hotspot source code - klass and oop

OOP-Klass 二分模型

1. OOP是指ordinary object pointer，普通对象指针，用来描述 JVM 中的实例对象的实例信息。
2. Klass是 Java 类在 C++ 中的对等体，用来描述 Java 类，在其中包括了类的相关信息。

这个模型也体现了编程设计的最根本的一个设计原则：分离变化的与不变化的部分。其中 Java 实例信息相对 Java 类而言，是包含了变化的部分数据，Java 类描述了类的行为和类信息，为所有实例所共享。这个二分模型中的 Klass 向 JVM 提供了二个功能：

1. 实现语言层面的 Java 类
2. 实现 Java 对象的方法分发(dispatch)功能

oop 基类

vm/oops/oop.hpp 第35行，**OopDesc类型都继承自class oopDesc，源码如下：

// oopDesc is the top baseclass for objects classes.  The {name}Desc classes describe
// the format of Java objects so the fields can be accessed from C++.
// oopDesc is abstract.
// (see oopHierarchy for complete oop class hierarchy)
//
// no virtual functions allowed

// store into oop with store check
template <class T> void oop_store(T* p, oop v);
template <class T> void oop_store(volatile T* p, oop v);

extern bool always_do_update_barrier;

// Forward declarations.
class OopClosure;
class ScanClosure;
class FastScanClosure;
class FilteringClosure;
class BarrierSet;
class CMSIsAliveClosure;

class PSPromotionManager;
class ParCompactionManager;

class oopDesc {
  friend class VMStructs;
 private:
  volatile markOop  _mark;
  union _metadata {
    Klass*      _klass;
    narrowKlass _compressed_klass;
  } _metadata;

  // Fast access to barrier set.  Must be initialized.
  static BarrierSet* _bs;

 public:
  markOop  mark() const         { return _mark; }
  markOop* mark_addr() const    { return (markOop*) &_mark; }

  void set_mark(volatile markOop m)      { _mark = m;   }

  void    release_set_mark(markOop m);
  markOop cas_set_mark(markOop new_mark, markOop old_mark);

  // Used only to re-initialize the mark word (e.g., of promoted
  // objects during a GC) -- requires a valid klass pointer
  void init_mark();

  Klass* klass() const;
  Klass* klass_or_null() const volatile;
  Klass** klass_addr();
  narrowKlass* compressed_klass_addr();

  void set_klass(Klass* k);

  // For klass field compression
  int klass_gap() const;
  void set_klass_gap(int z);
  // For when the klass pointer is being used as a linked list "next" field.
  void set_klass_to_list_ptr(oop k);
  oop list_ptr_from_klass();

  // size of object header, aligned to platform wordSize
  static int header_size()          { return sizeof(oopDesc)/HeapWordSize; }

  // Returns whether this is an instance of k or an instance of a subclass of k
  bool is_a(Klass* k)  const;

  // Returns the actual oop size of the object
  int size();

  // Sometimes (for complicated concurrency-related reasons), it is useful
  // to be able to figure out the size of an object knowing its klass.
  int size_given_klass(Klass* klass);

  // type test operations (inlined in oop.inline.h)
  bool is_instance()            const;
  bool is_instanceMirror()      const;
  bool is_instanceClassLoader() const;
  bool is_instanceRef()         const;
  bool is_array()               const;
  bool is_objArray()            const;
  bool is_typeArray()           const;

Klass 数据结构

vm/oops/klass.hpp 第44行，Klass类型数据结构说明，源码如下：

/
// A Klass provides:
//  1: language level class object (method dictionary etc.)
//  2: provide vm dispatch behavior for the object
// Both functions are combined into one C++ class.

// One reason for the oop/klass dichotomy in the implementation is
// that we don't want a C++ vtbl pointer in every object.  Thus,
// normal oops don't have any virtual functions.  Instead, they
// forward all "virtual" functions to their klass, which does have
// a vtbl and does the C++ dispatch depending on the object's
// actual type.  (See oop.inline.hpp for some of the forwarding code.)
// ALL FUNCTIONS IMPLEMENTING THIS DISPATCH ARE PREFIXED WITH "oop_"!

//  Klass layout:
//    [C++ vtbl ptr  ] (contained in Metadata)
//    [layout_helper ]
//    [super_check_offset   ] for fast subtype checks
//    [name          ]
//    [secondary_super_cache] for fast subtype checks
//    [secondary_supers     ] array of 2ndary supertypes
//    [primary_supers 0]
//    [primary_supers 1]
//    [primary_supers 2]
//    ...
//    [primary_supers 7]
//    [java_mirror   ]
//    [super         ]
//    [subklass      ] first subclass
//    [next_sibling  ] link to chain additional subklasses
//    [next_link     ]
//    [class_loader_data]
//    [modifier_flags]
//    [access_flags  ]
//    [last_biased_lock_bulk_revocation_time] (64 bits)
//    [prototype_header]
//    [biased_lock_revocation_count]
//    [_modified_oops]
//    [_accumulated_modified_oops]
//    [trace_id]


// Forward declarations.
template <class T> class Array;
template <class T> class GrowableArray;
class ClassLoaderData;
class klassVtable;
class ParCompactionManager;
class KlassSizeStats;
class fieldDescriptor;

class Klass : public Metadata {
  friend class VMStructs;
 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;

  jint        _modifier_flags;  // Processed access flags, for use by Class.getModifiers.
  AccessFlags _access_flags;    // Access flags. The class/interface distinction is stored here.

  // Biased locking implementation and statistics
  // (the 64-bit chunk goes first, to avoid some fragmentation)
  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;

  TRACE_DEFINE_KLASS_TRACE_ID;

  // Remembered sets support for the oops in the klasses.
  jbyte _modified_oops;             // Card Table Equivalent (YC/CMS support)
  jbyte _accumulated_modified_oops; // Mod Union Equivalent (CMS support)

instanceKlass

vm/oops/instanceKlass.hpp 第43行，instanceKlass是 JVM 中 Java 类的对等体，instanceKlass类型数据结构说明，源码如下：

// An InstanceKlass is the VM level representation of a Java class.
// It contains all information needed for at class at execution runtime.

//  InstanceKlass layout:
//    [C++ vtbl pointer           ] Klass
//    [subtype cache              ] Klass
//    [instance size              ] Klass
//    [java mirror                ] Klass
//    [super                      ] Klass
//    [access_flags               ] Klass
//    [name                       ] Klass
//    [first subklass             ] Klass
//    [next sibling               ] Klass
//    [array klasses              ]
//    [methods                    ]
//    [local interfaces           ]
//    [transitive interfaces      ]
//    [fields                     ]
//    [constants                  ]
//    [class loader               ]
//    [source file name           ]
//    [inner classes              ]
//    [static field size          ]
//    [nonstatic field size       ]
//    [static oop fields size     ]
//    [nonstatic oop maps size    ]
//    [has finalize method        ]
//    [deoptimization mark bit    ]
//    [initialization state       ]
//    [initializing thread        ]
//    [Java vtable length         ]
//    [oop map cache (stack maps) ]
//    [EMBEDDED Java vtable             ] size in words = vtable_len
//    [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.
//    [EMBEDDED implementor of the interface] only exist for interface
//    [EMBEDDED host klass        ] only exist for an anonymous class (JSR 292 enabled)


// forward declaration for class -- see below for definition
class SuperTypeClosure;
class JNIid;
class jniIdMapBase;
class BreakpointInfo;
class fieldDescriptor;
class DepChange;
class nmethodBucket;
class JvmtiCachedClassFieldMap;
class MemberNameTable;

// This is used in iterators below.
class FieldClosure: public StackObj {
public:
  virtual void do_field(fieldDescriptor* fd) = 0;
};

#ifndef PRODUCT
// Print fields.
// If "obj" argument to constructor is NULL, prints static fields, otherwise prints non-static fields.
class FieldPrinter: public FieldClosure {
   oop _obj;
   outputStream* _st;
 public:
   FieldPrinter(outputStream* st, oop obj = NULL) : _obj(obj), _st(st) {}
   void do_field(fieldDescriptor* fd);
};
#endif  // !PRODUCT

// ValueObjs embedded in klass. Describes where oops are located in instances of
// this klass.
class OopMapBlock VALUE_OBJ_CLASS_SPEC {
 public:
  // Byte offset of the first oop mapped by this block.
  int offset() const          { return _offset; }
  void set_offset(int offset) { _offset = offset; }

  // Number of oops in this block.
  uint count() const         { return _count; }
  void set_count(uint count) { _count = count; }

  // sizeof(OopMapBlock) in HeapWords.
  static const int size_in_words() {
    return align_size_up(int(sizeof(OopMapBlock)), HeapWordSize) >>
      LogHeapWordSize;
  }

 private:
  int  _offset;
  uint _count;
};

struct JvmtiCachedClassFileData;

class InstanceKlass: public Klass {
  friend class VMStructs;
  friend class ClassFileParser;
  friend class CompileReplay;

References

1. vm/oops/oop.hpp
2. vm/oops/klass.hpp
3. vm/oops/instanceKlass.hpp