- 在java語言層面的一個對象實(shí)例對應(yīng)一個JVM中的
instanceOopDesc；
參考/openjdk/hotspot/src/share/vm/oops/instanceOopDesc.hpp文件可知：
// An instanceOop is an instance of a Java Class
// Evaluating "new HashTable()" will create an instanceOop.
class instanceOopDesc : public oopDesc {
// 此處省略好多內(nèi)容
}
- 參考
/openjdk/hotspot/src/share/vm/oops/oop.hpp文件可知：-
對象頭
markOop弓熏，參考/openjdk/hotspot/src/share/vm/oops/markOop.hpp文件：// The markOop describes the header of an object. // // Note that the mark is not a real oop but just a word. // It is placed in the oop hierarchy for historical reasons. // // Bit-format of an object header (most significant first, big endian layout below): // // 32 bits: // -------- // hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object) // JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object) // size:32 ------------------------------------------>| (CMS free block) // PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object) // // 64 bits: // -------- // unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object) // JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object) // PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object) // size:64 ----------------------------------------------------->| (CMS free block) // // unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object) // JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object) // narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object) // unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block) // // - hash contains the identity hash value: largest value is // 31 bits, see os::random(). Also, 64-bit vm's require // a hash value no bigger than 32 bits because they will not // properly generate a mask larger than that: see library_call.cpp // and c1_CodePatterns_sparc.cpp. // // - the biased lock pattern is used to bias a lock toward a given // thread. When this pattern is set in the low three bits, the lock // is either biased toward a given thread or "anonymously" biased, // indicating that it is possible for it to be biased. When the // lock is biased toward a given thread, locking and unlocking can // be performed by that thread without using atomic operations. // When a lock's bias is revoked, it reverts back to the normal // locking scheme described below. // // Note that we are overloading the meaning of the "unlocked" state // of the header. Because we steal a bit from the age we can // guarantee that the bias pattern will never be seen for a truly // unlocked object. // // Note also that the biased state contains the age bits normally // contained in the object header. Large increases in scavenge // times were seen when these bits were absent and an arbitrary age // assigned to all biased objects, because they tended to consume a // significant fraction of the eden semispaces and were not // promoted promptly, causing an increase in the amount of copying // performed. The runtime system aligns all JavaThread* pointers to // a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM)) // to make room for the age bits & the epoch bits (used in support of // biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs). // // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread // [0 | epoch | age | 1 | 01] lock is anonymously biased // // - the two lock bits are used to describe three states: locked/unlocked and monitor. // // [ptr | 00] locked ptr points to real header on stack // [header | 0 | 01] unlocked regular object header // [ptr | 10] monitor inflated lock (header is wapped out) // [ptr | 11] marked used by markSweep to mark an object // not valid at any other time // // We assume that stack/thread pointers have the lowest two bits cleared. class markOopDesc: public oopDesc { public: // Constants enum { age_bits = 4, lock_bits = 2, biased_lock_bits = 1, max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits, hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits, cms_bits = LP64_ONLY(1) NOT_LP64(0), epoch_bits = 2 }; // The biased locking code currently requires that the age bits be // contiguous to the lock bits. enum { lock_shift = 0, biased_lock_shift = lock_bits, age_shift = lock_bits + biased_lock_bits, cms_shift = age_shift + age_bits, hash_shift = cms_shift + cms_bits, epoch_shift = hash_shift }; enum { lock_mask = right_n_bits(lock_bits), lock_mask_in_place = lock_mask << lock_shift, biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits), biased_lock_mask_in_place= biased_lock_mask << lock_shift, biased_lock_bit_in_place = 1 << biased_lock_shift, age_mask = right_n_bits(age_bits), age_mask_in_place = age_mask << age_shift, epoch_mask = right_n_bits(epoch_bits), epoch_mask_in_place = epoch_mask << epoch_shift, cms_mask = right_n_bits(cms_bits), cms_mask_in_place = cms_mask << cms_shift #ifndef _WIN64 ,hash_mask = right_n_bits(hash_bits), hash_mask_in_place = (address_word)hash_mask << hash_shift #endif }; // Alignment of JavaThread pointers encoded in object header required by biased locking enum { biased_lock_alignment = 2 << (epoch_shift + epoch_bits) }; #ifdef _WIN64 // These values are too big for Win64 const static uintptr_t hash_mask = right_n_bits(hash_bits); const static uintptr_t hash_mask_in_place = (address_word)hash_mask << hash_shift; #endif enum { locked_value = 0, unlocked_value = 1, monitor_value = 2, marked_value = 3, biased_lock_pattern = 5 }; enum { no_hash = 0 }; // no hash value assigned enum { no_hash_in_place = (address_word)no_hash << hash_shift, no_lock_in_place = unlocked_value }; enum { max_age = age_mask }; enum { max_bias_epoch = epoch_mask }; }?
-
元數(shù)據(jù)
_metadata娇跟，參考/openjdk/hotspot/src/share/vm/oops/kclass.hpp和/openjdk/hotspot/src/share/vm/oops/instanceKlass.hpp癌蚁，比如// 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) 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; // the source debug extension for this klass, NULL if not specified. // Specified as UTF-8 string without terminating zero byte in the classfile, // it is stored in the instanceklass as a NULL-terminated UTF-8 string char* _source_debug_extension; // 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 // _is_marked_dependent can be set concurrently, thus cannot be part of the // _misc_flags. bool _is_marked_dependent; // used for marking during flushing and deoptimization enum { _misc_rewritten = 1 << 0, // methods rewritten. _misc_has_nonstatic_fields = 1 << 1, // for sizing with UseCompressedOops _misc_should_verify_class = 1 << 2, // allow caching of preverification _misc_is_anonymous = 1 << 3, // has embedded _host_klass field _misc_is_contended = 1 << 4, // marked with contended annotation _misc_has_default_methods = 1 << 5 // class/superclass/implemented interfaces has default methods }; u2 _misc_flags; 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) MemberNameTable* _member_names; // Member names 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 BreakpointInfo* _breakpoints; // bpt lists, managed by Method* // Array of interesting part(s) of the previous version(s) of this // InstanceKlass. See PreviousVersionWalker below. GrowableArray<PreviousVersionNode *>* _previous_versions; // JVMTI fields can be moved to their own structure - see 6315920 // JVMTI: cached class file, before retransformable agent modified it in CFLH JvmtiCachedClassFileData* _cached_class_file; volatile u2 _idnum_allocated_count; // JNI/JVMTI: increments with the addition of methods, old ids don't change // 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 JvmtiCachedClassFieldMap* _jvmti_cached_class_field_map; // JVMTI: used during heap iteration NOT_PRODUCT(int _verify_count;) // to avoid redundant verifies // 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 original order of method in the class file (for JVMTI). Array<int>* _method_ordering; // 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. // }和
// // 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] 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; 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) }?
實(shí)例數(shù)據(jù)
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// 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
class oopDesc {
private:
volatile markOop _mark;
union _metadata {
Klass* _klass;
narrowKlass _compressed_klass;
} _metadata;
private:
// field addresses in oop
void* field_base(int offset) const;
jbyte* byte_field_addr(int offset) const;
jchar* char_field_addr(int offset) const;
jboolean* bool_field_addr(int offset) const;
jint* int_field_addr(int offset) const;
jshort* short_field_addr(int offset) const;
jlong* long_field_addr(int offset) const;
jfloat* float_field_addr(int offset) const;
jdouble* double_field_addr(int offset) const;
Metadata** metadata_field_addr(int offset) const;
}
總算把對象的內(nèi)存布局搞清楚了赶站。
