为什么这个while循环不终止？

Question

I'm writing code to simulate the structure of an in memory distributed database. I have a central server class and a slave server class. The central server instance communicates with all of the slave instances to get information from them. The method makeSlaveRequest() is part of the central server class and is shown below:

private ArrayList<String> makeSlaveRequest(SlaveServer slave, String artist) {
            int port = slave.getPort();
            ArrayList<String> result = new ArrayList<String>();
            try(Socket slaveConnection = new Socket("localhost", port)) {
                System.out.println("Entered makeSlaveRequest");
                PrintWriter outToSlave = new PrintWriter(slaveConnection.getOutputStream(), true);
                BufferedReader inFromSlave = new BufferedReader(new InputStreamReader(slaveConnection.getInputStream()));
                outToSlave.println(artist);
                String line = inFromSlave.readLine();
                while(line != null) { // I'm pretty sure that we're getting stuck here
                    if(line.equals("No songs by that artist")) {
                        break;
                    }
                    System.out.println("Reading from slave: " + line);
                    result.add(line);
                    System.out.println(result);
                    line = inFromSlave.readLine();
                }
                System.out.println("Slave connection closed");
                slaveConnection.close();

            } catch(UnknownHostException uhe) {
                System.out.println("unkown host");
                uhe.printStackTrace();
            } catch(IOException ioe) {
                System.out.println("IOException: " + ioe);
                ioe.printStackTrace();
            }
            System.out.println(result);
            return result;
        }

All calls to System.out.println() are purely for debugging purposes.

The point the code gets stuck at is the while loop. Once the last relevant song has been read in from the slave server, I'm expecting the while loop to terminate and "Slave connection closed" to be printed, however the while loop never terminates. I think this must be due to the fact that line never equals null, however, I would expect line to equal null once the last song has been sent from the slave and read in by readLine(). Here is the code for the SlaveServer's handleConnection() method, which is the method communicating with makeSlaveRequest():

protected void handleConnection(Socket connection) throws IOException {
            BufferedReader inFromCentral = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            PrintWriter outToCentral = new PrintWriter(connection.getOutputStream(), true);
            System.out.println("Entered the slave server handle connection method");
            String reqArtist = inFromCentral.readLine();
            System.out.println(reqArtist);
            if(songDir.containsValue(reqArtist.trim())) {
                System.out.println("Contains reqArtist");
                List<String> songList = new ArrayList<String>();
                System.out.println("songList created");
                for(Map.Entry<String, String> entry : songDir.entrySet()) {
                    System.out.println("entered for loop");
                    if(entry.getValue().equals(reqArtist)) {
                        songList.add(entry.getKey());
                        System.out.println("entry added");
                    }
                }
                for(String song : songList) {
                    System.out.println("slave output: " + song);
                    outToCentral.println(song);
                }
            } else {
                System.out.println("No Req Artist");
                outToCentral.println("No songs by that artist");
            }
            System.out.println("flushing");
            outToCentral.flush();
            System.out.println("EOM");
        }
    }

It should be noted that each SlaveServer instance is running in it's own thread, and whenever a connection is made to SlaveServer, it uses a thread from a thread pool to handle it. CentralServer is the same.

Why is the while loop not terminating?

Answer 1

从服务器永远不会关闭连接/流，因此中央服务器无法知道在接收到的最后一行之后没有下一行，因此readLine()阻塞，等待从服务器发送下一行，或者以便关闭连接并因此到达流的末端。

Answer 2

readLine() will only return null when it reaches EOF, which for a socket connection will be when remote side closes. If the slaves don't close connection and stop, you will be looping forever (well, readLine() will block). So, you need to decide on a method that indicates that the slave is done (if you don't want it to close socket when it's done).