Synchronized是怎么实现的？

//同步方法，对象锁  
public synchronized void doSth(){
    System.out.println("Hello World");
}

//同步方法，类锁  
public synchronized static void doSth(){
    System.out.println("Hello World");
}

//同步代码块，类锁
public void doSth1(){
    synchronized (Demo.class){
        System.out.println("Hello World");
    }
}

//同步代码块，对象锁
public void doSth1(){
    synchronized (this){
        System.out.println("Hello World");
    }
}

ObjectMonitor() {
    _header       = NULL;
    _count        = 0;
    _waiters      = 0,
    _recursions   = 0;
    _object       = NULL;
    _owner        = NULL;
    _WaitSet      = NULL;
    _WaitSetLock  = 0 ;
    _Responsible  = NULL ;
    _succ         = NULL ;
    _cxq          = NULL ;
    FreeNext      = NULL ;
    _EntryList    = NULL ;
    _SpinFreq     = 0 ;
    _SpinClock    = 0 ;
    OwnerIsThread = 0 ;
}

void ATTR ObjectMonitor::enter(TRAPS) {
    Thread * const Self = THREAD ;
    void * cur ;
    //通过CAS尝试把monitor的`_owner`字段设置为当前线程
    cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
    //获取锁失败
    if (cur == NULL) {         
        assert (_recursions == 0   , "invariant") ;
        assert (_owner      == Self, "invariant") ;
        // CONSIDER: set or assert OwnerIsThread == 1
        return ;
    }
    // 如果旧值和当前线程一样，说明当前线程已经持有锁，此次为重入，_recursions自增，并获得锁。
    if (cur == Self) { 
        // TODO-FIXME: check for integer overflow!  BUGID 6557169.
        _recursions ++ ;
        return ;
    }

    // 如果当前线程是第一次进入该monitor，设置_recursions为1，_owner为当前线程
    if (Self->is_lock_owned ((address)cur)) { 
        assert (_recursions == 0, "internal state error");
        _recursions = 1 ;
        // Commute owner from a thread-specific on-stack BasicLockObject address to
        // a full-fledged "Thread *".
        _owner = Self ;
        OwnerIsThread = 1 ;
        return ;
    }

    // 省略部分代码。
    // 通过自旋执行ObjectMonitor::EnterI方法等待锁的释放
    for (;;) {
        jt->set_suspend_equivalent();
        // cleared by handle_special_suspend_equivalent_condition()
        // or java_suspend_self()

        EnterI (THREAD) ;

        if (!ExitSuspendEquivalent(jt)) break ;

        //
        // We have acquired the contended monitor, but while we were
        // waiting another thread suspended us. We don't want to enter
        // the monitor while suspended because that would surprise the
        // thread that suspended us.
        //
        _recursions = 0 ;
        _succ = NULL ;
        exit (Self) ;

        jt->java_suspend_self();
    }
}

void ATTR ObjectMonitor::exit(TRAPS) {
   Thread * Self = THREAD ;
   //如果当前线程不是Monitor的所有者
   if (THREAD != _owner) { 
     if (THREAD->is_lock_owned((address) _owner)) { // 
       // Transmute _owner from a BasicLock pointer to a Thread address.
       // We don't need to hold _mutex for this transition.
       // Non-null to Non-null is safe as long as all readers can
       // tolerate either flavor.
       assert (_recursions == 0, "invariant") ;
       _owner = THREAD ;
       _recursions = 0 ;
       OwnerIsThread = 1 ;
     } else {
       // NOTE: we need to handle unbalanced monitor enter/exit
       // in native code by throwing an exception.
       // TODO: Throw an IllegalMonitorStateException ?
       TEVENT (Exit - Throw IMSX) ;
       assert(false, "Non-balanced monitor enter/exit!");
       if (false) {
          THROW(vmSymbols::java_lang_IllegalMonitorStateException());
       }
       return;
     }
   }
    // 如果_recursions次数不为0.自减
   if (_recursions != 0) {
     _recursions--;        // this is simple recursive enter
     TEVENT (Inflated exit - recursive) ;
     return ;
   }

   //省略部分代码，根据不同的策略（由QMode指定），从cxq或EntryList中获取头节点，通过ObjectMonitor::ExitEpilog方法唤醒该节点封装的线程，唤醒操作最终由unpark完成。