socket 基础问题

Question

package test;
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.*;public class test {
    public static void main(String[] args) {
        try {
            ServerSocket server=new ServerSocket(1991);
            Socket soc=server.accept();
            
            DataOutputStream output=new DataOutputStream(soc.getOutputStream());
            output.write("hello".getBytes());
            
            BufferedReader reader=new BufferedReader(new InputStreamReader(soc.getInputStream()));
            
            String in=null;
            while((in=reader.readLine())!=null)   
            {
                System.out.println(in);
            }
            
        }catch (Exception ex) {
            System.out.println(ex.getMessage());
        }
            }
}问题在这里:            while((in=reader.readLine())!=null)   
            {
                System.out.println(in);
            }我不明白它为什么会一直循环等待输入。 如果我不输入。那in 不就是null了么 。。 
谢谢回答！