java - 从单个线程修改 HashMap 并从多个线程读取？

Question

我有一个类，其中我每 30 秒从一个后台线程填充一个 map liveSocketsByDatacenter，然后我有一个方法 getNextSocket，它将被多个调用阅读器线程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);
    }
  }
}

正如你在我上面的类(class)中看到的那样:

• 从一个每 30 秒运行一次的后台线程，我用所有实时套接字填充 liveSocketsByDatacenter 映射。
• 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 liveSocketsByDatacenter 映射来获取所需的信息。

我上面的代码线程安全吗？所有的阅读器线程都能准确地看到 liveSocketsByDatacenter 吗？由于我每 30 秒从一个后台线程修改一次 liveSocketsByDatacenter 映射，然后从许多读取器线程修改一次，因此我调用了 getNextSocket 方法，所以我不确定我是否这样做了这里有什么问题。

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 map 中获取一个共享的 ArrayList 并将其洗牌？而且可能还有一些我可能错过的地方。在我的代码中修复这些线程安全问题的最佳方法是什么？

如果有任何更好的方法来重写它，那么我也愿意接受。

Answer 1

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。

给大家分享liveSocketsByDatacenter , HashMap 的实例Map 的非线程安全 实现可以同时读取(通过 updateLiveSockets 和 getNextSocket )和修改(通过 connectToZMQSockets 和 updateLiveSockets )而不同步任何已经足以使您的代码非线程安全的访问。此外，这个 Map 的值是 ArrayList 的实例List 的非线程安全 实现也可以同时读取(由 getNextSocket 和 updateLiveSockets )和修改(由 getLiveSocket 更准确地由 Collections.shuffle )。

解决 2 线程安全问题的简单方法可能是:

1. 使用ConcurrentHashMap而不是 HashMap对于你的变量 liveSocketsByDatacenter因为它是 Map 的 native 线程安全实现.
2. 将您的ArrayList 的不可修改 版本放入使用 Collections.unmodifiableList(List<? extends T> list) 作为 map 值的实例，那么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. 重写你的方法getLiveSocket避免调用 Collections.shuffle直接在您的列表上，例如，您可以仅随机播放 Activity 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 map ，您可以考虑重建您的 map ，然后每 30 秒共享其不可变版本(使用 Collections.unmodifiableMap(Map<? extends K,? extends V> m) )，这种方法是在大部分阅读场景中非常高效，因为您不再需要为访问 map 内容而支付任何同步机制的费用。

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);
}

你的领域liveSocketsByDatacenter也可以是 AtomicReference<Map<Datacenters, List<SocketHolder>>> 类型, 那么它将是 final ，您的 map 仍将存储在 volatile 中变量但在类内 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));
}