BufferedReader readLine方法挂起并阻止程序 [英] BufferedReader readLine method hangs and block program

问题描述

我正在尝试使用Java学习套接字，并且成功地将数据发送到了在我自己的机器上运行的ServerSocket.当我尝试使用readline从此套接字读取数据(以便我可以回显自己发送的消息)时，我的程序挂起，不会返回.

代码如下:

  public static void main(String [] args)引发UnknownHostException，IOException {TCPClient cli =新的TCPClient("127.0.0.1"，"15000");尝试 {cli.ostream.writeUTF("Teste");字符串回显= cli.istream.readLine();//它挂在这行System.out.println(echo);} 

TCPClient是我定义的类，因此我可以在使用homwework上的swing之前在更简单的接口上测试程序.这是代码:

 公共类TCPClient {public DataOutputStream ostream = null;public BufferedReader istream = null;公共TCPClient(字符串主机，字符串端口)抛出UnknownHostException {InetAddress ip = InetAddress.getByName(host);尝试 {套接字套接字=新套接字(主机，Integer.parseInt(端口));ostream =新的DataOutputStream(socket.getOutputStream());istream = new BufferedReader(new InputStreamReader(socket.getInputStream()));} catch(IOException ex){Logger.getLogger(TCPClient.class.getName()).log(Level.SEVERE，null，ex);}} 

我的服务器非常简单.建立连接后，它将进入此循环并停留在此处，直到我关闭客户端为止(由于无限循环).之后，一些异常处理会将其返回到连接开始之前的位置.

  while(true){字符串msg = istream.readLine();System.out.println(已到达服务器:" + msg);//只在调试时有效ostream.writeUTF("ACK:" + msg);ostream.flush();} 

我看不到我想念的东西.

PS:比较奇怪的是，如果我调试服务器，则可以看到到达那里的消息(例如，可以打印它)，但是如果我只运行此代码，则不可能.这是否有我忽略的并发关系?

thx

解决方案

问题是 readLine 尝试读取一行.在确定到达行尾之前，它不会返回该行.这意味着它期望换行符或通信结束.由于服务器不会发送任何换行符，也不会关闭流，因此客户端将无限期等待.

I'm trying to learn sockets using Java and I sucessfully sent data to a ServerSocket running in my own machine. When I try to read from this socket using readline (so I can just echo my own sent message) my program hangs and won't return.

Here's the code:

public static void main(String[] args) throws UnknownHostException, IOException {

    TCPClient cli = new TCPClient("127.0.0.1", "15000");
    try {
        cli.ostream.writeUTF("Teste");
        String echo = cli.istream.readLine(); //it hangs in this line
        System.out.println(echo);
    }

TCPClient is a class I defined so I can test my program on a simpler interface before using swing on my homwework. here's the code:

public class TCPClient {

public DataOutputStream ostream = null;
public BufferedReader istream = null;

public TCPClient(String host, String port) throws UnknownHostException {
    InetAddress ip = InetAddress.getByName(host);

    try {
        Socket socket = new Socket(host, Integer.parseInt(port));

        ostream = new DataOutputStream(socket.getOutputStream());
        istream = new BufferedReader(new InputStreamReader(socket.getInputStream()));


    } catch (IOException ex) {
        Logger.getLogger(TCPClient.class.getName()).log(Level.SEVERE, null, ex);
    }

}

My server is pretty simple. After the connection is estabilished, it enters in this loop and stays here until I close the client (because of the infinite loop). Afterwards, some exception handling returns it to the point before the connection started.

            while(true){
                String msg = istream.readLine();
                System.out.println("Arrived on server: " + msg); //just works on debug
                ostream.writeUTF("ACK: " + msg);
                ostream.flush();
            }

I don't see what am I missing.

PS: the wierd stuff is that if I debug the server, I can see the message arriving there (I can print it, for example), but this isn't possible if I just run this code. Does this have some concurrency relation I'm overlooking?

thx

解决方案

The problem is that readLine tries reading a line. It will not return the line until it's sure that the end of line has been reached. This means that it expects either a newline character, or the end of the communication. Since the server doesn't send any newline char and doesn't close the stream, the client waits indefinitely.

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