java BufferedReader readLine 方法挂起并阻塞程序
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BufferedReader readLine method hangs and block program
提问by Thiago Moraes
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.
我正在尝试使用 Java 学习套接字，并且成功地将数据发送到在我自己的机器上运行的 ServerSocket。当我尝试使用 readline 从这个套接字读取时（所以我可以只回显我自己发送的消息）我的程序挂起并且不会返回。
Here's the code:
这是代码：
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:
TCPClient 是我定义的一个类，因此我可以在我的工作中使用 Swing 之前在更简单的界面上测试我的程序。这是代码：
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.
我的服务器很简单。连接建立后，它进入这个循环并一直停留在这里直到我关闭客户端（因为无限循环）。之后，一些异常处理将其返回到连接开始之前的点。
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.
我不明白我错过了什么。
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?
PS：奇怪的是，如果我调试服务器，我可以看到到达那里的消息（例如，我可以打印它），但是如果我只运行此代码，这是不可能的。这是否有一些我忽略的并发关系？
thx
谢谢
回答by JB Nizet
The problem is that readLinetries 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.
问题是readLine尝试阅读一行。在确定到达行尾之前，它不会返回该行。这意味着它需要换行符或通信结束。由于服务器不发送任何换行符并且不关闭流，客户端无限期地等待。
回答by Ashwinee K Jha
cli.ostream.writeUTF("Teste");
Shouldn't this be containing a new line? Otherwise read method will be waiting for new line I think.
这不应该包含一个新行吗？否则我认为 read 方法将等待新行。
Also as suggested you can try flushing the ostream after writing to it.
同样按照建议，您可以在写入后尝试刷新 ostream。
回答by user207421
writeUTF() doesn't write a line. See the Javadoc. writeUTF() is for use with readUTF(). And Readers are for use with Writers. So change the DataOutputStream to a BufferedWriter and call write() and then newline().
writeUTF() 不写一行。请参阅 Javadoc。writeUTF() 与 readUTF() 一起使用。Readers 供 Writers 使用。因此，将 DataOutputStream 更改为 BufferedWriter 并调用 write()，然后调用 newline()。
