java管程sychronized

通过sychronized给对象加锁，每个对象在实例化后，会同时实例出一个ObjectMoniter，同时对象有对象头（markOop
），在对象头中会存有当前锁状态信息，包含线程id。
ObjectMoniter
objectMonitor.hpp

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 ;
    _previous_owner_tid = 0;
  }

_count:当有对象获取锁，count+1，对象释放锁count-1
_EntryList:当有对象尝试获取锁，加入entryList中
_WaitSet:当调用wait方法，进入等待队列
_owner:拥有锁当线程，当锁释放是，count-1同时owner置为NULL
当使用sychronized给对象加锁时，字节码中是monitorenter给指定对象加锁，给静态方法加锁时，使用ACC_SYNCHRONIZED标志给当前对象加锁。

public class SychronizedTest {

    private final Object lock = new Object();

    private volatile int count;

    public synchronized void method1() {
        count++;
    }

    public void method2() {
        synchronized (lock) {
            count++;
        }
    }
}

bytecode

public synchronized void method1();
    descriptor: ()V
    flags: ACC_PUBLIC, ACC_SYNCHRONIZED
    Code:
      stack=3, locals=1, args_size=1
         0: aload_0
         1: dup
         2: getfield      #4                  // Field count:I
         5: iconst_1
         6: iadd
         7: putfield      #4                  // Field count:I
        10: return
      LineNumberTable:
        line 14: 0
        line 15: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lcom/chao/SychronizedTest;

public void method2();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=3, args_size=1
         0: aload_0
         1: getfield      #3                  // Field lock:Ljava/lang/Object;
         4: dup
         5: astore_1
         6: monitorenter
         7: aload_0
         8: dup
         9: getfield      #4                  // Field count:I
        12: iconst_1
        13: iadd
        14: putfield      #4                  // Field count:I
        17: aload_1
        18: monitorexit
        19: goto          27
        22: astore_2
        23: aload_1
        24: monitorexit
        25: aload_2
        26: athrow
        27: return
      Exception table:
         from    to  target type
             7    19    22   any
            22    25    22   any
      LineNumberTable:
........