对象头源码讲解，原来，指向objectMonitor的指针在这里

markword

注释

该文件目录在：

\openjdk-jdk8u\hotspot\src\share\vm\oops\markOop.hpp

#ifndef SHARE_VM_OOPS_MARKOOP_HPP
#define SHARE_VM_OOPS_MARKOOP_HPP
#include "oops/oop.hpp"
// 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)

这里咱们翻译下

markOop描述对象头。

注意的是，对象头不是一个真正的oop，而只是一个word（大家可以回想下学计算机基础课程的时候，一个word大概是32字节，64位机器上，则是64字节。）

它只是因为历史原因而被放在oop的继承结构中。

对象头的格式（32字节）：

正常时：

25bit的hash---------------------------------- 4bit的 gc 年龄 -------偏向锁标志1个bit------------lock标志，2个bit

偏向时：

23个bit，存放偏向的线程的指针；2bit，存放epoch；4bit，存放gc年龄；1bit，偏向锁标志；2bit，锁标志

cms free block：

跳过，不懂

CMS promoted object（cms提升后的对象，是指从新生代提升到老年代的对象？）

29bit，存放PromotedObject的指针；3bit，存放promo_bits

继续下一段：

//  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)

翻译：

64 bit：
正常对象：
25bit,未使用；31bit，hashcode；1bit，没使用；4bit,gc年龄；1bit，偏向锁标志；2bit,lock标志
偏向锁状态时：
54bit，当前偏向的线程的指针；2bit，epoch；1bit，没使用；4bit，gc年龄；1bit，偏向锁标志；2bit,lock标志
cms提升后的对象：
61bit，PromotedObject*；3bit，promo_bits
cms free obj：
不懂，跳过。

然后我们先说下，上面还有几个没翻译，插播个名词，COOPs，压缩对象指针技术，对象指针压缩在Java SE 6u23 默认开启。在此之前，可以使用-XX:+UseCompressedOops来开启。可以看看这个链接：

https://blog.csdn.net/superfjj/article/details/107455559

ok，我们继续：

//
//  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)

翻译：

64 bit：
正常对象（和前面没开coops时比，没变化）：
25bit,未使用；31bit，hashcode；1bit，没使用；4bit,gc年龄；1bit，偏向锁标志；2bit,lock标志
偏向锁状态时（和前面没开coops时比，没变化）：
54bit，当前偏向的线程的指针；2bit，epoch；1bit，没使用；4bit，gc年龄；1bit，偏向锁标志；2bit,lock标志
COOPs && CMS promoted object:
narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3
COOPs && CMS free block:
unused:21 size:35 -->| cms_free:1 unused:7

//  - 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).

翻译：

hash字段，包含了唯一的hash value：最大的值是31bit。另外，64bit的虚拟机时，hash value也不能超过32bit；因为不能恰当地生成一个大于它的掩码。
偏向锁标志，是用来把一个锁，偏向一个指定的现场。当在最后三位，设置了该模式后，这个锁，要么偏向一个指定的现场，要么被匿名偏向（表示可能被偏向）。当这个锁，被偏向一个指定的线程时，该线程进行加锁和解锁时，无需原子操作（有点费解）
当该锁的偏向标志被撤销时，它会回到正常的锁定的模式。
注意，我们这里重载了header中，未锁定状态的意义。因为我们从age中偷了一位，这样我们就可以保证，对于一个真正没被锁定的对象，偏向标志不会被看到。
译者补充：我们再把偏向标志的拿过来看一下：
//  JavaThread*:54 epoch:2 cms_free:1 age:4    biased_lock:1 lock:2 (biased object)
从age偷了一位，难道之前age是5bit，最大gc年龄32吗，现在4位，所以最大gc年龄为16.
不是很理解这句话。
注意的是，偏向状态时，包含了age的bit位。当这几个位缺失时，且给所有被偏向对象，赋值一个任意的gc 年龄时，我们会看到，清理垃圾的时间大幅上升，因为他们将消耗一部分的eden空间，且不能被迅速提升到老年代，导致了一定量的拷贝工作（译者：从eden拷贝到s区）。

//
//    [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.

翻译：

lock偏向某个线程时：
[JavaThread* | epoch | age | 1 | 01]
lock is anonymously biased（可能被偏向时）
[0           | epoch | age | 1 | 01]
后面的两位，被用来描述三种状态：锁定、未锁定、monitor。
锁定时：
[ptr             | 00]
ptr指向一个栈上的header
未锁定时：
[header      | 0 | 01]
header就是常规的对象头
monitor时：
[ptr             | 10]
ptr指向膨胀后的lock， header被包装起来了
marked：
[ptr             | 11]  marked
markSweep，即标记清理时使用，标记一个对象无效

正文

以下为全文，下面的1处，我先讲解下，这个定义了一个field：

uintptr_t value()

我也不熟c++，查了一下，https://blog.csdn.net/cs_zhanyb/article/details/16973379

在64位的机器上，intptr_t和uintptr_t分别是long int、unsigned long int的别名；在32位的机器上，intptr_t和uintptr_t分别是int、unsigned int的别名。

也就是说，在64位机器上，这个类型代表了unsigned long int，那既然是long，肯定是64位了，也就是说，此时它就是个无符号的long类型；

32位上，则代表了unsigned int，此时，它就是个无符号的int（32位）。

有同学问我，锁膨胀时，指向objectmonitor的指针在哪里，ok，就是在这个里面。

大家继续看下面：

class BasicLock;
class ObjectMonitor;
class JavaThread;
class markOopDesc: public oopDesc {
 private:
  // Conversion
  // 1
  uintptr_t value() const { return (uintptr_t) this; }
 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
  };
  // Alignment of JavaThread pointers encoded in object header required by biased locking
  enum { biased_lock_alignment    = 2 << (epoch_shift + epoch_bits)
  };
  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 };
  // Prototype mark for initialization
  static markOop biased_locking_prototype() {
    return markOop( biased_lock_pattern );
  }
  // lock accessors (note that these assume lock_shift == 0)
  bool is_locked()   const {
    return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
  }
  bool is_unlocked() const {
    return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value);
  }
  bool is_marked()   const {
    return (mask_bits(value(), lock_mask_in_place) == marked_value);
  }
  bool is_neutral()  const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }
  // Special temporary state of the markOop while being inflated.
  // Code that looks at mark outside a lock need to take this into account.
  bool is_being_inflated() const { return (value() == 0); }
  // Distinguished markword value - used when inflating over
  // an existing stacklock.  0 indicates the markword is "BUSY".
  // Lockword mutators that use a LD...CAS idiom should always
  // check for and avoid overwriting a 0 value installed by some
  // other thread.  (They should spin or block instead.  The 0 value
  // is transient and *should* be short-lived).
  static markOop INFLATING() { return (markOop) 0; }    // inflate-in-progress
  // Should this header be preserved during GC?
  inline bool must_be_preserved(oop obj_containing_mark) const;
  inline bool must_be_preserved_with_bias(oop obj_containing_mark) const;
  // Should this header (including its age bits) be preserved in the
  // case of a promotion failure during scavenge?
  // Note that we special case this situation. We want to avoid
  // calling BiasedLocking::preserve_marks()/restore_marks() (which
  // decrease the number of mark words that need to be preserved
  // during GC) during each scavenge. During scavenges in which there
  // is no promotion failure, we actually don't need to call the above
  // routines at all, since we don't mutate and re-initialize the
  // marks of promoted objects using init_mark(). However, during
  // scavenges which result in promotion failure, we do re-initialize
  // the mark words of objects, meaning that we should have called
  // these mark word preservation routines. Currently there's no good
  // place in which to call them in any of the scavengers (although
  // guarded by appropriate locks we could make one), but the
  // observation is that promotion failures are quite rare and
  // reducing the number of mark words preserved during them isn't a
  // high priority.
  inline bool must_be_preserved_for_promotion_failure(oop obj_containing_mark) const;
  inline bool must_be_preserved_with_bias_for_promotion_failure(oop obj_containing_mark) const;
  // Should this header be preserved during a scavenge where CMS is
  // the old generation?
  // (This is basically the same body as must_be_preserved_for_promotion_failure(),
  // but takes the Klass* as argument instead)
  inline bool must_be_preserved_for_cms_scavenge(Klass* klass_of_obj_containing_mark) const;
  inline bool must_be_preserved_with_bias_for_cms_scavenge(Klass* klass_of_obj_containing_mark) const;
  // WARNING: The following routines are used EXCLUSIVELY by
  // synchronization functions. They are not really gc safe.
  // They must get updated if markOop layout get changed.
  markOop set_unlocked() const {
    return markOop(value() | unlocked_value);
  }
  bool has_locker() const {
    return ((value() & lock_mask_in_place) == locked_value);
  }
  BasicLock* locker() const {
    assert(has_locker(), "check");
    return (BasicLock*) value();
  }
  bool has_displaced_mark_helper() const {
    return ((value() & unlocked_value) == 0);
  }
  markOop displaced_mark_helper() const {
    assert(has_displaced_mark_helper(), "check");
    intptr_t ptr = (value() & ~monitor_value);
    return *(markOop*)ptr;
  }
  void set_displaced_mark_helper(markOop m) const {
    assert(has_displaced_mark_helper(), "check");
    intptr_t ptr = (value() & ~monitor_value);
    *(markOop*)ptr = m;
  }
  markOop copy_set_hash(intptr_t hash) const {
    intptr_t tmp = value() & (~hash_mask_in_place);
    tmp |= ((hash & hash_mask) << hash_shift);
    return (markOop)tmp;
  }
  // it is only used to be stored into BasicLock as the
  // indicator that the lock is using heavyweight monitor
  static markOop unused_mark() {
    return (markOop) marked_value;
  }
  // the following two functions create the markOop to be
  // stored into object header, it encodes monitor info
  static markOop encode(BasicLock* lock) {
    return (markOop) lock;
  }
  static markOop encode(ObjectMonitor* monitor) {
    intptr_t tmp = (intptr_t) monitor;
    return (markOop) (tmp | monitor_value);
  }
  static markOop encode(JavaThread* thread, uint age, int bias_epoch) {
    intptr_t tmp = (intptr_t) thread;
    assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
    assert(age <= max_age, "age too large");
    assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
    return (markOop) (tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
  }
  // used to encode pointers during GC
  markOop clear_lock_bits() { return markOop(value() & ~lock_mask_in_place); }
  // age operations
  markOop set_marked()   { return markOop((value() & ~lock_mask_in_place) | marked_value); }
  markOop set_unmarked() { return markOop((value() & ~lock_mask_in_place) | unlocked_value); }
  uint    age()               const { return mask_bits(value() >> age_shift, age_mask); }
  markOop set_age(uint v) const {
    assert((v & ~age_mask) == 0, "shouldn't overflow age field");
    return markOop((value() & ~age_mask_in_place) | (((uintptr_t)v & age_mask) << age_shift));
  }
  markOop incr_age()          const { return age() == max_age ? markOop(this) : set_age(age() + 1); }
  // hash operations
  intptr_t hash() const {
    return mask_bits(value() >> hash_shift, hash_mask);
  }
  bool has_no_hash() const {
    return hash() == no_hash;
  }
  // Prototype mark for initialization
  static markOop prototype() {
    return markOop( no_hash_in_place | no_lock_in_place );
  }
  // Helper function for restoration of unmarked mark oops during GC
  static inline markOop prototype_for_object(oop obj);
  // Debugging
  void print_on(outputStream* st) const;
  // Prepare address of oop for placement into mark
  inline static markOop encode_pointer_as_mark(void* p) { return markOop(p)->set_marked(); }
  // Recover address of oop from encoded form used in mark
  inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return clear_lock_bits(); }
  // These markOops indicate cms free chunk blocks and not objects.
  // In 64 bit, the markOop is set to distinguish them from oops.
  // These are defined in 32 bit mode for vmStructs.
  const static uintptr_t cms_free_chunk_pattern  = 0x1;
  // Constants for the size field.
  enum { size_shift                = cms_shift + cms_bits,
         size_bits                 = 35    // need for compressed oops 32G
       };
  // These values are too big for Win64
  const static uintptr_t size_mask = LP64_ONLY(right_n_bits(size_bits))
                                     NOT_LP64(0);
  const static uintptr_t size_mask_in_place =
                                     (address_word)size_mask << size_shift;
#ifdef _LP64
  static markOop cms_free_prototype() {
    return markOop(((intptr_t)prototype() & ~cms_mask_in_place) |
                   ((cms_free_chunk_pattern & cms_mask) << cms_shift));
  }
  uintptr_t cms_encoding() const {
    return mask_bits(value() >> cms_shift, cms_mask);
  }
  bool is_cms_free_chunk() const {
    return is_neutral() &&
           (cms_encoding() & cms_free_chunk_pattern) == cms_free_chunk_pattern;
  }
  size_t get_size() const       { return (size_t)(value() >> size_shift); }
  static markOop set_size_and_free(size_t size) {
    assert((size & ~size_mask) == 0, "shouldn't overflow size field");
    return markOop(((intptr_t)cms_free_prototype() & ~size_mask_in_place) |
                   (((intptr_t)size & size_mask) << size_shift));
  }
#endif // _LP64
};
#endif // SHARE_VM_OOPS_MARKOOP_HPP

然后我们讲解下部分方法：

// Biased Locking accessors.
  // These must be checked by all code which calls into the
  // ObjectSynchronizer and other code. The biasing is not understood
  // by the lower-level CAS-based locking code, although the runtime
  // fixes up biased locks to be compatible with it when a bias is
  // revoked.
  // 是否设置了偏向标志
  bool has_bias_pattern() const {
    return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
  }
  // 获取偏向的线程的指针
  JavaThread* biased_locker() const {
    assert(has_bias_pattern(), "should not call this otherwise");
    return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place))));
  }
  // Indicates that the mark has the bias bit set but that it has not
  // yet been biased toward a particular thread
  // 是否可以偏向；但当前还没有偏向任何线程
  bool is_biased_anonymously() const {
    return (has_bias_pattern() && (biased_locker() == NULL));
  }

下面可以看到epoch的简单意思：

如果因为太多次发生偏向锁撤销，那么epoch会发生变化。

  // Indicates epoch in which this bias was acquired. If the epoch
  // changes due to too many bias revocations occurring, the biases
  // from the previous epochs are all considered invalid.
  int bias_epoch() const {
    assert(has_bias_pattern(), "should not call this otherwise");
    return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
  }
  markOop set_bias_epoch(int epoch) {
    assert(has_bias_pattern(), "should not call this otherwise");
    assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
    return markOop(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
  }
  markOop incr_bias_epoch() {
    return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
  }

是否升级为monitor及返回对应的monitor的指针：

  bool has_monitor() const {
    return ((value() & monitor_value) != 0);
  }
  ObjectMonitor* monitor() const {
    assert(has_monitor(), "check");
    // Use xor instead of &~ to provide one extra tag-bit check.
    return (ObjectMonitor*) (value() ^ monitor_value);
  }