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java juc

· 5 min read

背景​

了解java容器类

juc主要类​

Collection 接口:

类描述
List列表
Queue队列
Set集合

Map接口:

类描述线程安全是否可以由null值
HashMaphanshmap否是
PriorityQueue优先队列,implement Queue否否,因为要排序,null会抛异常
HashSet一个set的实现否是 This class permits the null element.

aqs​

aqs 主要是提供了三个property:

  • state: int 表示资源 ,维护一个volatile 的int
  • queue: 来block队列,一个普通队列,用来塞线程Thread这个变量
  • block: 获取不到资源就阻塞 , 使用park和unpark, jni实现

block​

park 和unpark 使用的是pthread_cond_wait 和pthread_cond_notify

condition variable vs semaphore​

信号量semaphore ：

  • 描述的是state 计数器 state 描述的是资源数
  • state>0 表示还有资源
  • state<= 0 表示没有资源

条件变量condition variable:

  • 更多是偏向描述生产者和消费者的等待

条件变量： 为了解决等待 信号量： 为了解决线程/进程同步（线程/进程通信）

区别：

条件变量： queue + block 信号量： state+block

park 源码分析​

// Parker::park decrements count if > 0, else does a condvar wait.  Unpark
// sets count to 1 and signals condvar.  Only one thread ever waits
// on the condvar. Contention seen when trying to park implies that someone
// is unparking you, so don't wait. And spurious returns are fine, so there
// is no need to track notifications.

void Parker::park(bool isAbsolute, jlong time) {

  // 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(&_counter, 0) > 0) return;

  JavaThread *jt = JavaThread::current();

  // Optional optimization -- avoid state transitions if there's
  // an interrupt pending.
  if (jt->is_interrupted(false)) {
    return;
  }

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

  // 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);

  // Can't access interrupt state now that we are _thread_blocked. If we've
  // been interrupted since we checked above then _counter will be > 0.

  // Don't wait if cannot get lock since interference arises from
  // unparking.
  if (pthread_mutex_trylock(_mutex) != 0) {
    return;
  }

  int status;
  if (_counter > 0)  { // no wait needed
    _counter = 0;
    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();
    return;
  }

  OSThreadWaitState osts(jt->osthread(), false /* not Object.wait() */);

  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);
    assert_status(status == 0 MACOS_ONLY(|| status == ETIMEDOUT),
                  status, "cond_wait");
  }
  else {
    _cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
    status = pthread_cond_timedwait(&_cond[_cur_index], _mutex, &absTime);
    assert_status(status == 0 || status == ETIMEDOUT,
                  status, "cond_timedwait");
  }
  _cur_index = -1;

  _counter = 0;
  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();
}

aqs 内容​

aqs 主要包括三个内容:

  • state
  • queue
  • park/unpark

state 描述了资源的数量,一般是用这个来判断是否应该阻塞, 一般是state=0 就要被唤醒或者被注释
queue 为了唤醒的时候可以找到下一个需要被唤醒的元素 park/unpark LockSupport 的这两个函数,主要是实现了线程的阻塞和唤醒,在linux下是封装了pthread_cond_wait 和pthread_cond_signal.除此之外,为了避免丢失通知,多了一个_counter变量,比普通的pthread_cond_wait和pthread_cond_signal好一点

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