多线程Java数组元素在多个线程之间的可见性

Question

创建数组：

Uses the thread provided on startup since we only create the array once.

public static final Player[] PLAYERS = new Player[10];

写入数组：

Uses several different threads from a pool so that players can load concurrently.

public static Player loadPlayer(int freeIndex) {
    //slow code that loads the player's saved data.
    return PLAYERS[freeIndex] = player;
}

从数组读取：

Uses a single thread to process players in order of their index.

public static void process() {
    for(Player player : PLAYERS)
        player.process();
}

问题：在方法loadPlayer执行后，玩家是否会立即在方法过程中可见？如果不是（我怀疑是这样），我应该采取什么方法来解决问题

Answer 1

不，如果没有同步，在一个线程中设置的值可能无法用于另一个线程。您可以查看here以了解有关内存屏障的更多详细信息

在这种情况下，可以使用^{}进行同步

• 在初始化中，创建一个CountDownLatch：

  public static final Player[] PLAYERS = new Player[10];
  public static CountDownLatch lacth = new CountDownLatch(10);
  

• 加载新的Player后，调用countDown

  public static Player loadPlayer(int freeIndex) {
      //slow code that loads the player's saved data.
      PLAYERS[freeIndex] = player;
      latch.countDown();
      return player;
  }
  

• 在进程线程中，只需等待。在加载所有Player之后 线程将由最后一个countDown线程调用：

  public static void process() {
      latch.await();
      for(Player player : PLAYERS)
          player.process();
  }
  

Answer 2

你说得对，玩家可能不会立即出现

可以使用^{}而不是标准数组，以确保对players的更改对其他线程可见

从documentation

The memory effects for accesses and updates of atomics generally follow the rules for volatiles, as stated in The Java Language Specification (17.4 Memory Model):

get has the memory effects of reading a volatile variable.

set has the memory effects of writing (assigning) a volatile variable.

lazySet has the memory effects of writing (assigning) a volatile variable except that it permits reorderings with subsequent (but not previous) memory actions that do not themselves impose reordering constraints with ordinary non-volatile writes. Among other usage contexts, lazySet may apply when nulling out, for the sake of garbage collection, a reference that is never accessed again.

weakCompareAndSet atomically reads and conditionally writes a variable but does not create any happens-before orderings, so provides no guarantees with respect to previous or subsequent reads and writes of any variables other than the target of the weakCompareAndSet. compareAndSet and all other read-and-update operations such as getAndIncrement have the memory effects of both reading and writing volatile variables.

Answer 3

当您使用不同的线程加载播放器时，请修改共享集合。当并发程序编写不正确时，错误往往会分为三类：原子性、可见性或顺序

为了使代码线程安全，需要使用lock或其他非阻塞集合，如AtomicReferenceArray，以确保线程可见性和线程安全。另外，volatile变量不能确保原子性，但变量将确保可见性

原子性涉及哪些动作和动作集具有不可分割的效果。这是程序员最熟悉的并发方面：它通常被认为是互斥的。可见性决定了一个线程的效果何时可以被另一个线程看到。排序决定了一个线程中的操作何时会相对于另一个线程出现无序