从套接字读取直到在Java中\\ r \\ n

Question

I want to implement a POP3 client. I know that in POP3 protocol, each message ends with \\r\\n . I want to write a recvuntil method in Java, which will be able to receive a whole message at once.

It super easy in Python:

def recv_all_until_ver3(s, crlf):
    data = ""
    while data[-len(crlf):] != crlf:
        data += s.recv(1)
    return data

However, when I tried to do the same in Java:

class JavaApplication1 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {
        // TODO code application logic here
        String serverAddress = "interia.pl";
        int port = 110;

        Socket s = new Socket(serverAddress, port);
        BufferedReader input = new BufferedReader(new InputStreamReader(s.getInputStream()));
        String answer = "";
        while (!answer.contains("\r\n")) {
            answer = input.readLine();
            System.out.println(answer);
        }
    }
}

It receives one message and hands instead of end itself. Is there any way to implement this method in Java? Can I just read characters one by one, instead of whole line until I find \\r\\n in my message? Thank you.

Answer 1

The Python code reads one octet at a time, immediately appends it to answer and signals it's done if at the current iteration answer ends with CRLF .

The Java code hangs because BufferedReader.readLine() returns the line content after stripping the line termination. At this point answer does not contain CRLF , so readLine() is called once again, and since the stream is not closed from the other end (your program would then throw a NullPointerException ) and you have an infinite read timeout (you would see a SocketTimeoutException ), it hangs, forever waiting for the next line.

The correct version is

Socket socket = connect(); // Implement this
Scanner scanner = new Scanner(new InputStreamReader(socket.getInputStream(), "ASCII"));
scanner.useDelimiter("\r\n");
String response = scanner.nextLine();
socket.close();

Notes:

• always set a timeout on sockets.
• you may want to account for multiline messages .

Answer 2

you can use the Scanner class (it accepts an InputStream as a constructor param ) and set the delimiter using useDelimiter method. to read messages that end with \\r\\n