java - "java.net.BindException: Address already in use"尝试使用与前一个线程相同的新线程创建套接字时

Question

我正在使用 Java 进行客户端-服务器模拟，其中客户端(线程)连接到服务器以获取一些数据。几秒钟后，需要杀死随机选择的客户端(线程)之一。我关闭了它用于与服务器通信的套接字并让他死了(通过退出 run() 方法)。问题是当新创建的线程试图创建与前一个线程相同的套接字(相同的地址和相同的端口)以连接到服务器时，我得到:

java.net.BindException: Address already in use
    at java.net.PlainSocketImpl.socketBind(Native Method)
    at java.net.AbstractPlainSocketImpl.bind(AbstractPlainSocketImpl.java:374)
    at java.net.Socket.bind(Socket.java:627)
    at java.net.Socket.<init>(Socket.java:423)
    at java.net.Socket.<init>(Socket.java:319)

创建套接字的代码:

private void createNewSocket(InetAddress sIP, int sPort, 
        InetAddress cIP, int cPort) {
    try {
        socket = new Socket(sIP, sPort, cIP, cPort);
    } catch (IOException e) {
        e.printStackTrace();
        System.err.println("Socket unsuccessfully created");
    }
    try {
        in = new BufferedReader(new InputStreamReader(
                socket.getInputStream()));

        out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(
                socket.getOutputStream())), true);

    } catch (IOException e) {
        e.printStackTrace();    
        try {
            socket.close();
        } catch (IOException e2) {
            System.err.println("Socket unsuccessfully closed");
        }
    }
}

public void run() {

    createNewSocket(gprsServerIP, Util.PORT_SERVER_PORT,
            clientIP, sendPort);

    out.println(REQUEST);   
    try {
        serverPort = Integer.parseInt(in.readLine());   
        TCPClient.serverPort[clientID] = serverPort;
        System.out.println("Server port: " + serverPort + '\n' + 
                            "Send port: " + sendPort + '\n');
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            socket.close();
        } catch (IOException e) {
            System.err.println("Socket unsuccessfully closed");
        }
    }

    while (true) {      

        if (clientID == TCPClient.selectedID) {

            TCPClient.selectedID = -1;

            createNewSocket(gprsServerIP, Util.PORT_SERVER_PORT, 
                    clientIP, sendPort);

            out.println(FREE_PORT + serverPort);
            try {
                socket.close();
            } catch (IOException e2) {
                System.err.println("Socket unsuccessfully closed");
            }
            //System.out.println(socket.isClosed());
            System.out.println("Port:" + serverPort + " is free");
            TCPClient.id[clientID] = -1;
            break;              
        }                           
    }
    clientCount--;

}

Answer 1

很可能客户端套接字仍在 CLOSE_WAIT or TIME_WAIT status 中.操作系统确保在另一个应用程序可以重新使用它之前，所有数据都已通过套接字传送。否则，新客户端可能会得到先前连接遗留下来的垃圾重复数据包。

我建议您的客户使用一系列端口而不是一个常量。然后他们可以使用范围中的下一个端口并在到达范围末尾时循环。

但是，如果您不需要设置客户端端口，那么您根本不应该通过将端口 0 传递给 Socket 来在代码中设置客户端端口。在这种情况下，JDK 和操作系统会做正确的事情并为您选择合适的自由端口。

引自wikipedia :

CLOSE-WAIT: The server receives notice from the local application that it is done. The server sends its fin to the client.

TIME-WAIT: Represents waiting for enough time to pass to be sure the remote peer received the acknowledgment of its connection termination request. According to RFC 793 a connection can stay in TIME-WAIT for a maximum of four minutes known as a MSL (maximum segment lifetime).