Java while（true）循环仅在线程内执行一次

Question

我试图用Java实现一个简单的客户机-服务器应用程序。

这是代码：

客户端.java

public class Client implements Runnable {
    private String hostName;
    private int portNumber;
    private String message;

    private Socket socket;
    private PrintWriter writer;
    private BufferedReader reader;

    public Client(String hostName, int portNumber, String message) {
        this.hostName = hostName;
        this.portNumber = portNumber;
        this.message = message;
    }

    public void connect() {
        try {
            socket = new Socket(hostName, portNumber);
            writer = new PrintWriter(socket.getOutputStream(), true);
            reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));

            writer.println(message);
        } catch (UnknownHostException e) {
            System.err.println("Could not resolve the host name '" + hostName + "'.");
        } catch (IOException e) {
            System.err.println("Could not get the I/O for the connection to '" + hostName + "'.");
        }
    }

    private void listenForMessages() {
        while (true) {
            try {
                System.out.println("In loop!");
                String line;
                while ((line = reader.readLine()) != null) {
                    System.out.println(line);
                }
            } catch (IOException e) {
                System.err.println(e.getMessage());
            }
        }
    }

    public void run() {
        connect();
        listenForMessages();
    }
}

服务器.java

public class Server implements Runnable {
    private int portNumber;
    private String message;

    private ServerSocket serverSocket;
    private Socket clientSocket;
    private PrintWriter writer;
    private BufferedReader reader;

    public Server(int portNumber, String message) {
        this.portNumber = portNumber;
        this.message = message;
    }

    private void listen() {
        try {
            serverSocket = new ServerSocket(portNumber);
        } catch (IOException e) {
            System.err.println(e.getMessage());
        }

        while (true) {
            try {
                clientSocket = serverSocket.accept();
                writer = new PrintWriter(clientSocket.getOutputStream(), true);
                reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

                String line;
                while ((line = reader.readLine()) != null) {
                    System.out.println(line);
                }

                writer.println(message);
            } catch (IOException e) {
                System.err.println(e.getMessage());
                break;
            }
        }
    }

    public void run() {
        listen();
    }
}

这是主要类：

public class Test {
    public static void main(String[] args) {
        Client client = new Client("localhost", 4444, "Hello from client!");
        Server server = new Server(4444, "Hello from server!");

        Thread serverThread = new Thread(server);
        serverThread.start();

        Thread clientThread = new Thread(client);
        clientThread.start();
    }
}

代码的逻辑很简单:客户端和服务器都在等待< code>while(true)循环中的消息。

服务器的< code>listen方法中的< code>while循环执行得很好。但是，在< code>listenForMessages方法中，循环似乎只执行一次。我只看到一个“在循环”印在屏幕上。

你能找出问题所在吗？

提前谢谢你！

Answer 1

然而，在listenForMessages方法中，循环似乎只执行一次。我只看到屏幕上印着一个“循环”。

实际上，这并不是因为循环只执行一次，而是因为< code>reader.readLine()会让当前线程等待，直到它接收到一整行，在这里，如果您检查< code>Server的代码，它会先读取，并在一个无限循环中读取，因为< code>reader.readLine()只会在流的末尾返回< code>null，所以在这种情况下，当套接字被关闭时。

如果要在客户端和服务器之间实现某种乒乓球，只需读取然后在一侧写入，然后写入和读取，另一侧如下：

客户端代码：

public void connect() {
    try {
        socket = new Socket(hostName, portNumber);
        writer = new PrintWriter(socket.getOutputStream(), true);
        reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));

    } catch (UnknownHostException e) {
        System.err.println("Could not resolve the host name '" + hostName + "'.");
    } catch (IOException e) {
        System.err.println(
            "Could not get the I/O for the connection to '" + hostName + "'."
        );
    }
}

private void listenForMessages() {
    while (true) {
        try {
            System.out.println("In loop!");
            // Write the message for the server
            writer.println(message);
            // Read the message from the server
            System.out.println(reader.readLine());
        } catch (IOException e) {
            System.err.println(e.getMessage());
        }
    }
}

< code >服务器代码:

while (true) {
    try {
        clientSocket = serverSocket.accept();
        writer = new PrintWriter(clientSocket.getOutputStream(), true);
        reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

        while (true) {
            // Read the message from the client
            System.out.println(reader.readLine());
            // Write the message for the client
            writer.println(message);
        }


    } catch (IOException e) {
        System.err.println(e.getMessage());
        break;
    }
}

输出:

In loop!
Hello from client!
Hello from server!
In loop!
Hello from client!
Hello from server!
...