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，其中liveSocketsByDatacenter每30秒从一个后台线程填充一个映射，然后有一个方法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映射来获取此信息。

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);
    }
  }
}

如您在以上课程中所见：

• 从每30秒运行一次的单个后台线程，我liveSocketsByDatacenter使用所有活动套接字填充map。
• 然后，从多个线程中，我调用getNextSocket方法给我一个可用的实时套接字，该套接字使用liveSocketsByDatacentermap获取所需的信息。

我上面的代码线程安全liveSocketsByDatacenter吗，所有阅读器线程都可以正确看到？由于我liveSocketsByDatacenter每30秒从一个后台线程然后从许多读取器线程中修改map，所以我在调用getNextSocketmethod，所以不确定在这里是否做错了什么。

好像我的“
getLiveSocket”方法中可能存在线程安全问题，因为每次读取都ArrayList从映射中共享出来并对其进行洗牌？我可能已经错过了更多其他地方。解决我的代码中这些线程安全问题的最佳方法是什么？

如果有更好的方法可以重写它，那么我也很乐意。

Answer 1

为了线程安全，您的代码必须将对所有共享可变状态的任何访问同步。

在这里，您共享了的 非线程安全*
实现liveSocketsByDatacenter的实例，可以潜在地同时读取（通过和）和修改（通过和）而无需同步任何足以使您的代码非线程安全的访问。此外，此值是a的
非线程安全 实现的实例，也可以潜在地同时读取（由和）和修改（由精确地由）。HashMap


Map``updateLiveSockets``getNextSocket``connectToZMQSockets``updateLiveSockets``Map``ArrayList
***List``getNextSocket``updateLiveSockets``getLiveSocket``Collections.shuffle

解决您的2个线程安全问题的简单方法是：

1. 对变量使用a ConcurrentHashMap而不是a HashMap，liveSocketsByDatacenter因为它是的本机线程安全实现Map。
2. 使用将实例的 不可修改 版本ArrayList作为地图的值Collections.unmodifiableList(List<? extends T> list)，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)
);`

1. 重写您的方法getLiveSocket以避免Collections.shuffle直接在列表上调用，例如，您可以仅对活动套接字列表进行洗牌，而不是对所有套接字进行洗牌，或者使用列表的副本（例如使用new ArrayList<>(listOfEndPoints)）代替列表本身。

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();
}

对于您似乎经常阅读并且很少（每30秒仅一次）修改地图的＃1，您可以考虑重建地图，然后Collections.unmodifiableMap(Map<? extends K,? extends V> m)每30秒共享其不可变版本（使用），这种方法在大多数情况下是非常有效的因为您不再为访问地图内容付出任何同步机制的代价。

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也可以是类型AtomicReference<Map<Datacenters, List<SocketHolder>>>，那么final您的地图仍将存储在volatile变量中，但在类内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));
}