How to use setKeepAlive() ?

[Ralf Coby]

Hi,

I have written a little, simple Protocol to test the "setKeepAlive()"-method.

So, a little, simple echo-Server and a client.

I want that the Server keeps the connection to the client stable, wether or not the server has send a message to the client.

So, I try to work with the "KeepAlive()-Method" from the java.net-library.

But I am not able to find any examples with keepAlive() or any documentations.

Here is my example-protocol:

Server:

 ServerSocket serSo = new ServerSocket(25000);
            
            //Server Accept Client            
            Socket accClient = serSo.accept();
     
            InputStream is = accClient.getInputStream();
            ObjectInputStream ois = new ObjectInputStream(is);
            
                   
            String str = (String) ois.readObject();
           
            System.out.println(str);
            
            OutputStream os = accClient.getOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(os);
            
            oos.writeObject(str);
            
            os.flush();
          
            
            //serSo.setSoTimeout(100);
            is.close();
            os.close();

Client:

  Socket toServer = new Socket("127.0.0.1", 25000);
            //toServer.setKeepAlive(true);
            
            
            OutputStream os = toServer.getOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(os);
            
            String str1 = "Hallo";            
            oos.writeObject(str1);            
            oos.flush();            
            
            
            
            //Empfange vom Server
            
            InputStream is = toServer.getInputStream();
            ObjectInputStream ois = new ObjectInputStream(is);
            
            String str2 = (String) ois.readObject();
            
            System.out.println(str2);
            
            is.close();
            os.close();

I hope you can help me.

Thanks in advance

[Stephan van Hulst]

I believe the problem is that you're trying to set KeepAlive after the connection has been established. Try a constructor that doesn't immediately connect to the endpoint (such as the parameterless constructor), call setKeepAlive(), and finally call connect().

[Ralf Coby]

Stephan van Hulst wrote:I believe the problem is that you're trying to set KeepAlive after the connection has been established. Try a constructor that doesn't immediately connect to the endpoint (such as the parameterless constructor), call setKeepAlive(), and finally call connect().

Thanks for your help. I will try it tomorrow and then I will reply.

Greetz

[Ralf Coby]

I have set setKeepAlive() before the connect-method connects to the server.

Surprisingly, on the one hand, a ACK-Flag hasn't been set by invoking the method.

Now, I get the exception below:

  java.net.SocketException: Socket is closed
	at java.net.Socket.getKeepAlive(Socket.java:1325)
	at org.packt.testserver.TestClient.<init>(TestClient.java:34)
	at org.packt.testserver.TestClient.main(TestClient.java:84)
  

So, the client gets the message from the server, which the Client has send.
Then, I think, the client trys to reconnect to the server, but it isn't possible, because the client has closed its InputStreams and OutputStreams.

On the other hand, this exception tells me, that the client trys to get a "keepAlive"-Connection to the server.

So, i have to figure out how to close the Streams in the right moments.

by now, my code looks like this:

Client:

 public class TestClient {
   private Socket toServer;
    public TestClient(){
        try {
            toServer = new Socket();
            SocketAddress soAddrr = new InetSocketAddress("127.0.0.1", 25000);
            toServer.setKeepAlive(true);
            toServer.setSoTimeout(100);
            toServer.connect(soAddrr, 25000);
            connect();
            if(toServer.getKeepAlive()){
                
                            
               if(toServer.getSoTimeout()==50){
               
               new TestClient();
                   
               }
              
              
               
             }
            
        } catch (IOException ex) {
            Logger.getLogger(TestClient.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
    public void connect(){
        try{
            OutputStream os = toServer.getOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(os);
            
            String str1 = "Hallo";            
            oos.writeObject(str1);            
            oos.flush();            
            
            //Empfange vom Server
            
            InputStream is = toServer.getInputStream();
            ObjectInputStream ois = new ObjectInputStream(is);
            
            String str2 = (String) ois.readObject();
      
            System.out.println(str2);
            
            System.out.println(toServer.isConnected());
            if(toServer.getSoTimeout() == 100){
                is.close();
                os.close();
                
            }
            
          
        }catch(Exception e){
            System.out.println(e);
        }
    }
    
    public static void main(String[] args) {
        
      new TestClient();
           // InetAddress ia = InetAddress.getByName("127.0.0.1");
            
        
    }
    
    
}
 

Server:

 

public class TestServer extends Thread{
    private ServerSocket serSo;
     private Socket accClient;
    
    public TestServer(){
        try {
            serSo = new ServerSocket(25000);
           startServer();
            
            
            
        } catch (IOException ex) {
            Logger.getLogger(TestServer.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
    
   
    public void startServer(){
        
        try {
            while(true){
            accClient = serSo.accept();
            start();    
            }
            
            
        } catch (IOException ex) {
            Logger.getLogger(TestServer.class.getName()).log(Level.SEVERE, null, ex);
        }
        
    }
   

    @Override
    public void run() {       
        
         try {
            //Server Accept Client  
           
             
     
            InputStream is = accClient.getInputStream();
            ObjectInputStream ois = new ObjectInputStream(is);
            
            String str = (String) ois.readObject();
        
            System.out.println(str);
            
            OutputStream os = accClient.getOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(os);
            
            oos.writeObject(str);
            
            os.flush();
          
            
            //serSo.setSoTimeout(100);
          os.close();
          is.close();
            
            
            
                    
                    } catch (IOException ex) {
                     Logger.getLogger(TestServer.class.getName()).log(Level.SEVERE, null, ex);
                     } catch (ClassNotFoundException ex) {
                        Logger.getLogger(TestServer.class.getName()).log(Level.SEVERE, null, ex);
               }
        
        
        
    }
     public static void main(String[] args) {
       
        new TestServer();
       
       }
}


  

[Ralf Coby]

RESOLVED!
This works fiiiinnee!
There is no setKeepAlive(), but actually, this code does what I wanna do!

For anyone, who ist interessted about:

Client:

public class TestClientSocket {

	public static void main(String args[]) throws IOException {
		final String host = "localhost";
		final int portNumber = 81;
		System.out.println("Creating socket to '" + host + "' on port " + portNumber);

		while (true) {
			Socket socket = new Socket(host, portNumber);
			BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
			PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

			System.out.println("server says:" + br.readLine());

			BufferedReader userInputBR = new BufferedReader(new InputStreamReader(System.in));
			String userInput = userInputBR.readLine();

			out.println(userInput);

			System.out.println("server says:" + br.readLine());

			if ("exit".equalsIgnoreCase(userInput)) {
				socket.close();
				break;
			}
		}
	}
}

Server:

public class TestServerSocket {

	public static void main(String args[]) throws IOException {
		final int portNumber = 81;
		System.out.println("Creating server socket on port " + portNumber);
		ServerSocket serverSocket = new ServerSocket(portNumber);
		while (true) {
			Socket socket = serverSocket.accept();
			OutputStream os = socket.getOutputStream();
			PrintWriter pw = new PrintWriter(os, true);
			pw.println("What's you name?");

			BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
			String str = br.readLine();

			pw.println("Hello, " + str);
			pw.close();
			socket.close();

			System.out.println("Just said hello to:" + str);
		}
	}
}

[Henry Wong]

Ralf Coby wrote:
For anyone, who ist interessted about:

Thanks for posting the final code. Have a cow !!

Ralf Coby wrote:
There is no setKeepAlive(), but actually, this code does what I wanna do!

The purpose of keep alive is *not* to prevent the other side (application) from closing the connection due to inactivity. After all, you are writing both sides -- if you don't want the other side to close the connection, then simply don't close the connection.

In general, the most common purpose is to prevent the connection from being torn down due to inactivity (by the network). This can happen if the connection is going through lots of routes / firewalls. Another purpose is if the network gets a partial disconnect (one side loses the connection, perhaps via a crash, but the other side doesn't detect it). This mechanism helps the network detect it, so that it can close the connection.

Henry