我有一个类,我每隔30秒从一个后台线程填充一个mapSocketByDatacenter,然后我有一个方法getNextSocket,它将被多个读者线程调用以获得一个可用的实时套接字,它使用相同的地图来获取此信息.

public class SocketManager {
  private static final Random random = new Random();
  private final ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
  private final Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter = new HashMap<>();
  private final ZContext ctx = new ZContext();

  // Lazy Loaded Singleton Pattern
  private static class Holder {
    private static final SocketManager instance = new SocketManager();
  }

  public static SocketManager getInstance() {
    return Holder.instance;
  }

  private SocketManager() {
    connectToZMQSockets();
    scheduler.scheduleAtFixedRate(new Runnable() {
      public void run() {
        updateLiveSockets();
      }
    }, 30, 30, TimeUnit.SECONDS);
  }

  private void connectToZMQSockets() {
    Map<Datacenters, ImmutableList<String>> socketsByDatacenter = Utils.SERVERS;
    for (Map.Entry<Datacenters, ImmutableList<String>> entry : socketsByDatacenter.entrySet()) {
      List<SocketHolder> addedColoSockets = connect(entry.getKey(), entry.getValue(), ZMQ.PUSH);
      liveSocketsByDatacenter.put(entry.getKey(), addedColoSockets);
    }
  }

  private List<SocketHolder> connect(Datacenters colo, List<String> addresses, int socketType) {
    List<SocketHolder> socketList = new ArrayList<>();
    for (String address : addresses) {
      try {
        Socket client = ctx.createSocket(socketType);
        // Set random identity to make tracing easier
        String identity = String.format("%04X-%04X", random.nextInt(), random.nextInt());
        client.setIdentity(identity.getBytes(ZMQ.CHARSET));
        client.setTCPKeepAlive(1);
        client.setSendTimeOut(7);
        client.setLinger(0);
        client.connect(address);

        SocketHolder zmq = new SocketHolder(client, ctx, address, true);
        socketList.add(zmq);
      } catch (Exception ex) {
        // log error
      }
    }
    return socketList;
  }

  // this method will be called by multiple threads to get the next live socket
  public Optional<SocketHolder> getNextSocket() {
    Optional<SocketHolder> liveSocket = Optional.absent();
    List<Datacenters> dcs = Datacenters.getOrderedDatacenters();
    for (Datacenters dc : dcs) {
      liveSocket = getLiveSocket(liveSocketsByDatacenter.get(dc));
      if (liveSocket.isPresent()) {
        break;
      }
    }
    return liveSocket;
  }

  private Optional<SocketHolder> getLiveSocket(final List<SocketHolder> listOfEndPoints) {
    if (!CollectionUtils.isEmpty(listOfEndPoints)) {
      Collections.shuffle(listOfEndPoints);
      for (SocketHolder obj : listOfEndPoints) {
        if (obj.isLive()) {
          return Optional.of(obj);
        }
      }
    }
    return Optional.absent();
  }

  private void updateLiveSockets() {
    Map<Datacenters, ImmutableList<String>> socketsByDatacenter = Utils.SERVERS;

    for (Entry<Datacenters, ImmutableList<String>> entry : socketsByDatacenter.entrySet()) {
      List<SocketHolder> liveSockets = liveSocketsByDatacenter.get(entry.getKey());
      List<SocketHolder> liveUpdatedSockets = new ArrayList<>();
      for (SocketHolder liveSocket : liveSockets) {
        Socket socket = liveSocket.getSocket();
        String endpoint = liveSocket.getEndpoint();
        Map<byte[], byte[]> holder = populateMap();

        boolean status = SendToSocket.getInstance().execute(3, holder, socket);
        boolean isLive = (status) ? true : false;
        SocketHolder zmq = new SocketHolder(socket, liveSocket.getContext(), endpoint, isLive);
        liveUpdatedSockets.add(zmq);
      }
      liveSocketsByDatacenter.put(entry.getKey(), liveUpdatedSockets);
    }
  }
}

正如你在我上面的课程中看到的那样：

>从每30秒运行一次的后台线程中,我使用所有实时套接字填充liveSocketsByDatacenter映射.
>然后从多个线程,我调用getNextSocket方法给我实时套接字可用,它使用liveSocketsByDatacenter映射来获取所需的信息.

我上面的代码线程是否安全,所有读者线程都会准确地看到liveSocketsByDatacenter？由于我每30秒从一个后台线程修改liveSocketsByDatacenter映射,然后从很多读者线程修改,我调用getNextSocket方法,所以我不确定我是否在这里做错了.

看起来我的“getLiveSocket”方法中可能存在线程安全问题,因为每次读取都会从地图中获取共享的ArrayList并将其洗牌？而且可能还有一些我可能错过的地方.在我的代码中解决这些线程安全问题的最佳方法是什么？

如果有更好的方法来重写这个,那么我也是开放的.

最佳答案 为了线程安全,您的代码必须同步对所有共享可变状态的任何访问.

在这里,你共享liveSocketsByDatacenter,HashMap的一个实例,一个非线程安全的Map实现,可以同时读取(通过updateLiveSockets和getNextSocket)并修改(通过connectToZMQSockets和updateLiveSockets),而无需同步任何已经足以生成代码的访问权限非线程安全.此外,此Map的值是ArrayList的实例,List的非线程安全实现也可以同时读取(通过getNextSocket和updateLiveSockets)并修改(通过Collections.shuffle更精确地通过getLiveSocket).

解决2线程安全问题的简单方法可能是：

>为变量liveSocketsByDatacenter使用ConcurrentHashMap而不是HashMap,因为它是Map的本机线程安全实现.
>使用Collections.unmodifiableList(List<? extends T> list)将您的ArrayList实例的不可修改版本作为地图的值,您的列表将是不可变的,因此线程安全.

例如：

liveSocketsByDatacenter.put(
    entry.getKey(), Collections.unmodifiableList(liveUpdatedSockets)
);`

>重写你的方法getLiveSocket以避免直接在你的列表上调用Collections.shuffle,你可以例如只调整实时套接字列表而不是所有套接字或使用列表的副本(例如新的ArrayList<>(listOfEndPoints)而不是列表本身.

例如：

private Optional<SocketHolder> getLiveSocket(final List<SocketHolder> listOfEndPoints) {
    if (!CollectionUtils.isEmpty(listOfEndPoints)) {
        // The list of live sockets
        List<SocketHolder> liveOnly = new ArrayList<>(listOfEndPoints.size());
        for (SocketHolder obj : listOfEndPoints) {
            if (obj.isLive()) {
                liveOnly.add(obj);
            }
        }
        if (!liveOnly.isEmpty()) {
            // The list is not empty so we shuffle it an return the first element
            Collections.shuffle(liveOnly);
            return Optional.of(liveOnly.get(0));
        }
    }
    return Optional.absent();
}

对于#1,你似乎经常阅读,很少(每30秒一次)修改你的地图,你可以考虑重建你的地图然后每30秒分享一次不可变版本(使用Collections.unmodifiableMap(Map<? extends K,? extends V> m)),这种方法非常有效场景,因为您不再支付任何同步机制的价格来访问您的地图内容.

您的代码将是：

// Your variable is no more final, it is now volatile to ensure that all 
// threads will see the same thing at all time by getting it from
// the main memory instead of the CPU cache
private volatile Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter 
    = Collections.unmodifiableMap(new HashMap<>());

private void connectToZMQSockets() {
    Map<Datacenters, ImmutableList<String>> socketsByDatacenter = Utils.SERVERS;
    // The map in which I put all the live sockets
    Map<Datacenters, List<SocketHolder>> liveSockets = new HashMap<>();
    for (Map.Entry<Datacenters, ImmutableList<String>> entry : 
        socketsByDatacenter.entrySet()) {

        List<SocketHolder> addedColoSockets = connect(
            entry.getKey(), entry.getValue(), ZMQ.PUSH
        );
        liveSockets.put(entry.getKey(), Collections.unmodifiableList(addedColoSockets));
    }
    // Set the new content of my map as an unmodifiable map
    this.liveSocketsByDatacenter = Collections.unmodifiableMap(liveSockets);
}

public Optional<SocketHolder> getNextSocket() {
    // For the sake of consistency make sure to use the same map instance
    // in the whole implementation of my method by getting my entries
    // from the local variable instead of the member variable
    Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter = 
        this.liveSocketsByDatacenter;
    ...
}
...
// Added the modifier synchronized to prevent concurrent modification
// it is needed because to build the new map we first need to get the
// old one so both must be done atomically to prevent concistency issues
private synchronized void updateLiveSockets() {
    // Initialize my new map with the current map content
    Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter = 
        new HashMap<>(this.liveSocketsByDatacenter);
    Map<Datacenters, ImmutableList<String>> socketsByDatacenter = Utils.SERVERS;
    // The map in which I put all the live sockets
    Map<Datacenters, List<SocketHolder>> liveSockets = new HashMap<>();
    for (Entry<Datacenters, ImmutableList<String>> entry : socketsByDatacenter.entrySet()) {
        ...
        liveSockets.put(entry.getKey(), Collections.unmodifiableList(liveUpdatedSockets));
    }
    // Set the new content of my map as an unmodifiable map
    this.liveSocketsByDatacenter = Collections.unmodifiableMap(liveSocketsByDatacenter);
}

您的字段liveSocketsByDatacenter也可以是AtomicReference类型< Map< Datacenters,List< SocketHolder>>> ,它将是最终的,您的地图仍将存储在易变量中,但在AtomicReference类中.

之前的代码将是：

private final AtomicReference<Map<Datacenters, List<SocketHolder>>> liveSocketsByDatacenter 
    = new AtomicReference<>(Collections.unmodifiableMap(new HashMap<>()));

...

private void connectToZMQSockets() {
    ...
    // Update the map content
    this.liveSocketsByDatacenter.set(Collections.unmodifiableMap(liveSockets));
}

public Optional<SocketHolder> getNextSocket() {
    // For the sake of consistency make sure to use the same map instance
    // in the whole implementation of my method by getting my entries
    // from the local variable instead of the member variable
    Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter = 
        this.liveSocketsByDatacenter.get();
    ...
}

// Added the modifier synchronized to prevent concurrent modification
// it is needed because to build the new map we first need to get the
// old one so both must be done atomically to prevent concistency issues
private synchronized void updateLiveSockets() {
    // Initialize my new map with the current map content
    Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter = 
        new HashMap<>(this.liveSocketsByDatacenter.get());
    ...
    // Update the map content
    this.liveSocketsByDatacenter.set(Collections.unmodifiableMap(liveSocketsByDatacenter));
}