TCP server and TCP client in java

I don't want to be rude, but you are basing all the responses on 'keywords'. And the application itself is blocking the network port which you are trying to utilize. It would be much easier to use a TCPClient which invokes an acutal function that's accessible on the server side, you can even pass the arguments and get the return value. It's much more flexible and reliable.

Here's the TCPServer code:

public class TCPServer {
/**
 * Class Constants
 */
public static int TCP_CLIENTS_INFINITE = 0;

/**
 * Class Variables
 */
private int serverPort; // Port on which the server will listen for incoming connections
private String serverIP; // IP that the server will be listening on
private int maxClients; // If set to 0 the max client amount is infinite
private int threadCount; // Displays the current amount of running TCP threads, can be interpreted as amount of connected clients
private Class classInstance; // Instance of a class that contains the functions needed for the server to operate
private boolean serverWorking = false; // Indicates if the server is online

/**
 * Class Functions
 */
private ServerSocket serverSocket;

/**
 * TCPServer class constructor
 *
 * @param serverPort           Port on which the server will listen to for incoming connections
 * @param maxClients           Maximum number of connected clients
 * @param serverFunctionsClass Class which contains all the server functions
 */
public TCPServer(int serverPort, int maxClients, Class serverFunctionsClass) {
    try {

        this.serverPort = serverPort;
        System.out.println("Server port has been set to: " + serverPort);
        this.maxClients = maxClients;
        System.out.println("Max connected clients has been set to: " + maxClients);
        this.classInstance = serverFunctionsClass;
        System.out.println("Server functions class has been set");
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

/**
 * Bind the TCP Server Socket to a specific ip address
 *
 * @param ip String representation of an IP Address
 */
public void bindSocketToIP(String ip) {
    try {
        this.serverIP = ip;
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

/**
 * Start the server and begin listening for connections
 */
public void start() {
    try {
        if (!serverWorking) {

            if (serverIP != null) {
                serverSocket = new ServerSocket(serverPort, 50, InetAddress.getByName(serverIP));
            } else {
                serverSocket = new ServerSocket(serverPort);
            }
            System.out.println("TCP Server is now listening for new connections, on port: " + serverSocket);

            serverWorking = true;

            if (maxClients == 0) {
                maxClients = 999;
            }

            while (true) {
                // Open Client socket to handle client connections
                Socket clientSocket = serverSocket.accept();

                if (clientSocket != null && getThreadCount() < getMaxClients()) {
                    new Thread(() -> new TCPClientHandler(clientSocket)).start();
                } else {
                    System.out.println(String.format("Exceeded maximum connected clients limit: %d / %d", getThreadCount(), getMaxClients()));
                }
            }
        } else {
            return;
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

/**
 * Stop the server
 */
public void stop() {
    try {
        serverSocket.close();
        serverWorking = false;
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

/**
 * @return Current amount of connected clients a.k.a running threads
 */
public synchronized int getThreadCount() {
    return threadCount;
}

/**
 * @return Defined number of maximum clients accepted by the server at any given time
 */
public synchronized int getMaxClients() {
    return maxClients;
}


/**
 * Class which processes the client requests
 */
class TCPClientHandler {

    /**
     * Handle Client in a separate thread
     *
     * @param clientSocket TCP ServerSocket instance
     */
    public TCPClientHandler(Socket clientSocket) {

        // Inner Variables
        String clientSentence = null, clientIP = null;
        ObjectOutputStream toClient = null;
        ObjectInputStream fromClient = null;
        Object[] argumentsObjectArray;
        ArrayList argumentsCollectionArray;

        try {
            if (clientSocket.isConnected() && !clientSocket.isClosed()) {

                // Incline the threadNumber
                threadCount++;

                // Get connection input stream
                fromClient = new ObjectInputStream(clientSocket.getInputStream());

                // Get connection output stream
                toClient = new ObjectOutputStream(clientSocket.getOutputStream());

                while (true) {
                    try {
                        // Process fromClient
                        argumentsObjectArray = (Object[]) fromClient.readObject();

                        // Read client sentence
                        clientSentence = (String) argumentsObjectArray[0];

                        // Remove the client Sentence from argument array
                        argumentsCollectionArray = new ArrayList(Arrays.asList(argumentsObjectArray));
                        Collection c = argumentsCollectionArray;
                        c.remove(clientSentence);
                        argumentsObjectArray = c.toArray();

                        // Print was was received by the server
                        System.out.println("Function to be invoked: '" + clientSentence + "'");

                        for (Object o : argumentsObjectArray) {
                            System.out.println("Parameter: '" + o + "' Type: '" + o.getClass().getTypeName() + "'");
                        }

                        // Get Client IP Address
                        clientIP = clientSocket.getInetAddress().toString();

                        // Write data to client
                        toClient.writeObject(processServerFunction(clientSentence, argumentsObjectArray));
                    } catch (Exception ex) {
                        System.out.println("Connection with client: " + clientIP + ", has been terminated, closing sockets..." + '\n');
                        if (clientSocket != null) {
                            try {
                                clientSocket.close();
                            } catch (Exception e) {
                                // DO NOTHING
                            }
                        }
                        if (fromClient != null) {
                            try {
                                fromClient.close();
                            } catch (Exception e) {
                                // DO NOTHING
                            }
                        }
                        if (toClient != null) {
                            try {
                                toClient.close();
                            } catch (Exception e) {
                                // DO NOTHING
                            }
                        }
                        threadCount--;
                        break;
                    }

                }
            }
        } catch (Exception ex) {
            try {
                System.out.println("Connection with client: " + clientIP + ", has been terminated, closing sockets..." + '\n');
                if (clientSocket != null) {
                    try {
                        clientSocket.close();
                    } catch (Exception e) {
                        // DO NOTHING
                    }
                }
                if (fromClient != null) {
                    try {
                        fromClient.close();
                    } catch (Exception e) {
                        // DO NOTHING
                    }
                }
                if (toClient != null) {
                    try {
                        toClient.close();
                    } catch (Exception e) {
                        // DO NOTHING
                    }
                }
                threadCount--;
                ex.printStackTrace();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * Process Server Function, by it's String name
     *
     * @param functionName Name of the function or method interpreted as a String
     * @param args         Array of objects that which represent both function name and it's arguments if any
     * @return String value received from a invoked function
     */
    public Object processServerFunction(String functionName, Object[] args) {
        try {

            Method functionInvoker;
            Object clientEcho;

            if (args.length >= 1) {
                functionInvoker = classInstance.getDeclaredMethod(functionName, getParameterTypes(classInstance, functionName));
                clientEcho = functionInvoker.invoke(classInstance, args);
            } else {
                functionInvoker = classInstance.getDeclaredMethod(functionName);
                clientEcho = functionInvoker.invoke(classInstance);
            }

            if (clientEcho != null) {
                System.out.println("Sending to client: " + clientEcho);
            }
            return clientEcho;
        } catch (Exception ex) {
            ex.printStackTrace();
            System.out.println("Client requested function does not exists on the server");
            return "Client requested function does not exists on the server";
        }
    }

    /**
     * Get all the function parameter types dynamically
     *
     * @param aClass     class in which the function is located
     * @param methodName function name
     * @return Class[] of parameter types
     */
    private Class<?>[] getParameterTypes(Class<? extends Object> aClass, String methodName) {
        try {
            for (Method m : aClass.getDeclaredMethods()) {
                if (m.getName().equals(methodName)) {
                    return m.getParameterTypes();
                }
            }

            // If nothing have been found return null value
            return null;

        } catch (Exception ex) {
            ex.printStackTrace();
            return null;
        }
    }
}

Here's the TCPClient code:

public class TCPClient {
/**
 * Variables
 */
private String serverIP;
private int serverPort;
private Socket clientSocket;
private boolean connected = false;
private ObjectOutputStream outToServer;
private ObjectInputStream inFromServer;

/**
 * Class constructor
 *
 * @param serverIP   String value that's representing the server ip
 * @param serverPort Integer value that represents server port
 */
public TCPClient(String serverIP, int serverPort) {
    try {
        this.serverIP = serverIP;
        this.serverPort = serverPort;
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

/**
 * Establish connection with the specified TCP server
 */
public void connect() {
    try {
        if (!connected) {
            try {
                System.out.println(String.format("Trying to connect to: %s on port: %d", getServerIP(), getServerPort()));
                clientSocket = new Socket(getServerIP(), getServerPort());
                System.out.println("Connected !");
                connected = true;
                // Create ObjectStreams
                outToServer = new ObjectOutputStream(clientSocket.getOutputStream());
                inFromServer = new ObjectInputStream(clientSocket.getInputStream());
            } catch (Exception e) {
                connected = false;
                if (clientSocket != null) {
                    clientSocket.close();
                    System.out.println(String.format("Could not connect to: %s on port: %d", getServerIP(), getServerPort()));
                }
            }
        } else {
            return;
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

/**
 * Disconnect client for the currently connected server
 */
public void disconnect() {
    try {
        if (connected) {
            clientSocket.close();
            connected = false;
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

/**
 * Invoke server side functions
 *
 * @param serverSideFunction Object representing both function name and its parameters
 * @return String vale of a server side function return value
 */
public Object invokeFunction(Object[] serverSideFunction) {
    try {
        if (clientSocket != null && connected) {
            outToServer.writeObject(serverSideFunction);
            Object modifiedSentence = inFromServer.readObject();
            if (modifiedSentence != null) {
                System.out.println("Received from server: " + modifiedSentence);
            }
            return modifiedSentence;
        }
        return null;
    } catch (Exception ex) {
        ex.printStackTrace();
        return null;
    }
}

/**
 * Get server ip data
 *
 * @return String value of Server IP
 */
public String getServerIP() {
    return serverIP;
}

/**
 * Get server port data
 *
 * @return Integer value of Server Port
 */
public int getServerPort() {
    return serverPort;
 }

Here's how to use it:

1. Create a class that will hold all the server-side functions and methods, call it MyFuntions or something
   2. Create instance of TCPServer and provide the needed data, then start the server on a new Thread
   3. Create the TCPClient instance, provide all the data and call .connect()
   4. Use TCPClient.invokeFunction() to invoke server-side function

Here's example code:

// This is how you start the server, yes it's that simple, just don't start it //everytime you call a function. Start it once.

TCPServer tcpServer = new TCPServer(6969, 0, MyFuntions.class);

new Thread(() -> {
   tcpServer.start();
}).start();


// Now create a TCPClient instance
TCPClient tcpClient = new TCPClient("Server IP as String", 6969);

tcpClient.connect(); // You can call it in a different Thread if you want

/*
Now lets say you have a server side function that's called: 'doMath(int x, int y)'

what you want to do to call it, and pass parameters is quite simple:
*/

tcpClient.invokeFunction(new Object[]{"doMath", 5, 10);

/*
The server will invoke this function do what you wanted with those number 
and if you want to return the outcome to the client it will do so aswell.

Just remember that the server-side functions and methods have to be public 
and static
*/

The code should work, I was writing it using Notepad++ so that's why exceptions >are a bit generic since I can't remeber which i should use without the IDE.