How to continuously send messages between clients?

I have a simple client/server application that groups 2 clients and makes them communicate between eachother and if a third client got connected it would not be able to communicate with the 2 other clients but it would create another group of 2 clients and so on... However the problem occurs when i send a message from a client it doesn't immediately send it over to the other client instead it waits for the first client input and it also waits for the second clients input and only when both clients have entered an input it sends them over to eachother. Any help is very appriciated.

Server:

public class ChatServer {

	Socket previousSocket = null;
	
	public static void main(String[] args) throws Exception {
		ServerSocket serverSocket = new ServerSocket(9001);
		System.out.println("The chat server is running.");
		Socket previousSocket = null;
		while (true) {
		    Socket newSocket = serverSocket.accept();
		    
		    /**
		     * if (previousSocket == null) occurs if only 1 client is connected and its being set as a previousSocket
		     * and we wait for a second client to connect so meanwhile nothing happens
		     * 
		     * if second clients joins in we can start communicating in groups of 2
		     */
		    
		    if (previousSocket == null) { 
		        previousSocket = newSocket;
		    } else {
		        new Handler(previousSocket, newSocket).start();
		        new Handler(newSocket, previousSocket).start();
		        previousSocket = null;
		    }
		}
	}

	
	private static class Handler extends Thread {
	    private String name;
	    private Socket socket;
	    private Socket peerSocket;
	    private DataInputStream in;
	    private DataOutputStream out;

	    public Handler(Socket socket, Socket peerSocket) {
	        this.socket = socket;
	        this.peerSocket = peerSocket;
	    }

	    public void run() {
	        try {
	        	while (true) {
				in = new DataInputStream(socket.getInputStream());
				out = new DataOutputStream(peerSocket.getOutputStream());
				
				
				

					try {
						String input = in.readUTF();
						

				            out.writeUTF(input);
					} catch (IOException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
		           
		        }
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
	        
	    }
	}
}

Client:

public class ChatClient {

	public static void main(String[] args) throws UnknownHostException, IOException {
		new ChatClient();
	}
	
	public ChatClient() throws UnknownHostException, IOException  {
		Socket socket = new Socket("127.0.0.1", 9001);
		
		System.out.println("You can start typing:");
		
		while(true) {
			
			
			Scanner scanner = new Scanner(System.in);
			
			Thread input = new Thread() {
				@Override
				public void run() {
					while(true) {
						try {
							DataInputStream inputStream = new DataInputStream(socket.getInputStream());
							
							while(scanner.hasNextLine()) {
								String message = scanner.nextLine();
								
								System.out.println(message);
								
								//reading messages from server
								String received = inputStream.readUTF();
								System.out.println(received);
							}
						} catch (IOException e) {
							
							e.printStackTrace();
						}
					}
				}
			};
			input.start();
			
			Thread output = new Thread() {
				public void run() {
					try {
						DataOutputStream outputStream = new DataOutputStream(socket.getOutputStream());
						while(scanner.hasNextLine()) {
							String message = scanner.nextLine();
							
							if(message.equalsIgnoreCase("quit")) {
								socket.close();
								break;
							}
							
							outputStream.writeUTF(message);
						}
					} catch (IOException e) {
						
						e.printStackTrace();
					}
					
					
				};
			};
			output.start();

			
		}
	}
}

Remove top level while loop and remove scanner.nextLine() while reading from server, in client.
I have updated your client code below, this should work fine.

public class ChatClient {

    public static void main(String[] args) throws UnknownHostException, IOException {
        new ChatClient();
    }
    
    public ChatClient() throws UnknownHostException, IOException  {
        Socket socket = new Socket("127.0.0.1", 9001);
        
        System.out.println("You can start typing:");
            
            
            Scanner scanner = new Scanner(System.in);
            System.out.println("got it");
            Thread input = new Thread() {
                @Override
                public void run() {
                    while(true) {
                        try {
                            DataInputStream inputStream = new DataInputStream(socket.getInputStream());
                            
                            while(true) {
                                //String message = scanner.nextLine();
                                
                                //System.out.println(message);
                                
                                //reading messages from server
                                String received = inputStream.readUTF();
                                System.out.println(received);
                            }
                        } catch (IOException e) {
                            
                            e.printStackTrace();
                        }
                    }
                }
            };
            input.start();
            
            Thread output = new Thread() {
                public void run() {
                    try {
                        DataOutputStream outputStream = new DataOutputStream(socket.getOutputStream());
                        while(scanner.hasNextLine()) {
                            String message = scanner.nextLine();
                            
                            if(message.equalsIgnoreCase("quit")) {
                                socket.close();
                                break;
                            }
                            
                            outputStream.writeUTF(message);
                        }
                    } catch (IOException e) {
                        
                        e.printStackTrace();
                    }
                    
                    
                };
            };
            output.start();
    }
}

You need to separate the input and output streams in the client into two separate threads. The issue is that your client is waiting for input from the scanner before you read the input. Scanner is a blocking call, so it needs it own thread.

Also, you should move the creation of the data streams in the loop to the constructor...that will increase your efficiency.