LockSupport

是 jsr 166 中新增的 juc 工具类。 LockSupport 类主要用于创建锁和其他同步类来实现线程阻塞。这个类与他使用的每个线程进行关联, 如果可用就立即 park , 我们可以通过 unpack 方法进行唤醒。

park 方法分析

除非许可证可用，否则出于线程调度目的禁用当前线程。如果许可证可用，则该许可证被消耗，呼叫立即返回；否则，出于线程调度目的，当前线程将被禁用，并处于休眠状态，直到发生以下三种情况之一：

其他线程以当前线程为目标调用 unpark；
或其他线程中断当前线程；
或呼叫错误地（即，没有原因地）返回。

此方法不报告导致方法返回的原因。调用方应首先重新检查导致线程停止的条件。调用方还可以在返回时确定线程的中断状态。

public static void park(Object blocker) {
    Thread t = Thread.currentThread();
    setBlocker(t, blocker);
    UNSAFE.park(false, 0L);
    setBlocker(t, null);
}

unpark 方法分析

使给定线程的许可证可用（如果尚未可用）。如果线程在 park上被阻塞，那么它将解除阻塞。否则，它对 park 的下一次呼叫保证不会被阻塞。如果给定的线程尚未启动，则不能保证此操作有任何效果。

public static void unpark(Thread thread) {
    if (thread != null)
        UNSAFE.unpark(thread);
}

JVM 源码分析

park 方法源码分析

通过上面的代码我们可以看到，在 LockSupport 的底层主要是调用 Unsafa 类的 park, unpark 方法实现(源码文件：unsafe.cpp 链接)：

UNSAFE_ENTRY(void, Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute, jlong time))
  UnsafeWrapper("Unsafe_Park");
  EventThreadPark event;
#ifndef USDT2
  HS_DTRACE_PROBE3(hotspot, thread__park__begin, thread->parker(), (int) isAbsolute, time);
#else /* USDT2 */
   HOTSPOT_THREAD_PARK_BEGIN(
                             (uintptr_t) thread->parker(), (int) isAbsolute, time);
#endif /* USDT2 */
  JavaThreadParkedState jtps(thread, time != 0);
  thread->parker()->park(isAbsolute != 0, time);
#ifndef USDT2
  HS_DTRACE_PROBE1(hotspot, thread__park__end, thread->parker());
#else /* USDT2 */
  HOTSPOT_THREAD_PARK_END(
                          (uintptr_t) thread->parker());
#endif /* USDT2 */
  if (event.should_commit()) {
    oop obj = thread->current_park_blocker();
    event.set_klass((obj != NULL) ? obj->klass() : NULL);
    event.set_timeout(time);
    event.set_address((obj != NULL) ? (TYPE_ADDRESS) cast_from_oop<uintptr_t>(obj) : 0);
    event.commit();
  }
UNSAFE_END

UNSAFE_ENTRY(void, Unsafe_Unpark(JNIEnv *env, jobject unsafe, jobject jthread))
  UnsafeWrapper("Unsafe_Unpark");
  Parker* p = NULL;
  if (jthread != NULL) {
    oop java_thread = JNIHandles::resolve_non_null(jthread);
    if (java_thread != NULL) {
      jlong lp = java_lang_Thread::park_event(java_thread);
      if (lp != 0) {
        // This cast is OK even though the jlong might have been read
        // non-atomically on 32bit systems, since there, one word will
        // always be zero anyway and the value set is always the same
        p = (Parker*)addr_from_java(lp);
      } else {
        // Grab lock if apparently null or using older version of library
        MutexLocker mu(Threads_lock);
        java_thread = JNIHandles::resolve_non_null(jthread);
        if (java_thread != NULL) {
          JavaThread* thr = java_lang_Thread::thread(java_thread);
          if (thr != NULL) {
            p = thr->parker();
            if (p != NULL) { // Bind to Java thread for next time.
              java_lang_Thread::set_park_event(java_thread, addr_to_java(p));
            }
          }
        }
      }
    }
  }
  if (p != NULL) {
#ifndef USDT2
    HS_DTRACE_PROBE1(hotspot, thread__unpark, p);
#else /* USDT2 */
    HOTSPOT_THREAD_UNPARK(
                          (uintptr_t) p);
#endif /* USDT2 */
    p->unpark();
  }
UNSAFE_END

每个线程对象都有一个 Parker 实例（源码文件：thread.hpp）

// JSR166 per-thread parker
private:
  Parker*    _parker;
public:
  Parker*     parker() { return _parker; }

parker 类的定义如下，我们可以看到（源码文件 park .hpp）。

Parker 类继承 os::PlatformParker。应该是一个针对不同操作系统适配的
有一个 _counter 属性，可以理解为是否可以调用 park 方法的许可证，只有 _count > 0 的时候才能调用；
提供了公开的 park 和 unpark 方法

class Parker : public os::PlatformParker {
private:
  volatile int _counter ;
  Parker * FreeNext ;
  JavaThread * AssociatedWith ; // Current association

public:
  Parker() : PlatformParker() {
    _counter       = 0 ;
    FreeNext       = NULL ;
    AssociatedWith = NULL ;
  }
protected:
  ~Parker() { ShouldNotReachHere(); }
public:
  // For simplicity of interface with Java, all forms of park (indefinite,
  // relative, and absolute) are multiplexed into one call.
  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 ;

};

前面我提到 Parker 的父类是 PlatformParker (源码文件 os_linux.cpp)。我们可以看下他在 Linunx 下 Parker 的 park 方法的实现过程：

判断是否需要阻塞等待，如果已经是 _counter >0, 不需要等待，将 _counter = 0 ，返回
如果 1 不成立，构造当前线程的 ThreadBlockInVM ，检查 _counter > 0 是否成立，成立则将 _counter 设置为 0， unlock mutex 返回；
如果 2 不成立，更具需要时间进行不同的函数等待，如果等待正确返回，则将 _counter 设置为0， unlock mutex ， park 调用成功。

void Parker::park(bool isAbsolute, jlong time) {
  // Ideally we'd do something useful while spinning, such
  // as calling unpackTime().

  // Optional fast-path check:
  // Return immediately if a permit is available.
  // We depend on Atomic::xchg() having full barrier semantics
  // since we are doing a lock-free update to _counter.
  if (Atomic::xchg(0, &_counter) > 0) return; // _counter > 0, 使用 xchg 指令修改为 0 返回

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

  // Optional optimization -- avoid state transitions if there's an interrupt pending.
  // Check interrupt before trying to wait
  if (Thread::is_interrupted(thread, false)) { // 如果线程处于中断状态，直接返回
    return;
  }

  // Next, demultiplex/decode time arguments
  timespec absTime;
  if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
    return;
  }
  if (time > 0) {
    unpackTime(&absTime, isAbsolute, time);
  }


  // Enter safepoint region
  // Beware of deadlocks such as 6317397.
  // The per-thread Parker:: mutex is a classic leaf-lock.
  // In particular a thread must never block on the Threads_lock while
  // holding the Parker:: mutex.  If safepoints are pending both the
  // the ThreadBlockInVM() CTOR and DTOR may grab Threads_lock.
  ThreadBlockInVM tbivm(jt); // 构造当前线程的 ThreadBlockInVM, 为了防止死锁等特殊场景

  // Don't wait if cannot get lock since interference arises from
  // unblocking.  Also. check interrupt before trying wait
  if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
    return;
  }

  int status ;
  if (_counter > 0)  { // no wait needed
    _counter = 0; // 重置
    status = pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
    // Paranoia to ensure our locked and lock-free paths interact
    // correctly with each other and Java-level accesses.
    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.)
  sigset_t oldsigs;
  sigset_t* allowdebug_blocked = os::Linux::allowdebug_blocked_signals();
  pthread_sigmask(SIG_BLOCK, allowdebug_blocked, &oldsigs);
#endif

  OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);
  jt->set_suspend_equivalent();
  // cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()

  assert(_cur_index == -1, "invariant");
  if (time == 0) {
    _cur_index = REL_INDEX; // arbitrary choice when not timed
    status = pthread_cond_wait (&_cond[_cur_index], _mutex) ;
  } else {
    _cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
    // 调用 safe_cond_timedwait 进入线程阻塞
    status = os::Linux::safe_cond_timedwait (&_cond[_cur_index], _mutex, &absTime) ;
    if (status != 0 && WorkAroundNPTLTimedWaitHang) {
      pthread_cond_destroy (&_cond[_cur_index]) ;
      pthread_cond_init    (&_cond[_cur_index], isAbsolute ? NULL : os::Linux::condAttr());
    }
  }
  _cur_index = -1;
  assert_status(status == 0 || status == EINTR ||
                status == ETIME || status == ETIMEDOUT,
                status, "cond_timedwait");

#ifdef ASSERT
  pthread_sigmask(SIG_SETMASK, &oldsigs, NULL);
#endif

  _counter = 0 ; // 返回后 _counter 状态位重置
  status = pthread_mutex_unlock(_mutex) ;
  assert_status(status == 0, status, "invariant") ;
  // Paranoia to ensure our locked and lock-free paths interact
  // correctly with each other and Java-level accesses.
  OrderAccess::fence();

  // If externally suspended while waiting, re-suspend
  if (jt->handle_special_suspend_equivalent_condition()) {
    jt->java_suspend_self();
  }
}

unpark 方法源码分析

我们再来看看 unpark (源码文件 os_linux.cpp)。主要是西安的流程如下：

pthread_mutex_lock 获取锁
_counter 设置为 1
判断 _counter 的旧值：

小于 1 时，调用 pthread_cond_signal 唤醒在 park 阻塞的线程；
等于 1 时，直接返回

void Parker::unpark() {
  int s, status ;
  status = pthread_mutex_lock(_mutex);
  assert (status == 0, "invariant") ;
  s = _counter;
  _counter = 1;
  if (s < 1) {
    // thread might be parked
    if (_cur_index != -1) {
      // thread is definitely parked
      if (WorkAroundNPTLTimedWaitHang) {
        status = pthread_cond_signal (&_cond[_cur_index]);
        assert (status == 0, "invariant");
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant");
      } else {
        status = pthread_mutex_unlock(_mutex);
        assert (status == 0, "invariant");
        status = pthread_cond_signal (&_cond[_cur_index]);
        assert (status == 0, "invariant");
      }
    } else {
      pthread_mutex_unlock(_mutex);
      assert (status == 0, "invariant") ;
    }
  } else {
    pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant") ;
  }
}

Parker 和 ParkEvent 的区别

ParkEvent 其实和 Parker 的功能类似，同样也可以提供线程阻塞和唤醒的功能。意思是说ParkEvent是用于java级别的synchronize关键字，Parker是JSR166来的并发工具集合,后面会统一使用ParkEvent。ParkerEvent 继承了PlatformEvent。基类PlatformEvent是特定于平台的，而ParkEvent则是平台无关的。Parker 继承自PlatformParker。

ParkerEvent中的park，unpark方法用于实现Java的object.wait()方法和object.notify()方法;
Parker中的park，unpark方法用于实现Java的Locksupprt.park()方法和Locksupprt.unpark()方法;

参考 park.hpp 的源码注释

// The base-class, PlatformEvent, is platform-specific while the ParkEvent is
// platform-independent.  PlatformEvent provides park(), unpark(), etc., and
// is abstract -- that is, a PlatformEvent should never be instantiated except
// as part of a ParkEvent.
// Equivalently we could have defined a platform-independent base-class that
// exported Allocate(), Release(), etc.  The platform-specific class would extend
// that base-class, adding park(), unpark(), etc.
//
// A word of caution: The JVM uses 2 very similar constructs:
// 1. ParkEvent are used for Java-level "monitor" synchronization.
// 2. Parkers are used by JSR166-JUC park-unpark.
//
// We'll want to eventually merge these redundant facilities and use ParkEvent.

使用案例

下面是一个先进先出的非重入方法的使用。

class FIFOMutex {
    private final AtomicBoolean locked = new AtomicBoolean(false);
    private final Queue<Thread> waiters
        = new ConcurrentLinkedQueue<Thread>();

    public void lock() {
        boolean wasInterrupted = false;
        Thread current = Thread.currentThread();
        waiters.add(current);

        // Block while not first in queue or cannot acquire lock
        while (waiters.peek() != current ||
               !locked.compareAndSet(false, true)) {
            LockSupport.park(this);
            if (Thread.interrupted()) // ignore interrupts while waiting
                wasInterrupted = true;
        }

        waiters.remove();
        if (wasInterrupted)          // reassert interrupt status on exit
            current.interrupt();
    }

    public void unlock() {
        locked.set(false);
        LockSupport.unpark(waiters.peek());
    }
}}

LockSupport 以及 park、unpark 方法源码分析