ServerSocket关闭方法不释放端口答案

【问题标题】：ServerSocket close method not releasing portServerSocket关闭方法不释放端口
【发布时间】：2018-07-01 03:04:20
【问题描述】：

我正在开发 2 个应用程序（网络和独立应用程序）。我在 Web 应用程序中内置了功能，以便用户能够从网页重新启动独立应用程序的某些模块。我实现这一点的方法是使用一个 ServerSocket 对象，该对象侦听在数据库中配置为参数的端口。这是侦听传入请求的服务器端的缩短版本：

try
    {
        int port = Integer.parseInt(globalParamService.findByName("serviceInterconnectPort").getValue());
        ServerSocket serverSocket = new ServerSocket(port);
        logEntryService.logInfo(LogEntry.CONNECTIVITY, "Successfully started web client connector on port " + port);
        while(running)
        {
            socket = serverSocket.accept();
            logEntryService.logInfo(LogEntry.CONNECTIVITY, "Incoming request from web client");
            InputStream is = socket.getInputStream();
            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader br = new BufferedReader(isr);
            String message = br.readLine();
            if (message.contains("Restart Web Client Connector"))
            {
                if (!main.isWebClientConnectorRestarting())
                {
                    main.restartWebClientConnector();
                    String returnMessage = "Done\n";
                    OutputStream os = socket.getOutputStream();
                    OutputStreamWriter osw = new OutputStreamWriter(os);
                    BufferedWriter bw = new BufferedWriter(osw);
                    bw.write(returnMessage);
                    bw.flush();

                    os.close();
                    osw.close();
                    bw.close();
                }
                else
                {
                    String returnMessage = "Request cancelled\n";
                    OutputStream os = socket.getOutputStream();
                    OutputStreamWriter osw = new OutputStreamWriter(os);
                    BufferedWriter bw = new BufferedWriter(osw);
                    bw.write(returnMessage);
                    bw.flush();

                    os.close();
                    osw.close();
                    bw.close();
                    logEntryService.logWarning(LogEntry.CONNECTIVITY, "Web client connector restart request cancelled, restart already in progress");
                }
            }

            is.close();
            isr.close();
            br.close();
            socket.close();
        }
    }
    catch (Exception ex)
    {
        logEntryService.logError(LogEntry.CONNECTIVITY, "Error processing restart request from web client : " + ex.getMessage());
    }

部署我的 2 个应用程序后，用户可能需要更改运行此侦听器的端口。当他们从网络应用程序中更改它时，我会在数据库中更新之前提取未更改的端口并将其发送到以下方法：

public void restartWebClientConnector(int oldPort)
{
    Thread t = new Thread(() -> 
    {
        try
        {
            logEntryService.logInfo(LogEntry.CONNECTIVITY, "Connecting to port " + oldPort + " to restart web client connector");
            InetAddress address = InetAddress.getByName("localhost");
            socket = new Socket(address, oldPort);
            logEntryService.logDebug(LogEntry.CONNECTIVITY, "Successfully connected to port " + oldPort);
            OutputStream os = socket.getOutputStream();
            OutputStreamWriter osw = new OutputStreamWriter(os);
            BufferedWriter bw = new BufferedWriter(osw);
            bw.write("Restart Web Client Connector\n");
            bw.flush();
            InputStream is = socket.getInputStream();
            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader br = new BufferedReader(isr);
            String message = br.readLine();
            if (message.compareTo("Done") == 0)
            {
                logEntryService.logInfo(LogEntry.CONNECTIVITY, "Web client connector restart request acknowledged");
            }
            else
            {
                logEntryService.logWarning(LogEntry.CONNECTIVITY, "Web client connector restart request cancelled, restart already in progress");
            }

            os.close();
            osw.close();
            bw.close();
            is.close();
            isr.close();
            br.close();
            socket.close();
        }
        catch (IOException | NumberFormatException ex)
        {
            logEntryService.logError(LogEntry.CONNECTIVITY, "Error sending web client connector restart command : " + ex.getMessage());
        }
    });
    t.start();
}

此方法然后调用以下代码，终止我的侦听器线程并在新的更新端口号上重新初始化它：

public void restartWebClientConnector()
{
    if (!webClientConnectorRestarting)
    {
        webClientConnectorRestarting = true;
        webClientConnector.setRunning(false);
        webClientConnectorThread.interrupt();
        initWebClientConnector();
        logEntryService.logInfo(LogEntry.CONNECTIVITY, "Successfully restarted web client connector");
        webClientConnectorRestarting = false;
    }
}

private void initWebClientConnector()
{
    logEntryService.logInfo(LogEntry.CORE, "Initializing web connector"); 

    try
    {
        webClientConnector = new WebClientConnector(this, globalParamService, logEntryService);
        webClientConnectorThread = new Thread(threads, webClientConnector);
        webClientConnectorThread.setName("Web Client Connector Thread");
        webClientConnectorThread.start();
    }
    catch (Exception ex)
    {
        logEntryService.logError(LogEntry.CORE, "Error initializing messaging process : " + ex.getMessage()); 
    }
}

除了一件事之外，一切都很好：尽管明确关闭了我能想到的所有内容，但旧端口号并未释放。从 web 客户端更改端口后执行 netstat -a 时，旧端口仍列在 LISTENING 状态。我可以连续更改几次，每次都能正常工作，但端口没有被释放。我花了相当长的时间研究这个，从我读到的内容看来我做的一切都是正确的（显然不是！）。

你们的任何意见都会有所帮助。

干杯！

【问题讨论】：

我再次仔细阅读了我的代码，我认为中断线程可能会导致它实际上没有运行 socket.close() 方法。我删除了该行，通过将运行变量设置为 false 来让线程自然终止，但即使这样也不会释放端口。

标签： java sockets server port release

【解决方案1】：

我建议您使用资源 try 块，或者至少 try-finally 块来确保资源关闭：

    int port = Integer.parseInt(globalParamService.findByName("serviceInterconnectPort").getValue());
    try (ServerSocket serverSocket = new ServerSocket(port)) {
        logEntryService.logInfo(LogEntry.CONNECTIVITY, "Successfully started web client connector on port " + port);
        while (running) {
            try (Socket socket = serverSocket.accept();
                    InputStream is = socket.getInputStream();
                    InputStreamReader isr = new InputStreamReader(is);
                    BufferedReader br = new BufferedReader(isr)) {
                String message = br.readLine();
                if (message.contains("Restart Web Client Connector")) {
                    if (!main.isWebClientConnectorRestarting()) {
                        main.restartWebClientConnector();
                        String returnMessage = "Done\n";
                        try (OutputStream os = socket.getOutputStream();
                                OutputStreamWriter osw = new OutputStreamWriter(os);
                                BufferedWriter bw = new BufferedWriter(osw)) {
                            bw.write(returnMessage);
                            bw.flush();
                        }
                    } else {
                        String returnMessage = "Request cancelled\n";
                        try (OutputStream os = socket.getOutputStream();
                                OutputStreamWriter osw = new OutputStreamWriter(os);
                                BufferedWriter bw = new BufferedWriter(osw)) {
                            bw.write(returnMessage);
                            bw.flush();
                        }
                        logEntryService.logWarning(LogEntry.CONNECTIVITY, "Web client connector restart request cancelled, restart already in progress");
                    }
                }
            }
        }
    } catch (IOException ex) {
        logEntryService.logError(LogEntry.CONNECTIVITY, "Error processing restart request from web client : " + ex.getMessage());
    }

【讨论】：

感谢您的建议！我刚刚想到的一件事是，当您在服务器套接字上调用 accept() 时，它会在该行停止，直到它接收到连接，对吗？如果是这种情况，将 running 设置为 false 实际上对此代码没有任何作用，因为它卡在这条线上等待客户端连接。我理解正确吗？
谢谢莫里斯，那很可能是我的问题。我需要找到一种方法来允许服务器套接字侦听传入请求，同时回复确认消息，然后才能正确终止自身。据我了解，在这种情况下，即使是 finally 块也不会执行 socket.close 方法，因为在确认消息被发送回 Web 应用程序之后执行仍然卡在 socket.accept() 行。看起来我需要使用某种线程中断逻辑来在我停止套接字时关闭它。

【解决方案2】：

我将 WebClientConnector 类的运行方法（它实现了 Runnable）更改为以下代码：

@Override
public void run() 
{
    try
    {
        initWebClientListener();
    }
    finally
    {
        try 
        {
            serverSocket.close();
        } 
        catch (IOException ex1) 
        {}
    }
}

并更改了我的重新启动方法以对其调用中断，并将运行变量设置为 false。 ServerSocket 现在可以正常关闭并释放端口。

【讨论】：

另外，你会注意到我在 Socket 对象上调用 close 而不是在 ServerSocket 对象上，这当然没有帮助！