卡在readLine（）上，即使该行为空

Question

I wrote this loop in my server, where he just sends some strings to a 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
}

The client has another loop to retrieve all these strings but never exit from there. For example, if I send him 4 strings the output will be:
-hi
-how
-are
-you
And then after this last string it hangs and I cannot do anything else than closing the server. When I close it, the client exit from the while. This is the loop that doesn't work:

        /* 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");

Here is the code of the client with the loop incriminated:

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");

Here is the server code:

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

Based on reading your client code, it looks like it's blocked waiting for another message, and it's not returning null because the end of the stream hasn't been reached. The fact that it continues once you kill the server process validates this.

As noted in the comments, you should make sure you close the PrintWriter on the server side. However, this by itself won't fix it, since the stream is on the socket, and as long as the socket is still open, this won't return null.

You can use specific control strings to communicate state back and forth (things that would never be user input, just to verify that round of communication is finished) then instead of checking for null, you'd check if the line matched the control string. Simply use that technique on both sides to pass control back and forth, and make sure to close the socket when done.