LockSupport中的park与unpark原理

JavaThread* thread=JavaThread::thread_from_jni_environment(env);
...
thread->parker()->park(isAbsolute != 0, time);
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class Parker : public os::PlatformParker {
private:
   //表示许可
  volatile int _counter ; 
  Parker * FreeNext ;
  JavaThread * AssociatedWith ; // Current association
public:
  Parker() : PlatformParker() {
    //初始化_counter
    _counter       = 0 ; 
    FreeNext       = NULL ;
    AssociatedWith = NULL ;
  }
protected:
  ~Parker() { ShouldNotReachHere(); }
public:
  void park(bool isAbsolute, jlong time);
  void unpark();

  // Lifecycle operators  
  static Parker * Allocate (JavaThread * t) ;
  static void Release (Parker * e) ;
private:
  static Parker * volatile FreeList ;
  static volatile int ListLock ;

};
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class PlatformParker : public CHeapObj {
  protected:
    //互斥变量类型
    pthread_mutex_t _mutex [1] ; 
   //条件变量类型
    pthread_cond_t  _cond  [1] ;

  public:        
     ~PlatformParker() { guarantee (0, "invariant") ; }

  public:
    PlatformParker() {
      int status;
     //初始化条件变量，使用    pthread_cond_t之前必须先执行初始化
      status = pthread_cond_init (_cond, NULL);
      assert_status(status == 0, status, "cond_init”);
      // 初始化互斥变量，使用    pthread_mutex_t之前必须先执行初始化
      status = pthread_mutex_init (_mutex, NULL);
      assert_status(status == 0, status, "mutex_init");
    }
} 
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void Parker::park(bool isAbsolute, jlong time) {
  if (_counter > 0) {
       //已经有许可了，用掉当前许可
      _counter = 0 ;
     //使用内存屏障，确保 _counter赋值为0(写入操作)能够被内存屏障之后的读操作获取内存屏障事前的结果，也就是能够正确的读到0
      OrderAccess::fence();
     //立即返回
      return ;
  }

  Thread* thread = Thread::current();
  assert(thread->is_Java_thread(), "Must be JavaThread");
  JavaThread *jt = (JavaThread *)thread;

 if (Thread::is_interrupted(thread, false)) {
 // 线程执行了中断，返回
    return;
  }

  if (time < 0 || (isAbsolute && time == 0) ) { 
    //时间到了，或者是代表绝对时间，同时绝对时间是0（此时也是时间到了），直接返回，java中的parkUtil传的就是绝对时间，其它都不是
   return;
  }
  if (time > 0) {
  //传入了时间参数，将其存入absTime，并解析成absTime->tv_sec(秒)和absTime->tv_nsec(纳秒)存储起来，存的是绝对时间
    unpackTime(&absTime, isAbsolute, time);
  }

 //进入safepoint region，更改线程为阻塞状态
  ThreadBlockInVM tbivm(jt);

 if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
  //如果线程被中断，或者是在尝试给互斥变量加锁的过程中，加锁失败，比如被其它线程锁住了，直接返回
    return;
  }
//这里表示线程互斥变量锁成功了
  int status ;
  if (_counter > 0)  {
    // 有许可了，返回
    _counter = 0;
    //对互斥变量解锁
    status = pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
    OrderAccess::fence();
    return;
  }

#ifdef ASSERT
  // Don't catch signals while blocked; let the running threads have the signals.  
// (This allows a debugger to break into the running thread.)  
 //debug用
sigset_t oldsigs;
  sigset_t* allowdebug_blocked = os::Linux::allowdebug_blocked_signals();
  pthread_sigmask(SIG_BLOCK, allowdebug_blocked, &oldsigs);
#endif
//将java线程所拥有的操作系统线程设置成 CONDVAR_WAIT状态 ，表示在等待某个条件的发生
OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);
//将java的_suspend_equivalent参数设置为true
  jt->set_suspend_equivalent();
  // cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()
  if (time == 0) {
    //把调用线程放到等待条件的线程列表上，然后对互斥变量解锁，（这两是原子操作），这个时候线程进入等待，当它返回时，互斥变量再次被锁住。
  //成功返回0，否则返回错误编号
    status = pthread_cond_wait (_cond, _mutex) ;
  } else {
  //同pthread_cond_wait，只是多了一个超时，如果超时还没有条件出现，那么重新获取胡吃两然后返回错误码 ETIMEDOUT
    status = os::Linux::safe_cond_timedwait (_cond, _mutex, &absTime) ;
    if (status != 0 && WorkAroundNPTLTimedWaitHang) {
   //WorkAroundNPTLTimedWaitHang 是JVM的运行参数，默认为1
  //去除初始化
      pthread_cond_destroy (_cond) ;
//重新初始化
      pthread_cond_init    (_cond, NULL);
    }
  }
  assert_status(status == 0 || status == EINTR ||
                status == ETIME || status == ETIMEDOUT,
                status, "cond_timedwait");

#ifdef ASSERT
  pthread_sigmask(SIG_SETMASK, &oldsigs, NULL);
#endif
 //等待结束后，许可被消耗，改为0  _counter = 0 ;
//释放互斥量的锁
  status = pthread_mutex_unlock(_mutex) ;
  assert_status(status == 0, status, "invariant") ;
  // If externally suspended while waiting, re-suspend 
    if (jt->handle_special_suspend_equivalent_condition()) {
    jt->java_suspend_self();
  }
//加入内存屏障指令
  OrderAccess::fence();
}

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inline void OrderAccess::fence() {
  if (os::is_MP()) {
#ifdef AMD64
  // 没有使用mfence,因为mfence有时候性能差于使用 locked addl
    __asm__ volatile ("lock; addl $0,0(%%rsp)" : : : "cc", "memory");
#else    __asm__ volatile ("lock; addl $0,0(%%esp)" : : : "cc", "memory");
#endif  }
}
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class ThreadBlockInVM : public ThreadStateTransition {
 public:
  ThreadBlockInVM(JavaThread *thread)
  : ThreadStateTransition(thread) {
    // Once we are blocked vm expects stack to be walkable    
    thread->frame_anchor()->make_walkable(thread);
   //把线程由运行状态转成阻塞状态
    trans_and_fence(_thread_in_vm, _thread_blocked);
  }
  ...
};
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//这个枚举是用来追踪线程在代码的那一块执行，用来给 safepoint code使用，有4种重要的类型，_thread_new/_thread_in_native/_thread_in_vm/_thread_in_Java。形如xxx_trans的状态都是中间状态，表示线程正在由一种状态变成另一种状态，这种方式使得 safepoint code在处理线程状态时，不需要对线程进行挂起，使得safe point code运行更快，而给定一个状态，通过+1就可以得到他的转换状态
enum JavaThreadState {
  _thread_uninitialized     =  0, // should never happen (missing initialization) 
_thread_new               =  2, // just starting up, i.e., in process of being initialized 
_thread_new_trans         =  3, // corresponding transition state (not used, included for completeness)  
_thread_in_native         =  4, // running in native code  . This is a safepoint region, since all oops will be in jobject handles
_thread_in_native_trans   =  5, // corresponding transition state  
_thread_in_vm             =  6, // running in VM 
_thread_in_vm_trans       =  7, // corresponding transition state 
_thread_in_Java           =  8, //  Executing either interpreted or compiled Java code running in Java or in stub code  
_thread_in_Java_trans     =  9, // corresponding transition state (not used, included for completeness) 
_thread_blocked           = 10, // blocked in vm 
_thread_blocked_trans     = 11, // corresponding transition state 
_thread_max_state         = 12  // maximum thread state+1 - used for statistics allocation
};
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void trans_and_fence(JavaThreadState from, JavaThreadState to) { transition_and_fence(_thread, from, to);} //_thread即构造函数传进来de thread
// transition_and_fence must be used on any thread state transition
// where there might not be a Java call stub on the stack, in
// particular on Windows where the Structured Exception Handler is
// set up in the call stub. os::write_memory_serialize_page() can
// fault and we can't recover from it on Windows without a SEH in
// place.
//transition_and_fence方法必须在任何线程状态转换的时候使用
static inline void transition_and_fence(JavaThread *thread, JavaThreadState from, JavaThreadState to) {
  assert(thread->thread_state() == from, "coming from wrong thread state");
  assert((from & 1) == 0 && (to & 1) == 0, "odd numbers are transitions states");
//标识线程转换中
    thread->set_thread_state((JavaThreadState)(from + 1));

  // 设置内存屏障，确保新的状态能够被VM 线程看到
if (os::is_MP()) {
    if (UseMembar) {
      // Force a fence between the write above and read below     
        OrderAccess::fence();
    } else {
      // Must use this rather than serialization page in particular on Windows      
        InterfaceSupport::serialize_memory(thread);
    }
  }

  if (SafepointSynchronize::do_call_back()) {
    SafepointSynchronize::block(thread);
  }
//线程状态转换成最终的状态,对待这里的场景就是阻塞
  thread->set_thread_state(to);

  CHECK_UNHANDLED_OOPS_ONLY(thread->clear_unhandled_oops();)
}
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enum ThreadState {
 ALLOCATED,                    // Memory has been allocated but not initialized  
INITIALIZED,                  // The thread has been initialized but yet started 
RUNNABLE,                     // Has been started and is runnable, but not necessarily running  
MONITOR_WAIT,                 // Waiting on a contended monitor lock  
CONDVAR_WAIT,                 // Waiting on a condition variable  
OBJECT_WAIT,                  // Waiting on an Object.wait() call  
BREAKPOINTED,                 // Suspended at breakpoint  
SLEEPING,                     // Thread.sleep()  
ZOMBIE                        // All done, but not reclaimed yet
};
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void Parker::unpark() {
  int s, status ;
 //给互斥量加锁，如果互斥量已经上锁，则阻塞到互斥量被解锁
//park进入wait时，_mutex会被释放
  status = pthread_mutex_lock(_mutex);
  assert (status == 0, "invariant") ; 
  //存储旧的_counter
  s = _counter; 
//许可改为1，每次调用都设置成发放许可
  _counter = 1;
  if (s < 1) {
     //之前没有许可
     if (WorkAroundNPTLTimedWaitHang) {
      //默认执行 ,释放信号，表明条件已经满足，将唤醒等待的线程
        status = pthread_cond_signal (_cond) ;
        assert (status == 0, "invariant") ;
        //释放锁
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant") ;
     } else {
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant") ;
        status = pthread_cond_signal (_cond) ;
        assert (status == 0, "invariant") ;
     }
  } else {
   //一直有许可，释放掉自己加的锁,有许可park本身就返回了
    pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
  }
}
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