java - 即使该行为空，也卡在 readLine() 上

Question

我在我的服务器中编写了这个循环，他只是将一些字符串发送到客户端:

PrintWriter out = new PrintWriter(incoming.getOutputStream(), true);
for (int j = 0; j < i; j++) {
    out.println(tmp[j]); // send the strings to the client
}

客户端有另一个循环来检索所有这些字符串，但永远不会从那里退出。例如，如果我向他发送 4 个字符串，则输出将是:
-嗨
-如何
-是
-你
然后在最后一个字符串之后它挂起，除了关闭服务器我不能做任何其他事情。当我关闭它时，客户端会退出。这是不起作用的循环:

        /* PHASE 2: The client receives the ArrayList with the emails */
        BufferedReader in = new BufferedReader(new InputStreamReader(s.getInputStream()));
        String line;
        String message[] = new String[5];
        for (int j=0; ((line = in.readLine()) != null) && (line.length())>0;) {
           System.out.println(line); //DEBUG
           message[j++] = line;
           if (j==5) {
                data = format.parse(message[3]);
                email.add(new Email((Integer.parseInt(message[0])), message[1], account, message[2], message[4], data));
                j=0;
            }
        }
        System.out.println("Out");

这是涉及循环的客户端代码:

public void loadData() throws IOException, ClassNotFoundException, ParseException {

    try {
        connect();
        ArrayList<Email> email = new ArrayList<Email>();
        DateFormat format = new SimpleDateFormat("dd/MM/yyyy");
        Date data;

        /* PHASE 1: The client sends a string to the server */
        try {
            PrintWriter out = new PrintWriter(s.getOutputStream(), true);
            out.println(account+"\n"); // send the account name to server

            /* PHASE 2: The client receives the ArrayList with the emails */
            BufferedReader in = new BufferedReader(new InputStreamReader(s.getInputStream()));
            String line;
            String message[] = new String[5];
            for (int j=0; ((line = in.readLine()) != null) && (line.length())>0;) {
                System.out.println(line); //DEBUG
                message[j++] = line;
                if (j==5) {
                    data = format.parse(message[3]);
                    email.add(new Email((Integer.parseInt(message[0])), message[1], account, message[2], message[4], data));
                    j=0;
                }
            }
            System.out.println("Out");

这是服务器代码:

class ThreadedEchoHandler implements Runnable {

    private Socket incoming;

    private String nomeAccount = "";

    public void run() {
        try {
            incoming = s.accept();
        } catch (IOException ex) {
            System.out.println("Unable to accept requests");
        }
        contenutoTextArea.append("Connected from: " + incoming.getLocalAddress() + "\n");
        textarea.setText(contenutoTextArea.toString());
        try {
            //PHASE 1: The server receives the email
            try {
                BufferedReader in = new BufferedReader(new InputStreamReader(incoming.getInputStream()));
                nomeAccount = in.readLine();
            } catch (IOException ex) {
                System.out.println("Not works");
            }

            //PHASE 2: I'm getting all the emails from the files
            File dir = new File("src/server/" + nomeAccount);
            String[] tmp = new String[100];
            int i = 0;
            for (File file : dir.listFiles()) {
                if (file.isFile() && !(file.getName().equals(".DS_Store"))) {
                    try (BufferedReader br = new BufferedReader(new FileReader(file))) {
                        String line;
                        while ((line = br.readLine()) != null) {
                            tmp[i++] = line;
                        }
                        br.close();
                    } catch (IOException ex) {
                        System.out.println("Cannot read from file");
                    }
                }
            }

            //PHASE 3: The server sends the ArrayList to the client
            PrintWriter out = new PrintWriter(incoming.getOutputStream(), true);
            for (int j = 0; j < i; j++) {
                out.println(tmp[j]); // send the strings to the client
            }

        } catch (IOException ex) {
            System.out.println("Cannot send the strings to the client");
        }

        //PHASE 4: Here I loop and wait for the client choise
        BufferedReader in;
        String op;
        boolean exit = false;
        try {
            in = new BufferedReader(new InputStreamReader(incoming.getInputStream()));
            while ((op = in.readLine()) != null) {
            System.out.println("OP: " + op);
            if (op.equals("Elimina")) {
                String tmp = in.readLine();
                contenutoTextArea.append("Ho eliminato la mail ").append(tmp).append(" \n");
                textarea.setText(contenutoTextArea.toString());
                File file = new File("src/server/" + nomeAccount + "/" + tmp + ".txt");
                file.delete();
            }
        }
            System.out.println("bbbbb");
        } catch (IOException ex) {
            System.out.println("Unable to read messages");
        } finally {
            try {
                incoming.close();
            } catch (IOException ex) {
                System.out.println("Cannot close the socket");
            }
        }
    }
}

Answer 1

根据读取您的客户端代码，它看起来像是被阻止等待另一条消息，并且它没有返回 null，因为尚未到达流的末尾。一旦您终止服务器进程，它就会继续，这一事实验证了这一点。

正如评论中所述，您应该确保关闭服务器端的 PrintWriter。但是，这本身并不能解决问题，因为流位于套接字上，并且只要套接字仍然打开，就不会返回 null。

您可以使用特定的控制字符串来回通信状态(永远不会是用户输入的东西，只是为了验证该轮通信是否已完成)，然后您无需检查是否为空，而是' d 检查该行是否与控制字符串匹配。只需在两侧使用该技术来来回传递控制，并确保完成后关闭套接字。