java - 在迭代器内的线程内删除集合的元素

Question

我有这个循环:

for (Iterator<Socket> it = collection.iterator(); it.hasNext();) {
    Object obj = it.next();
    new Thread( () -> {
        if(myCondition(obj)){
            it.remove();
        }
    }).start();
}

有时它可以工作，但有时我在调用 it.remove(); 时会收到 java.lang.IllegalStateException有安全的方法吗？

---更新---

我需要在服务器上运行的代码中执行此操作，该代码从许多客户端套接字接收连接。这段代码时不时地执行一些查询，每次执行查询时，结果都会发送到所有连接的套接字。因此，对于每个套接字，我创建一个新线程来执行此操作。我试图从列表中删除的是关闭的套接字。

这是我的服务器如何工作的示例:

public class SocketServidor {
    static ServerSocket serverSocket;
    static List<Socket> socketsConectados = new ArrayList<>();

    public static void main(String[] args) throws Exception {
        serverSocket = new ServerSocket(5963);

        new Thread( ()-> {
            lacoConexao:
            while(true) {
                try {
                    socketsConectados.add(serverSocket.accept());
                } catch (IOException ex) {
                    Logger.getLogger(SocketServidor.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }).start();

        lacoMensagens:
        while(true) {
            int valor = (int) (Math.random() * 3) + 1;
            lacoIterators:
            for (Iterator<Socket> it = socketsConectados.iterator(); it.hasNext();) {
                Socket socket = it.next();
                new Thread( () -> {
                    try {
                        if(socket.isClosed()) {
                            return;
                        }
                        PrintWriter out = new PrintWriter(socket.getOutputStream());
                        out.println("Você acabou de ganhar R$ "+new DecimalFormat("###,##0.00").format(valor)+" às "+new SimpleDateFormat("HH:mm:ss").format(new Date())+"!");
                        if(out.checkError()){
                            socket.close();
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }).start();
            }
            System.out.println("Sockets conectados: "+socketsConectados.size());
            Thread.sleep(valor*1000);
        }
    }
}

Answer 1

您应该阅读有关线程安全的更多信息。我可以告诉您要在代码中修复哪些内容，但是在您正确理解并发性之前，您每次编写一行代码都会不断添加错误...

你应该:

• 使用线程安全列表，例如 List<Socket> socketsConectados = new CopyOnWriteArrayList<>();
• 使用线程池而不是为每个套接字创建一个线程:ExecutorService executor = Executors.newCachedThreadPool();
• 将发送消息的任务提交到线程池，而不是手动创建线程
• 您使用的那些标签也很令人困惑，它们应该用于从外部循环中中断 - 如果您只想注释代码，请使用注释!

<小时/>

作为引用，代码可能如下所示:

public class SocketServidor {
  static ServerSocket serverSocket;
  private static final ExecutorService executor = Executors.newCachedThreadPool();
  private static final List<Socket> sockets = new CopyOnWriteArrayList<>();

  public static void main(String[] args) throws Exception {
    serverSocket = new ServerSocket(5963);
    new Thread(SocketServidor::acceptSockets).start();
    sendMessages();
  }

  private static void acceptSockets() {
    while (true) {
      try {
        sockets.add(serverSocket.accept());
      } catch (IOException ex) {
        Logger.getLogger(SocketServidor.class.getName()).log(Level.SEVERE, null, ex);
      }
    }
  }

  private static void sendMessages() throws InterruptedException {
    while (true) {
      int valor = (int) (Math.random() * 3) + 1;
      for (Socket socket : sockets) {
        executor.submit(() -> {
          if (socket.isClosed()) sockets.remove(socket);
          else sendMessage(socket, valor);
        });
      }
      System.out.println("Sockets conectados: " + sockets.size());
      Thread.sleep(valor * 1000);
    }
  }

  private static void sendMessage(Socket socket, int valor) {
    try {
      PrintWriter out = new PrintWriter(socket.getOutputStream());
      out.println("Você acabou de ganhar R$ " + new DecimalFormat("###,##0.00").format(valor) + " às " + new SimpleDateFormat("HH:mm:ss").format(
              new Date()) + "!");
      if (out.checkError()) {
        socket.close();
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}