只在匿名类中存在，为了支持JSR 292中的动态语言特性，会给匿名类生成一个host klass。 

HotSpot在解析一个类时会调用InstanceKlass::allocate_instance_klass()方法分配内存，而分配多大的内存则是通过调用InstanceKlass::size()计算出来的，调用语句如下：

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

调用的size()方法的实现如下：

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()为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)
		   );
  }

可以看到除了会为类中本身的属性分配内存，也会为vtable与itable等分配内存。调用的header_size()方法就是计算此类的对象所占用的内存大小，实现如下：

// Sizing (in words) 
static int header_size(){ 
  return align_object_offset(sizeof(InstanceKlass)/HeapWordSize); // 以HeapWordSize为单位，64位一个字为8字节，所以值为8 
}

调用的align_object_offset()方法是进行内存对齐，这是一块非常重要的C++知识点，后面会专门进行讲解。

3、InstanceKlass类的子类

InstanceKlass共有3个直接子类，这3个子类用来表示一些特殊的类，下面简单介绍一下这3个子类：

（1）InstanceRefKlass

java/lang/ref/Reference的子类需要使用InstanceRefKlass类来表示，因为这些类需要垃圾回收器特殊处理 ，在后续讲解强引用、弱引用、虚引用以及幽灵引用时在详细介绍。

（2） InstanceMirrorKlass 类

用于表示特殊的java.lang.Class类，我们需要分清相关类的表示方法，如下图所示。

java.lang.Class对象是通过对应的Oop对象来保存类的静态属性，因此他们的实例大小不同，需要特殊的方式来计算他们的大小以及属性遍历。

Klass的属性_java_mirror就指向保存该类静态字段的Oop对象，可通过该属性访问类的静态字段。 Oop是HotSpot的对象表示模型，在后面会详细介绍。

（3） InstanceClassLoaderKlass 类

没有添加新的字段，增加了新的oop遍历方法，主要用于类加载器依赖遍历使用。

创建InstanceKlass实例会调用InstanceKlass::allocate_instance_klass()方法。在创建时，会涉及到C++new运算符的重载，通过重载new运算符来分配对象的内存空间，也就是调用InstanceKlass::size()方法得到的大小，然后再调用对应类的构造函数初始化相应的属性。方法的实现如下：

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为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);
  }
  ///////////////////////////////////////////////////////////////////////

  // 添加所有类型到我们内部类加载器列表中，包括在根加载器中的类
  // Add all classes to our internal class loader list here,
  // including classes in the bootstrap (NULL) class loader.
  // loader_data的类型为ClassLoaderData*,通过ClassLoaderData中的_klasses保持通过InstanceKlass._next_link属性保持的列表
  loader_data->add_class(ik);

  return ik;
}

方法的实现比较简单，当rt等于REF_NONE时，也就是为非Reference类型时，会根据类名创建对应C++类的对象。Class类创建InstanceMirrorKlass、ClassLoader类或ClassLoader的子类创建InstanceClassLoaderKlass类、普通类通过InstanceKlass来表示。当rt不为REF_NONE时，会创建InstanceRefKlass对象。REF_NONE枚举常量的定义如下：

// 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及子类都会用InstanceRefKlass来表示。当无法判断到底是哪个子类时，会将Reference设置为REF_OTHER。