在单个后台线程定期修改地图的同时读取地图

Question

我有一个类，其中liveSocketsByDatacenter每隔30秒从一个后台线程中填充一个映射到updateLiveSockets()方法中，然后有一个方法getNextSocket()，该方法将被多个读取器线程调用以获取一个可用的实时套接字，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 AtomicReference<Map<Datacenters, List<SocketHolder>>> liveSocketsByDatacenter =
      new AtomicReference<>(Collections.unmodifiableMap(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);
  }

  // during startup, making a connection and populate once
  private void connectToZMQSockets() {
    Map<Datacenters, ImmutableList<String>> socketsByDatacenter = Utils.SERVERS;
    // The map in which I put all the live sockets
    Map<Datacenters, List<SocketHolder>> updatedLiveSocketsByDatacenter = new HashMap<>();
    for (Map.Entry<Datacenters, ImmutableList<String>> entry : socketsByDatacenter.entrySet()) {
      List<SocketHolder> addedColoSockets = connect(entry.getKey(), entry.getValue(), ZMQ.PUSH);
      updatedLiveSocketsByDatacenter.put(entry.getKey(),
          Collections.unmodifiableList(addedColoSockets));
    }
    // Update the map content
    this.liveSocketsByDatacenter.set(Collections.unmodifiableMap(updatedLiveSocketsByDatacenter));
  }

  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
  // is there any concurrency or thread safety issue or race condition here?
  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();
    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;
  }

  // is there any concurrency or thread safety issue or race condition here?
  private Optional<SocketHolder> getLiveSocketX(final List<SocketHolder> endpoints) {
    if (!CollectionUtils.isEmpty(endpoints)) {
      // The list of live sockets
      List<SocketHolder> liveOnly = new ArrayList<>(endpoints.size());
      for (SocketHolder obj : endpoints) {
        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();
  }

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

    // Initialize my new map with the current map content
    Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter =
        new HashMap<>(this.liveSocketsByDatacenter.get());

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

        boolean status = SendToSocket.getInstance().execute(message.getAdd(), holder, socket);
        boolean isLive = (status) ? true : false;
        // is there any problem the way I am using `SocketHolder` class?
        SocketHolder zmq = new SocketHolder(socket, liveSocket.getContext(), endpoint, isLive);
        liveUpdatedSockets.add(zmq);
      }
      liveSocketsByDatacenter.put(entry.getKey(),
          Collections.unmodifiableList(liveUpdatedSockets));
    }
    this.liveSocketsByDatacenter.set(Collections.unmodifiableMap(liveSocketsByDatacenter));
  }
}

如您在我班上所见：

• 从每30秒运行一次的单个后台线程，我liveSocketsByDatacenter使用方法中的所有活动套接字填充map updateLiveSockets()。
• 然后从多个线程调用该getNextSocket()方法，以提供一个可用的实时套接字，该套接字使用liveSocketsByDatacenter映射来获取所需的信息。

我的代码运行正常，没有任何问题，并且想看看是否有更好或更有效的方式编写此代码。我还想对线程安全性问题或任何竞争条件（如果有）提出意见，但到目前为止我还没有看到任何问题，但我可能是错的。

我主要担心updateLiveSockets()方法和getLiveSocketX()方法。我迭代liveSockets这是一个List的SocketHolder在A线，然后产生一个新的SocketHolder对象，并加入到另一个新的列表。这可以吗？

注意： SocketHolder是一个不可变的类。您可以忽略ZeroMQ我拥有的东西。

Answer 1

您使用以下同步技术。

1. 具有实时套接字数据的映射位于原子引用的后面，因此可以安全地切换映射。
2. 该updateLiveSockets()方法是同步的（隐式地），这将防止同时通过两个线程切换映射。
3. 如果在getNextSocket()方法期间发生切换，则在使用地图时要对地图进行本地引用以避免混淆。

是否像现在一样是线程安全的？

线程安全性始终取决于共享的可变数据上是否存在正确的同步。在这种情况下，共享的可变数据是数据中心到其SocketHolders列表的映射。

地图位于中AtomicReference，并制作本地副本以供使用的事实足以在地图上实现同步。您的方法采用了地图的一个版本，并使用该版本，由于的性质，切换版本是线程安全的AtomicReference。只需为地图创建member字段，也可以实现此目的volatile，因为您要做的只是更新参考（您无需对其进行任何先检查后操作操作）。

由于可以scheduleAtFixedRate()确保所传递的消息Runnable不会与自身并发运行，因此不需要synchronizedon
updateLiveSockets()，但是它也没有任何实质性的危害。

所以是的，该类是线程安全的。

但是，尚不清楚a是否SocketHolder可以同时被多个线程使用。照原样，此类仅尝试SocketHolder通过选择一个随机活动对象来最大程度地减少对s的并发使用（尽管无需重新整理整个数组以选择一个随机索引）。它实际上没有阻止并发使用。

可以提高效率吗？

我相信可以。当查看该updateLiveSockets()方法时，似乎它会构建完全相同的映射，只是SocketHolders的isLive标志值可能不同。这使我得出结论，我只想切换地图中的每个列表，而不是切换整个地图。为了以线程安全的方式更改映射中的条目，我可以使用ConcurrentHashMap。

如果我使用ConcurrentHashMap，并且不切换地图，而是切换地图中的值，则可以摆脱AtomicReference。

要更改映射，我可以构建新列表并将其直接放入映射中。这是更有效的，因为我可以更快地发布数据，创建的对象也更少，而同步仅基于现成的组件构建，这有利于提高可读性。

这是我的构建（为简洁起见，省略了一些不太重要的部分）

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

    // ...

    private SocketManager() {
      connectToZMQSockets();
      scheduler.scheduleAtFixedRate(this::updateLiveSockets, 30, 30, TimeUnit.SECONDS);
    }

    // during startup, making a connection and populate once
    private void connectToZMQSockets() {
      Map<Datacenters, List<String>> socketsByDatacenter = Utils.SERVERS;
      for (Map.Entry<Datacenters, List<String>> entry : socketsByDatacenter.entrySet()) {
        List<SocketHolder> addedColoSockets = connect(entry.getValue(), ZMQ.PUSH);
        liveSocketsByDatacenter.put(entry.getKey(), addedColoSockets); // we can put it straight into the map
      }
    }

    // ...

    // this method will be called by multiple threads to get the next live socket
    // is there any concurrency or thread safety issue or race condition here?
    public Optional<SocketHolder> getNextSocket() {
      for (Datacenters dc : Datacenters.getOrderedDatacenters()) {
        Optional<SocketHolder> liveSocket = getLiveSocket(liveSocketsByDatacenter.get(dc)); // no more need for a local copy, ConcurrentHashMap, makes sure I get the latest mapped List<SocketHolder>
        if (liveSocket.isPresent()) {
          return liveSocket;
        }
      }
      return Optional.absent();
    }

    // is there any concurrency or thread safety issue or race condition here?
    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
          return Optional.of(liveOnly.get(random.nextInt(liveOnly.size()))); // just pick one
        }
      }
      return Optional.absent();
    }

    // no need to make this synchronized
    private void updateLiveSockets() {
      Map<Datacenters, List<String>> socketsByDatacenter = Utils.SERVERS;

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

          boolean status = SendToSocket.getInstance().execute(message.getAdd(), holder, socket);
          boolean isLive = (status) ? true : false;

          SocketHolder zmq = new SocketHolder(socket, liveSocket.getContext(), endpoint, isLive);
          liveUpdatedSockets.add(zmq);
        }
        liveSocketsByDatacenter.put(entry.getKey(), Collections.unmodifiableList(liveUpdatedSockets)); // just put it straigth into the map, the mapping will be updated in a thread safe manner.
      }
    }

}