Implementar Tranferencia de Arquivos

Olá, me chamo Carlos,

Fiz este chat Multi usuários baseado em tutoriais na internet, mas agora estou tentando implementar nele a transferência de arquivos, mas nada que eu tento dá certo, sei que é algo um pouco amplo demais pra perguntar, e desculpa a quantidade de códigos no post não sei se pode postar o programa inteiro assim, mas se alguém puder me ajudar a colocar essa transferência de arquivos eu agradeço.

Aplicação Cliente :

Classe Cliente:

package com.tiete.app;

import com.tiete.app.frame.ClienteFrame;


/**
 *
 * @author Carlos Santana
 */
public class Cliente {

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new ClienteFrame().setVisible(true);
            }
        });
    }
}

Classe ChatMessage:

package com.tiete.app.bean;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

/**
 *
 * @author Carlos Santana
 */
public class ChatMessage implements Serializable {
    
    private String name;
    private String text;
    private String nameReserved;
    private Set<String> setOnlines;
    private Action action;

    public ChatMessage() {
        this.setOnlines = new HashSet<>();
    }
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getNameReserved() {
        return nameReserved;
    }

    public void setNameReserved(String nameReserved) {
        this.nameReserved = nameReserved;
    }

    public Set<String> getSetOnlines() {
        return setOnlines;
    }

    public void setSetOnlines(Set<String> setOnlines) {
        this.setOnlines = setOnlines;
    }

    public Action getAction() {
        return action;
    }

    public void setAction(Action action) {
        this.action = action;
    }
        
    public enum Action {
        CONNECT, DISCONNECT, SEND_ONE, SEND_ALL, USERS_ONLINE
    }
}

Classe ClienteFrame:

package com.tiete.app.frame;

import com.tiete.app.bean.ChatMessage;
import com.tiete.app.bean.ChatMessage.Action;
import com.tiete.app.bean.FileMessage;
import com.tiete.app.service.ClienteService;
import java.io.File;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.util.Set;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.*;

/**
 *
 * @author Carlos Santana
 */
public class ClienteFrame extends javax.swing.JFrame {

    private Socket socket;
    private ChatMessage message;
    private ClienteService service;
    private ObjectOutputStream output;
   
    /**
     * Creates new form ClienteFrame
     */
    public ClienteFrame() {
        initComponents();
    }

    private class ListenerSocket implements Runnable {

        private ObjectInputStream input;
        

        public ListenerSocket(Socket socket) {
            try {
                this.input = new ObjectInputStream(socket.getInputStream());
            } catch (IOException ex) {
                Logger.getLogger(ClienteFrame.class.getName()).log(Level.SEVERE, null, ex);
            }
        }

        @Override
        public void run() {
            ChatMessage message = null;
            try {
                while ((message = (ChatMessage) input.readObject()) != null) {
                    Action action = message.getAction();

                    switch (action) {
                        case CONNECT:
                            connected(message);
                            break;
                        case DISCONNECT:
                            disconnected(); //Chama Metodo de Desconexão
                            socket.close(); //Fecha o Socket
                            break;
                        case SEND_ONE:
                            System.out.println("::: " + message.getText() + " :::");
                            receive(message);
                            break;
                        case USERS_ONLINE:
                            refreshOnlines(message);
                            break;
                        default:
                            break;
                    }
                }
            } catch (IOException | ClassNotFoundException ex) {
                Logger.getLogger(ClienteFrame.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    private void connected(ChatMessage message) {
        if (message.getText().equals("Rejeitado")) {
            this.txtName.setText("");
            JOptionPane.showMessageDialog(this, "Conexão não realizada!\nTente novamente com um novo usuário.");
            return;
        }

        this.message = message;
        this.btnConnectar.setEnabled(false);
        this.txtName.setEditable(false);

        this.btnSair.setEnabled(true);
        this.txtAreaSend.setEnabled(true);
        this.txtAreaReceive.setEnabled(true);
        this.btnEnviar.setEnabled(true);
        this.btnLimpar.setEnabled(true);
        this.btnFile.setEnabled(true);
        
        
        JOptionPane.showMessageDialog(this, "Conectado com sucesso !");
    }

    private void disconnected() {

        this.btnConnectar.setEnabled(true);
        this.txtName.setEditable(true);

        this.btnSair.setEnabled(false);
        this.txtAreaSend.setEnabled(false);
        this.txtAreaReceive.setEnabled(false);
        this.btnEnviar.setEnabled(false);
        this.btnLimpar.setEnabled(false);
        this.btnFile.setEnabled(false);
        
        this.txtAreaReceive.setText("");
        this.txtAreaSend.setText("");

        JOptionPane.showMessageDialog(this, "Desconectado com sucesso !");
    }

    private void receive(ChatMessage message) {
        this.txtAreaReceive.append(message.getName() + " diz: \n" + message.getText() + "\n");
    }

    private void refreshOnlines(ChatMessage message) {
        System.out.println(message.getSetOnlines().toString());
        
        Set<String> names = message.getSetOnlines();
        
        names.remove(message.getName());
        
        String[] array = (String[]) names.toArray(new String[names.size()]);
        
        this.listOnlines.setListData(array);
        this.listOnlines.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        this.listOnlines.setLayoutOrientation(JList.VERTICAL);
    }

    
    
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jPanel1 = new javax.swing.JPanel();
        txtName = new javax.swing.JTextField();
        btnConnectar = new javax.swing.JButton();
        btnSair = new javax.swing.JButton();
        jPanel2 = new javax.swing.JPanel();
        jScrollPane3 = new javax.swing.JScrollPane();
        listOnlines = new javax.swing.JList();
        jPanel3 = new javax.swing.JPanel();
        jScrollPane1 = new javax.swing.JScrollPane();
        txtAreaReceive = new javax.swing.JTextArea();
        jScrollPane2 = new javax.swing.JScrollPane();
        txtAreaSend = new javax.swing.JTextArea();
        btnEnviar = new javax.swing.JButton();
        btnLimpar = new javax.swing.JButton();
        btnFile = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Tietê Com v 2.5");
        setBackground(new java.awt.Color(0, 204, 204));
        setResizable(false);

        jPanel1.setBorder(javax.swing.BorderFactory.createTitledBorder("Conectar"));

        btnConnectar.setText("Conectar");
        btnConnectar.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnConnectarActionPerformed(evt);
            }
        });

        btnSair.setText("Sair");
        btnSair.setEnabled(false);
        btnSair.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnSairActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(txtName)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(btnConnectar)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(btnSair)
                .addContainerGap())
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(txtName, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addComponent(btnConnectar)
                .addComponent(btnSair))
        );

        jPanel2.setBorder(javax.swing.BorderFactory.createTitledBorder("Online"));

        jScrollPane3.setViewportView(listOnlines);

        javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
        jPanel2.setLayout(jPanel2Layout);
        jPanel2Layout.setHorizontalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel2Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jScrollPane3, javax.swing.GroupLayout.DEFAULT_SIZE, 128, Short.MAX_VALUE)
                .addContainerGap())
        );
        jPanel2Layout.setVerticalGroup(
            jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jScrollPane3)
        );

        jPanel3.setBorder(javax.swing.BorderFactory.createEtchedBorder());

        txtAreaReceive.setEditable(false);
        txtAreaReceive.setColumns(20);
        txtAreaReceive.setRows(5);
        txtAreaReceive.setEnabled(false);
        jScrollPane1.setViewportView(txtAreaReceive);

        txtAreaSend.setColumns(20);
        txtAreaSend.setRows(5);
        txtAreaSend.setEnabled(false);
        jScrollPane2.setViewportView(txtAreaSend);

        btnEnviar.setText("Enviar");
        btnEnviar.setEnabled(false);
        btnEnviar.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnEnviarActionPerformed(evt);
            }
        });

        btnLimpar.setText("Limpar");
        btnLimpar.setEnabled(false);
        btnLimpar.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnLimparActionPerformed(evt);
            }
        });

        btnFile.setText("Enviar Arquivo");
        btnFile.setEnabled(false);
        btnFile.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnFileActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout jPanel3Layout = new javax.swing.GroupLayout(jPanel3);
        jPanel3.setLayout(jPanel3Layout);
        jPanel3Layout.setHorizontalGroup(
            jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel3Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jScrollPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 639, Short.MAX_VALUE)
                    .addComponent(jScrollPane2)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel3Layout.createSequentialGroup()
                        .addComponent(btnFile)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addComponent(btnLimpar)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(btnEnviar)))
                .addContainerGap())
        );
        jPanel3Layout.setVerticalGroup(
            jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel3Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jScrollPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 407, Short.MAX_VALUE)
                .addGap(18, 18, 18)
                .addComponent(jScrollPane2, javax.swing.GroupLayout.PREFERRED_SIZE, 66, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel3Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(btnEnviar)
                    .addComponent(btnLimpar)
                    .addComponent(btnFile))
                .addContainerGap())
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(jPanel3, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 185, Short.MAX_VALUE)
                .addComponent(jPanel2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jPanel3, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                    .addComponent(jPanel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                .addContainerGap())
        );

        pack();
    }// </editor-fold>                        

    private void btnConnectarActionPerformed(java.awt.event.ActionEvent evt) {                                             
        String name = this.txtName.getText();

        if (!name.isEmpty()) {
            this.message = new ChatMessage();
            this.message.setAction(Action.CONNECT);
            this.message.setName(name);

            this.service = new ClienteService();
            this.socket = this.service.connect();

            new Thread(new ListenerSocket(this.socket)).start();

            this.service.send(message);
        }
    }                                            

    private void btnSairActionPerformed(java.awt.event.ActionEvent evt) {                                        
        ChatMessage message = new ChatMessage();
        message.setName(this.message.getName());
        message.setAction(Action.DISCONNECT);
        this.service.send(message);
        disconnected();
    }                                       

    private void btnLimparActionPerformed(java.awt.event.ActionEvent evt) {                                          
        this.txtAreaSend.setText("");
    }                                         

    private void btnEnviarActionPerformed(java.awt.event.ActionEvent evt) {                                          
        String text = this.txtAreaSend.getText();
        String name = this.message.getName();
        
        this.message = new ChatMessage();
        
        if (this.listOnlines.getSelectedIndex() > -1) {
            this.message.setNameReserved((String) this.listOnlines.getSelectedValue());
            this.message.setAction(Action.SEND_ONE);
            this.listOnlines.clearSelection();
        } else {
            this.message.setAction(Action.SEND_ALL);
        }
        
        if (!text.isEmpty()) {
            this.message.setName(name);
            this.message.setText(text);

            this.txtAreaReceive.append("Você disse: \n" + text + "\n");
            
            this.service.send(this.message);
        }
        
        this.txtAreaSend.setText("");
    }                                         

    private void btnFileActionPerformed(java.awt.event.ActionEvent evt) {                                        
        
       
        JFileChooser ch = new JFileChooser();
        
        String nome = message.getName();
        int op = ch.showOpenDialog(null);
        
        if(op == JFileChooser.APPROVE_OPTION){
            
            try {
                File file = ch.getSelectedFile();
                
                this.output = new ObjectOutputStream(socket.getOutputStream());
                this.output.writeObject(new FileMessage(nome, file));
            } catch (IOException ex) {
                Logger.getLogger(ClienteFrame.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
        
        
        
    }                                       

    // Variables declaration - do not modify                     
    private javax.swing.JButton btnConnectar;
    private javax.swing.JButton btnEnviar;
    private javax.swing.JButton btnFile;
    private javax.swing.JButton btnLimpar;
    private javax.swing.JButton btnSair;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JPanel jPanel2;
    private javax.swing.JPanel jPanel3;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JScrollPane jScrollPane2;
    private javax.swing.JScrollPane jScrollPane3;
    private javax.swing.JList listOnlines;
    private javax.swing.JTextArea txtAreaReceive;
    private javax.swing.JTextArea txtAreaSend;
    private javax.swing.JTextField txtName;
    // End of variables declaration                   

}

Classe ClienteService:

package com.tiete.app.service;

import com.tiete.app.bean.ChatMessage;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author Carlos Santana
 */
public class ClienteService {
    
    private Socket socket;
    private ObjectOutputStream output;
    
    public Socket connect() {
        try {
            this.socket = new Socket("localhost", 5555);
            this.output = new ObjectOutputStream(socket.getOutputStream());
        } catch (UnknownHostException ex) {
            Logger.getLogger(ClienteService.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IOException ex) {
            Logger.getLogger(ClienteService.class.getName()).log(Level.SEVERE, null, ex);
        }
        
        return socket;
    }
    
    public void send(ChatMessage message) {
        try {
            output.writeObject(message);
        } catch (IOException ex) {
            Logger.getLogger(ClienteService.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

Aplicação Servidor:

Classe Servidor:

package com.tiete.app;

import com.tiete.app.service.ServidorService;

/**
 *
 * @author Carlos Santana
 */
public class Servidor {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        new ServidorService();
    }
}

Classe ServidorService:

package com.tiete.app.service;

import com.tiete.app.bean.ChatMessage;
import com.tiete.app.bean.ChatMessage.Action;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JOptionPane;


/**
 * @author Carlos Santana
 */
public class ServidorService {

    private ServerSocket serverSocket;
    private Socket socket;
    private Map<String, ObjectOutputStream> mapOnlines = new HashMap<String, ObjectOutputStream>();

    public ServidorService() {
        try {
            serverSocket = new ServerSocket(5555);

            JOptionPane.showMessageDialog(null, "Servidor online!");

            while (true) {
                socket = serverSocket.accept();

                new Thread(new ListenerSocket(socket)).start();
            }

        } catch (IOException ex) {
            Logger.getLogger(ServidorService.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    private class ListenerSocket implements Runnable {

        private ObjectOutputStream output;
        private ObjectInputStream input;

        public ListenerSocket(Socket socket) {
            try {
                this.output = new ObjectOutputStream(socket.getOutputStream());
                this.input = new ObjectInputStream(socket.getInputStream());
            } catch (IOException ex) {
                Logger.getLogger(ServidorService.class.getName()).log(Level.SEVERE, null, ex);
            }
        }

        @Override
        public void run() {
            ChatMessage message = null;
            try {
                while ((message = (ChatMessage) input.readObject()) != null) {
                    Action action = message.getAction();

                    switch (action) {
                        case CONNECT:
                            boolean isConnect = connect(message, output);
                            if (isConnect) {
                                mapOnlines.put(message.getName(), output);
                                sendOnlines();
                            }   break;
                        case DISCONNECT:
                            disconnect(message, output);
                            sendOnlines();
                            return;
                        case SEND_ONE:
                            sendOne(message);
                            break;
                        case SEND_ALL:
                            sendAll(message);
                            break;
                        default:
                            break;
                    }
                }
            } catch (IOException ex) {
                ChatMessage cm = new ChatMessage();
                cm.setName(message.getName());
                disconnect(cm, output);
                sendOnlines();
                System.out.println(message.getName() + " Desconectou!");
            } catch (ClassNotFoundException ex) {
                Logger.getLogger(ServidorService.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    private boolean connect(ChatMessage message, ObjectOutputStream output) {
        if (mapOnlines.isEmpty()) {
            message.setText("Aceito");
            send(message, output);
            message.setText(" Usuário Conectou - se!");

            message.setAction(Action.SEND_ONE);

            sendAll(message);
            return true;
        }

        if (mapOnlines.containsKey(message.getName())) {
            message.setText("Rejeitado");
            send(message, output);
            return false;
        } else {
            message.setText("Aceito");
            send(message, output);
            message.setText(" Usuário Conectou - se!");

            message.setAction(Action.SEND_ONE);

            sendAll(message);
            return true;
        }
    }

    private void disconnect(ChatMessage message, ObjectOutputStream output) {
        mapOnlines.remove(message.getName());

        message.setText(" Desconectou-se");

        message.setAction(Action.SEND_ONE);

        sendAll(message);

        System.out.println("Usuario " + message.getName() + " saiu da sala");
    }

    private void send(ChatMessage message, ObjectOutputStream output) {
        try {
            output.writeObject(message);
        } catch (IOException ex) {
            Logger.getLogger(ServidorService.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    private void sendOne(ChatMessage message) {
        for (Map.Entry<String, ObjectOutputStream> kv : mapOnlines.entrySet()) {
            if (kv.getKey().equals(message.getNameReserved())) {
                try {
                    kv.getValue().writeObject(message);
                } catch (IOException ex) {
                    Logger.getLogger(ServidorService.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }
    }

    private void sendAll(ChatMessage message) {
        for (Map.Entry<String, ObjectOutputStream> kv : mapOnlines.entrySet()) {
            if (!kv.getKey().equals(message.getName())) {
                message.setAction(Action.SEND_ONE);
                try {
                    kv.getValue().writeObject(message);
                } catch (IOException ex) {
                    Logger.getLogger(ServidorService.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }
    }

    private void sendOnlines() {
        Set<String> setNames = new HashSet<>();
        for (Map.Entry<String, ObjectOutputStream> kv : mapOnlines.entrySet()) {
            setNames.add(kv.getKey());
        }

        ChatMessage message = new ChatMessage();
        message.setAction(Action.USERS_ONLINE);
        message.setSetOnlines(setNames);

        for (Map.Entry<String, ObjectOutputStream> kv : mapOnlines.entrySet()) {
            message.setName(kv.getKey());
            try {
                kv.getValue().writeObject(message);
            } catch (IOException ex) {
                Logger.getLogger(ServidorService.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }
}

Classe ChatMessage (do Servidor):

package com.tiete.app.bean;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

/**
 *
 * @author Carlos Santana
 */
public class ChatMessage implements Serializable {
    
    private String name;
    private String text;
    private String nameReserved;
    private Set<String> setOnlines;
    private Action action;

    public ChatMessage() {
        this.setOnlines = new HashSet<>();
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getNameReserved() {
        return nameReserved;
    }

    public void setNameReserved(String nameReserved) {
        this.nameReserved = nameReserved;
    }

    public Set<String> getSetOnlines() {
        return setOnlines;
    }

    public void setSetOnlines(Set<String> setOnlines) {
        this.setOnlines = setOnlines;
    }

    public Action getAction() {
        return action;
    }

    public void setAction(Action action) {
        this.action = action;
    }
        
    public enum Action {
        CONNECT, DISCONNECT, SEND_ONE, SEND_ALL, USERS_ONLINE
    }
}

Topicos relacionados