HotSpot源码分析之类模型

源代码位置：hotspot/src/share/vm/oops/klass.hpp

class Klass : public Metadata {
 // ...
 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;
 
  AccessFlags _access_flags;    // Access flags. The class/interface distinction is stored here.
 
  markOop  _prototype_header;   // Used when biased locking is both enabled and disabled for this type
  
  ...
}

jint Klass::array_layout_helper(BasicType etype) {
  assert(etype >= T_BOOLEAN && etype <= T_OBJECT, "valid etype");

  bool  isobj = (etype == T_OBJECT);
  int   tag   =  isobj ? _lh_array_tag_obj_value : _lh_array_tag_type_value;
   // Note that T_ARRAY is not allowed here.
  // 在64位系统下，存放_metadata的空间大小是8字节，_mark是8字节，
  // length是4字节，对象头为20字节，由于要按8字节对齐，所以会填充4字节，最终占用24字节
  int   hsize = arrayOopDesc::base_offset_in_bytes(etype); // hsize表示数组头部大小
  int   esize = type2aelembytes(etype); // Java基本类型元素需要占用的字节数
  int   esz = exact_log2(esize); // 例如Java基本类型元素占用4个字节，则存储的是2
  int   lh = array_layout_helper(tag, hsize, etype, esz);

  return lh;
}

_lh_array_tag_type_value    = ~0x00, 
_lh_array_tag_obj_value     = ~0x01   

// Returns the offset of the first element.
static int base_offset_in_bytes(BasicType type) {
    return header_size(type) * HeapWordSize;
}

// Should only be called with constants as argument (will not constant fold otherwise)
// Returns the header size in words aligned to the requirements of the array object type.
static int header_size(BasicType type) {
    size_t  typesize_in_bytes = header_size_in_bytes();
    if( Universe::element_type_should_be_aligned(type) ){
    	return (int)align_object_offset( typesize_in_bytes/HeapWordSize );
    }else {
    	return (int)typesize_in_bytes/HeapWordSize;
    }
}

// 在64位系统下，存放_metadata的空间大小是8字节，_mark是8字节，length是4字节，对象头为20字节，
// 由于要按8字节对齐，所以会填充4字节，最终占用24字节
static int header_size_in_bytes() {
    intptr_t  temp =  length_offset_in_bytes() + sizeof(int); // sizeof(int)
    size_t    hs = align_size_up(temp,HeapWordSize);
    return (int)hs;
}

// The _length field is not declared in C++.  It is allocated after the
// declared nonstatic fields in arrayOopDesc if not compressed, otherwise
// it occupies the second half 下半场 of the _klass field in oopDesc.
static int length_offset_in_bytes() {
	if(UseCompressedClassPointers){
		return klass_gap_offset_in_bytes();
	}else{
		 sizeof(arrayOopDesc);
	}
}
　　

static jint array_layout_helper(jint tag, int hsize, BasicType etype, int log2_esize) {
    return (tag        << _lh_array_tag_shift)           // 左移30位
      |     (hsize      << _lh_header_size_shift)         // 左移16位
      |     ((int)etype << _lh_element_type_shift)       // 左移1位
      |     (log2_esize << _lh_log2_element_size_shift);  // 左移0位
  }

 static jint instance_layout_helper(jint size, bool slow_path_flag) {
	  if(slow_path_flag){
		  return  (size << LogHeapWordSize)  | _lh_instance_slow_path_bit;
	  }else{
		  return  (size << LogHeapWordSize)  | 0; // LogHeapWordSize=3
	  }
  }

源代码位置：hotspot/src/share/vm/oops/klass.cpp 

void Klass::initialize_supers(Klass* k, TRAPS) { // 
  // 当前类的父类k可能为NULL，例如Object的父类为NULL
  if (k == NULL) {
    set_super(NULL);           
    _primary_supers[0] = this;
  } 
  // k就是当前类的直接父类，如果有父类，那么super()一般为NULL，如果k为NULL，那么就是Object类，从下面的断言也可以看出
  else if (k != super() || k == SystemDictionary::Object_klass()) {

    set_super(k);    // 设置Klass的_super属性
    Klass* sup = k;

    int sup_depth = sup->super_depth();
    juint my_depth  = MIN2(sup_depth + 1, (int)primary_super_limit()); // primary_super_limit()方法得到的值一般默认为8
    // 当父类的的继承链长度大于等于primary_super_limit()时，当前的深度只能是primary_super_limit()，也就是8,因为_primary_supers中只存储8个类
    if (!can_be_primary_super_slow()){
      my_depth = primary_super_limit(); // 8
    }
    for (juint i = 0; i < my_depth; i++) { // my_depth默认的值为8
       _primary_supers[i] = sup->_primary_supers[i];
    }

    Klass* *super_check_cell;
    if (my_depth < primary_super_limit()) { // primary_super_limit()的默认为8
       _primary_supers[my_depth] = this;
       super_check_cell = &_primary_supers[my_depth];
    } else {
       // Overflow of the primary_supers array forces me to be secondary.
       super_check_cell = &_secondary_super_cache;
    }
    // 通过_super_check_offset这个偏移量可以快速定义到当前在_primary_supers中的位置
    juint  _super_check_offset = (address)super_check_cell - (address) this;
    set_super_check_offset( _super_check_offset ); // 设置Klass中的_super_check_offset属性
  }

  // 第2部分代码在下面
}

if (secondary_supers() == NULL) {
    KlassHandle this_kh (THREAD, this);

    // Now compute the list of secondary supertypes.
    // Secondaries can occasionally be on the super chain,
    // if the inline "_primary_supers" array overflows.
    int extras = 0;
    Klass* p;
    for (p = super();
           // 当p不为NULL并且p已经存储在了_secondary_supers数组中时，条件为true
           // 也就是当前类的父类多于8个，将多出来的存储到了_secondary_supers数组中了
    	  !(p == NULL || p->can_be_primary_super());  
    	  p = p->super()) {
      ++extras;
    }

    // 计算secondaries需要的大小，因为secondaries数组中还需要存储当前类的所有实现接口（包括直接和间接实现的接口）
    // Compute the "real" non-extra secondaries.
    GrowableArray<Klass*>* secondaries = compute_secondary_supers(extras);
    if (secondaries == NULL) { // extras为0时直接返回，不需要额外的处理
      // secondary_supers set by compute_secondary_supers
      return;
    }

    GrowableArray<Klass*>* primaries = new GrowableArray<Klass*>(extras);
    for ( p = this_kh->super();
    	  !(p == NULL || p->can_be_primary_super());  
    	  p = p->super()
    ){
      primaries->push(p);
    }

    // Combine the two arrays into a metadata object to pack the array.
    // The primaries are added in the reverse order, then the secondaries.
    int new_length = primaries->length() + secondaries->length();
    Array<Klass*>* s2 = MetadataFactory::new_array<Klass*>(class_loader_data(), new_length, CHECK);
    int fill_p = primaries->length();
    for (int j = 0; j < fill_p; j++) {
       s2->at_put(j, primaries->pop());  // add primaries in reverse order.也就是父类永远在数组前，子类永远在数组后
    }
    for( int j = 0; j < secondaries->length(); j++ ) {
       s2->at_put(j+fill_p, secondaries->at(j));  // add secondaries on the end.
    }

    this_kh->set_secondary_supers(s2); // 设置_secondary_supers属性
}

interface IA{}
interface IB{}

class A{}
class B extends A{}
class C extends B{}
class D extends C{}
public class Test extends D implements IA,IB {}

_primary_supers[Object,A,B]
_secondary_supers[C,D,Test,IA,IB]

// subtype check: true if is_subclass_of, or if k is interface and receiver implements it
bool is_subtype_of(Klass* k) const { // 判断当前类是否为k的子类
    juint    off = k->super_check_offset();
    Klass*   sup = *(Klass**)( (address)this + off );

    const juint secondary_offset = in_bytes(secondary_super_cache_offset());

    if (sup == k) {
      return true;
    } else if (off != secondary_offset) { 
      return false;
    } else {
      return search_secondary_supers(k);
    }
}

bool Klass::search_secondary_supers(Klass* k) const {
  // Put some extra logic here out-of-line, before the search proper.
  // This cuts down the size of the inline method.

  // This is necessary, since I am never in my own secondary_super list.
  if (this == k){
    return true;
  }
  // Scan the array-of-objects for a match
  int cnt = secondary_supers()->length();
  for (int i = 0; i < cnt; i++) {
    if (secondary_supers()->at(i) == k) {
      ((Klass*)this)->set_secondary_super_cache(k); // 设置_secondary_super_cache属性，保存这次查询的结果
      return true;
    }
  }
  return false;
}

void Klass::append_to_sibling_list() {  

  // add ourselves to superklass' subklass list
  InstanceKlass* super = superklass(); // 获取到_super属性的值
  if (super == NULL)
	  return;   // special case: class Object

  Klass* prev_first_subklass = super->subklass_oop(); // 获取_subklass属性的值
  if (prev_first_subklass != NULL) {
    // set our sibling to be the superklass' previous first subklass
    set_next_sibling(prev_first_subklass); // 设置_next_sibling属性的值
  }
  // make ourselves the superklass' first subklass
  super->set_subklass(this); // 设置_subklass属性的值
}