从单个线程修改哈希映射并从多个线程读取？

Question

我有一个类，我liveSocketsByDatacenter每 30 秒从单个后台线程填充一个地图，然后我有一个方法getNextSocket，该方法将由多个读取器线程调用以获取可用的实时套接字，该套接字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中看到的：

• 从每 30 秒运行一次的单个后台线程中，我liveSocketsByDatacenter用所有实时套接字填充地图。
• 然后从多个线程中，我调用getNextSocket方法来为我提供可用的实时套接字，它使用liveSocketsByDatacentermap 来获取所需的信息。

我上面的代码线程安全liveSocketsByDatacenter吗，所有读者线程都能准确看到吗？由于我liveSocketsByDatacenter每 30 秒从单个后台线程修改地图，然后从许多读取器线程修改地图，因此我正在调用getNextSocket方法，所以我不确定我是否在这里做错了什么。

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ArrayList从地图中获取共享并对其进行洗牌？而且可能还有一些我可能错过的地方。在我的代码中修复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，那么我也愿意。

Answer 1

为了实现线程安全，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，您共享a的非线程安全实现的liveSocketsByDatacenter实例，它可以同时读取（由和）和修改（由和），而无需同步任何访问，这已经足以使您的代码非线程安全。此外， this 的值是 a 的非线程安全实现的实例，也可以同时读取（由和）和修改（更准确地说由）。HashMapMapupdateLiveSocketsgetNextSocketconnectToZMQSocketsupdateLiveSocketsMapArrayListListgetNextSocketupdateLiveSocketsgetLiveSocketCollections.shuffle

解决两个线程安全问题的简单方法可能是：

1. 使用 aConcurrentHashMap而不是 aHashMap作为变量，liveSocketsByDatacenter因为它是 a 的本机线程安全实现Map。
2. 使用 ，将实例的不可修改版本作为映射的值，然后您的列表将是不可变的，因此线程安全。ArrayListCollections.unmodifiableList(List<? extends T> list)

例如：

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

3. 重写您的方法getLiveSocket以避免直接在列表上调用Collections.shuffle，例如，您可以仅混洗活动套接字列表而不是所有套接字，或者使用列表的副本（例如new 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，因为您似乎经常阅读并且很少（每 30 秒一次）修改您的地图，您可以考虑重建您的地图，然后Collections.unmodifiableMap(Map<? extends K,? extends V> m)每 30 秒共享其不可变版本（使用 ），这种方法在大多数阅读场景中非常有效因为您不再需要为访问地图内容而支付任何同步机制的费用。

那么你的代码将是：

// 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也可以是类型AtomicReference<Map<Datacenters, List<SocketHolder>>>，那么final您的映射仍将存储在volatile变量中，但在类中AtomicReference。

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));
}