AQS是java.conccurent包下諸多工具類的抽象基類，借以AQS抽象基類，實(shí)現(xiàn)如lock,unlock,condition.wait,condition.signal等重要功能。

其中ReentrantLock(可重入鎖)就是借以AQS實(shí)現(xiàn)加鎖，釋放鎖，線程阻塞，線程喚醒等功能。

對(duì)于獨(dú)占式(Exclusive)

lock()->acquire()--->tryAcquire()[模板方法]
                |---->acquireQueued(addWaiter())
unlock()->release()--->tryRelease()[模板方法]

對(duì)于共享式(Shard)
lock()->acquireShard()->tryAcquireShard()[模板方法]

unlock()->releaseShard()->tryReleaseShard()[模板方法]

在獨(dú)占式模式下，在線程A獲取鎖后，線程B也來(lái)?yè)屨兼i，但是此時(shí)線程A占有著鎖，線程B就會(huì)加入AQS的雙向鏈表中，在條件滿足的情況下（前驅(qū)節(jié)點(diǎn)有waitStatus < 0存在）就會(huì)進(jìn)行阻塞。

LockSupport.park()

AbstractQueuedSynchronizer中的shouldParkAfterFailedAcquire(..)方法判斷是否滿足阻塞條件

    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

線程調(diào)用LockSupport.park()阻塞線程

    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker); // 記錄線程對(duì)象
        UNSAFE.park(false, 0L);
        setBlocker(t, null); // 記錄線程對(duì)象
    }

調(diào)用UNSAFE.park(false, 0L);查看此方法已經(jīng)變成了本地方法

public native void park(boolean var1, long var2);

找到/hotspot/src/share/vm/prims/unsafe.cpp

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);
  // 調(diào)用thread對(duì)象下的parker下的park()方法
  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

找到\hotspot\src\share\vm\runtime\thread.cpp，\hotspot\src\share\vm\runtime\thread.hpp
通過(guò)thread對(duì)象下的parker函數(shù)返回parker對(duì)象

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

\hotspot\src\share\vm\runtime\park.hpp，對(duì)應(yīng)的Parker類

class Parker : public os::PlatformParker {
private:
  volatile int _counter ;   //計(jì)數(shù)
  Parker * FreeNext ;      //指向下一個(gè)Parker
  JavaThread * AssociatedWith ; // 指向parker所屬的線程。
 
public:
  Parker() : PlatformParker() {
    _counter       = 0 ;    //初始化為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 ;
 
};

最終調(diào)用到Parker類的park函數(shù)，Parker繼承了PlatformParker。

class PlatformParker : public CHeapObj<mtInternal> {
  protected:
    enum {
        REL_INDEX = 0,
        ABS_INDEX = 1
    };
    int _cur_index;  // 條件變量數(shù)組下標(biāo)，which cond is in use: -1, 0, 1
    pthread_mutex_t _mutex [1] ;  //pthread互斥鎖
    pthread_cond_t  _cond  [2] ; // pthread條件變量數(shù)組,一個(gè)用于相對(duì)時(shí)間，一個(gè)用于絕對(duì)時(shí)間。
 
  public:       // TODO-FIXME: make dtor private
    ~PlatformParker() { guarantee (0, "invariant") ; }
 
  public:
    PlatformParker() {
      int status;
      status = pthread_cond_init (&_cond[REL_INDEX], os::Linux::condAttr());
      assert_status(status == 0, status, "cond_init rel");
      status = pthread_cond_init (&_cond[ABS_INDEX], NULL);
      assert_status(status == 0, status, "cond_init abs");
      status = pthread_mutex_init (_mutex, NULL);
      assert_status(status == 0, status, "mutex_init");
      _cur_index = -1; // mark as unused
    }
};

PlatformParker主要看三個(gè)成員變量，_cur_index, _mutex, _cond。其中mutex和cond就是很熟悉的glibc nptl包中符合posix標(biāo)準(zhǔn)的線程同步工具，一個(gè)互斥鎖一個(gè)條件變量。再看thread和Parker的關(guān)系，在hotspot的Thread類的NameThread內(nèi)部類中有一個(gè) Parker成員變量。說(shuō)明parker是線程變量，在創(chuàng)建線程的時(shí)候就會(huì)生成一個(gè)parker實(shí)例。

以\hotspot\src\os\linux\vm\os_linux.cpp為例，再看park()實(shí)現(xiàn)

void Parker::park(bool isAbsolute, jlong time) {
  
  //原子交換，如果_counter > 0,則將_counter置為0，直接返回，否則_counter為0
  if (Atomic::xchg(0, &_counter) > 0) return;
  //獲取當(dāng)前線程
  Thread* thread = Thread::current();
  assert(thread->is_Java_thread(), "Must be JavaThread");
  //下轉(zhuǎn)型為java線程
  JavaThread *jt = (JavaThread *)thread;
  //如果當(dāng)前線程設(shè)置了中斷標(biāo)志，調(diào)用park則直接返回，所以如果在park之前調(diào)用了
  //interrupt就會(huì)直接返回
  if (Thread::is_interrupted(thread, false)) {
    return;
  }
 
  // 高精度絕對(duì)時(shí)間變量
  timespec absTime;
  //如果time小于0，或者isAbsolute是true并且time等于0則直接返回
  if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
    return;
  }
  //如果time大于0，則根據(jù)是否是高精度定時(shí)計(jì)算定時(shí)時(shí)間
  if (time > 0) {
    unpackTime(&absTime, isAbsolute, time);
  }
  //進(jìn)入安全點(diǎn)避免死鎖
// 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);
  //如果當(dāng)前線程設(shè)置了中斷標(biāo)志，或者獲取mutex互斥鎖失敗則直接返回
  //由于Parker是每個(gè)線程都有的，所以_counter cond mutex都是每個(gè)線程都有的，
  //不是所有線程共享的所以加鎖失敗只有兩種情況，第一unpark已經(jīng)加鎖這時(shí)只需要返回即可，
  //第二調(diào)用pthread_mutex_trylock出錯(cuò)。對(duì)于第一種情況就類似是unpark先調(diào)用的情況，所以
  //直接返回。
  if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
    return;
  }
 
  int status ;
  //如果_counter大于0，說(shuō)明unpark已經(jīng)調(diào)用完成了將_counter置為了1，
  //現(xiàn)在只需將_counter置0，解鎖，返回
  if (_counter > 0)  { // no wait needed
    _counter = 0;
    status = pthread_mutex_unlock(_mutex);
    assert (status == 0, "invariant");
    OrderAccess::fence();
    return;
  }
  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");
  //如果time等于0，說(shuō)明是相對(duì)時(shí)間也就是isAbsolute是fasle(否則前面就直接返回了),則直接掛起
  if (time == 0) {
    _cur_index = REL_INDEX; // arbitrary choice when not timed
    status = pthread_cond_wait (&_cond[_cur_index], _mutex) ;
  } else { //如果time非0
    //判斷isAbsolute是false還是true，false的話使用_cond[0]，否則用_cond[1]
    _cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
    //使用條件變量使得當(dāng)前線程掛起。
    status = os::Linux::safe_cond_timedwait (&_cond[_cur_index], _mutex, &absTime) ;
    //如果掛起失敗則銷毀當(dāng)前的條件變量重新初始化。
    if (status != 0 && WorkAroundNPTLTimedWaitHang) {
      pthread_cond_destroy (&_cond[_cur_index]) ;
      pthread_cond_init    (&_cond[_cur_index], isAbsolute ? NULL : os::Linux::condAttr());
    }
  }
 
  //如果pthread_cond_wait成功則以下代碼都是線程被喚醒后執(zhí)行的。
  _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
  //將_counter變量重新置為1
  _counter = 0 ;
  //解鎖
  status = pthread_mutex_unlock(_mutex) ;
  assert_status(status == 0, status, "invariant") ;
  // 使用內(nèi)存屏障使_counter對(duì)其它線程可見
  OrderAccess::fence();
 
  // 如果在park線程掛起的時(shí)候調(diào)用了stop或者suspend則還需要將線程掛起不能返回
  if (jt->handle_special_suspend_equivalent_condition()) {
    jt->java_suspend_self();
  }
}

其中：
/**加鎖**/
pthread\nptl\pthread_mutex_trylock.c
(..)，該函數(shù)表示，用來(lái)鎖住mutex 所指定的互斥體，但不阻塞。如果該互斥體已經(jīng)被上鎖，該調(diào)用不會(huì)阻塞等待，而會(huì)返回一個(gè)錯(cuò)誤代碼。

/**休眠**/
pthread_cond_wait(..)
/**解鎖**/
pthread_mutex_unlock(..)