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1 OOP-Klass(Ordinary Object Pointer)模型

OOP-Klass模型用来描述class的属性和行为

1.1 OOP

oopDesc是对象类的最高父类。 {name}Desc类描述了Java对象的格式，可从C++访问这些字段

• 路径: /hotspot/share/oops/oop.hpp
  

完整的类层次结构，请阅读src/hotspot/share/oops/oopsHierarchy.hpp

• OOP体系
  

1.2 Klass

• Klass体系
  
  Klass对象提供
• 语言级别的类对象（方法字典等）
• 为对象提供虚拟机调度行为

class Klass : public Metadata {
     friend class VMStructs;  friend class JVMCIVMStructs; protected:  // 如果添加指向任何元数据对象的新字段，则必须将此字段添加到Klass :: metaspace_pointers_do()  // 注意：在klass结构的起始处将常用字段放在一起，以获得更好的缓存行为（虽然可能不会有太大的区别，但可以肯定不会造成伤害）  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;  // Klass identifier used to implement devirtualized oop closure dispatching.  const KlassID _id;  // 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;  // 类名.  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
   
    * _secondary_supers;  // Ordered list of all primary supertypes  Klass*      _primary_supers[_primary_super_limit];  // java/lang/Class instance mirroring this class  OopHandle _java_mirror;  // Superclass  Klass*      _super;  // First subclass (NULL if none); _subklass->next_sibling() is next one  Klass* volatile _subklass;  // Sibling link (or NULL); links all subklasses of a klass  Klass* volatile _next_sibling;  // All klasses loaded by a class loader are chained through these links  Klass*      _next_link;    // 用于加载此类的VM对类加载器的表示。  //提供访问相应的java.lang.ClassLoader实例  ClassLoaderData* _class_loader_data;  jint        _modifier_flags;  // 处理的访问标志，由Class.getModifiers使用  AccessFlags _access_flags;    // 访问标志。 类/接口的区别就存储在这里  JFR_ONLY(DEFINE_TRACE_ID_FIELD;)  // 偏向锁定实现和统计  // 64位块优先，以避免碎片  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;  // 虚表长度  int _vtable_len;  ...
   

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