Question

Why am I getting "java.net.BindException: Address already in use: NET_Bind" error?

My code is below:

public ServerSocket server = null;
  
   public static void run() throws FileNotFoundException{
      
       PrintWriter logFile = new PrintWriter("C:OutFile.log.txt");
       var date = new java.util.Date();
       var nDate = date.toString();
      
       try {
       ServerSocket ss = new ServerSocket(1234);
       Socket s = ss.accept();
       //log connection time
       logFile.printf("Got a Connection: ", nDate);
      
       //listen on 2nd socket
       ServerSocket nn = new ServerSocket(2345);
       Socket n = nn.accept();
       //log connection time
       logFile.printf("Got a Connection: ", nDate);
         
       DataInputStream dataIn = new DataInputStream(s.getInputStream());
       DataOutputStream dataOut = new DataOutputStream(s.getOutputStream());
      
         
       String str = dataIn.readUTF();
       String cstr=null;
       // loop exit when it find "quit" in input
       while(!str.equals("quit")) {
           cstr = CipherServer.PoyAlphabet.cipher(cstr);
       dataOut.writeUTF(cstr);
       dataOut.flush();
       str = dataIn.readUTF();
       }
       dataOut.writeUTF("Good Bye!");
       s.close();
       ss.close();
       //log disconnect time
       logFile.printf("Connection Closed: ", nDate);
       logFile.close();
      
       } catch (IOException e) {
       // TODO Auto-generated catch block
       e.printStackTrace();
       }
      
       }

Answer 1

java.net.BindException: Address already in use: in Java occurs because you are trying to access a port which is already in use by another program. In your case the port number 2345 you have used might be being used by another program.

So one easy solution is to change the port number in both the places in your program and try again. For example try with another port number 73501 etc.

Another solution is to find out the program which is using the port you are using and kill that program. This will leave the port free for your use.

In windows to find out which program is using the port you require open the command program by pressing windows key and typing cmd to open command prompt. Then use command netstat -ano | find "2345"

In linux use the command netstat -nap | grep 2345

Note that even a previous instant of your own program might be running in another command prompt which is preventing you from accessing the port.

Even simply restarting the system can sometimes solve the issue if the program is not a startup program.