jvm源码解读--16 锁_开头

现在不太清楚,

    public static void main(String[] args) {

        Object object=new Object();
        System.out.println("before synchronized start");
        synchronized (object) {
            int am =object.hashCode();
            System.out.println("start");
            synchronized (object) {
                int a=object.hashCode();
                System.out.println("inner");

            }
            System.out.println("end");

        }

断点

//%note monitor_1
IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
  if (PrintBiasedLockingStatistics) {
    Atomic::inc(BiasedLocking::slow_path_entry_count_addr());
  }
  Handle h_obj(thread, elem->obj());
  assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
         "must be NULL or an object");
  if (UseBiasedLocking) {
    // Retry fast entry if bias is revoked to avoid unnecessary inflation
    ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
  } else {
    ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
  }
  assert(Universe::heap()->is_in_reserved_or_null(elem->obj()),
         "must be NULL or an object");
#ifdef ASSERT
  thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END

这个只会在嵌套的第二个里面,搞不懂,为什么第一次不会再第一次synchronized进入

通过thread打印栈帧

(gdb) x/50xg 0x7f8c1cd976d0
0x7f8c1cd976d0:    0xffffffffdb26d002     BasicObjectLock
                0x00000000f3886948
0x7f8c1cd976e0:    0x0000000000000001
                0x00000000f3886948   obj
0x7f8c1cd976f0:    0x00007f8c1cd976d0     -8
                0x00007f8c192357be   -7
0x7f8c1cd97700:    0x00007f8c1cd97798     -6
                0x00007f8c19235aa8   -5
0x7f8c1cd97710:    0x0000000000000000      -4
                0x00007f8c192359c8   -3
0x7f8c1cd97720:    0x0000000000000000     -2
                0x00007f8c1cd97798   -1
0x7f8c1cd97730:    0x00007f8c1cd97800     fp()
                0x00007f8c05000671
0x7f8c1cd97740:    0x0000000000000000
                0x0000000000000000
0x7f8c1cd97750:    0x0000000000000000
                0x0000000000000000
0x7f8c1cd97760:    0x0000000000000000
                0x0000000000000000
0x7f8c1cd97770:    0x0000000000000000
                0x00000000f3886948
0x7f8c1cd97780:    0x0000000033909752
                0x00000000f3886948   0bj
0x7f8c1cd97790:    0x00000000f3886948     obj
                0x00000000f3886938  args[]
0x7f8c1cd977a0:    0x00007f8c00001fa0     -12
                0x00007f8c1cd98700  -11
0x7f8c1cd977b0:    0x0000000000000000    -10
                0x00007f8c1cd97b40 -9
0x7f8c1cd977c0:    0x0000000000000001    -8
                0x00007f8c05000564 -7
0x7f8c1cd977d0:    0x00007f8c1cd97880    -6
                0x00007f8c1cd97d28 -5
0x7f8c1cd977e0:    0x00007f8c0000000a    -4
                0x00007f8c192359c8    -3
0x7f8c1cd977f0:    0x00007f8c0501e2e0    -2
                0x00007f8c1cd97b40  -1
0x7f8c1cd97800:    0x00007f8c1cd97a40       saved rbp
                0x00007f8c1b486e2e     return addr
0x7f8c1cd97810:    0x0000000000000001        param size
                0x00007f8c1400b800    thread
0x7f8c1cd97820:    0x00007f8c1400b800
                0x00007f8c1cd97b30
0x7f8c1cd97830:    0x00007f8c1cd97c50    0x00007f8c1cd97d20
0x7f8c1cd97840:    0x00007f8c1400b800    0x00007f8c1400c078
0x7f8c1cd97850:    0x00007f8c1400c0e8    0x00007f8c1400c108

sp头顶是是elem

(gdb) p * elem
$1 = {_lock = {_displaced_header = 0xffffffffdbae1002}, _obj = 0xf3886978}

那么看下BasicObjectLock定义

class BasicObjectLock VALUE_OBJ_CLASS_SPEC {
  friend class VMStructs;
 private:
  BasicLock _lock;                                    // the lock, must be double word aligned
  oop       _obj;                                     // object holds the lock;

 public:
  // Manipulation
....
};

(gdb) p elem._lock
$3 = {_displaced_header = 0xffffffffdbae1002}

那看下lock定义

class BasicLock VALUE_OBJ_CLASS_SPEC {
  friend class VMStructs;
 private:
  volatile markOop _displaced_header;
 public:
  markOop      displaced_header() const               { return _displaced_header; }
  void         set_displaced_header(markOop header)   { _displaced_header = header; }

  void print_on(outputStream* st) const;

  // move a basic lock (used during deoptimization
  void move_to(oop obj, BasicLock* dest);

  static int displaced_header_offset_in_bytes()       { return offset_of(BasicLock, _displaced_header); }
};

就是记录的一个markOop头

  enum { locked_value             = 0, //轻量级
         unlocked_value           = 1, //无锁
         monitor_value            = 2,  重量级
         marked_value             = 3,
         biased_lock_pattern      = 5  偏向锁
  };

那么打印一下这个oop中指示的互斥锁对象

(gdb) p * inf
$43 = {
  static SpinCallbackFunction = 0x0,
  static SpinCallbackArgument = 0,
  _header = 0x3390975201,
  _object = 0xf3886978,
  SharingPad = {-7.4786357953083842e+240},
  _owner = 0x7f24b05236e0,
  _previous_owner_tid = 0,
  _recursions = 0,
  OwnerIsThread = 0,
  _cxq = 0x0,
  _EntryList = 0x0,
  _succ = 0x0,
  _Responsible = 0x0,
  _PromptDrain = -235802127,
  _Spinner = -235802127,
  _SpinFreq = 0,
  _SpinClock = 0,
  _SpinDuration = 5000,
  _SpinState = -1012762419733073423,
  _count = 0,
  _waiters = 0,
  _WaitSet = 0x0,
  _WaitSetLock = 0,
  _QMix = -235802127,
  FreeNext = 0x0,
  StatA = -1012762419733073423,
  StatsB = -1012762419733073423,
  static _sync_ContendedLockAttempts = 0x7f24a800d8f8,
  static _sync_FutileWakeups = 0x7f24a800d9c8,
  static _sync_Parks = 0x7f24a800da88,
  static _sync_EmptyNotifications = 0x7f24a800db48,
  static _sync_Notifications = 0x7f24a800dc08,
  static _sync_SlowEnter = 0x7f24a800dcc8,
  static _sync_SlowExit = 0x7f24a800dd88,
  static _sync_SlowNotify = 0x7f24a800de48,
  static _sync_SlowNotifyAll = 0x7f24a800df08,
  static _sync_FailedSpins = 0x0,
  static _sync_SuccessfulSpins = 0x7f24a800e088,
  static _sync_PrivateA = 0x7f24a800e148,
  static _sync_PrivateB = 0x7f24a800e208,
  static _sync_MonInCirculation = 0x7f24a800e2c8,
  static _sync_MonScavenged = 0x7f24a800e388,
  static _sync_Inflations = 0x7f24a800d378,
  static _sync_Deflations = 0x7f24a800d838,
  static _sync_MonExtant = 0x7f24a800e448,
  static Knob_Verbose = 0,
  static Knob_SpinLimit = 5000
}
(gdb) p object
$44 = (oopDesc *) 0xf3886978