Добавил логгирование с помощью log4j2.

resources/log4j2.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <properties>
+        <Property name="basePath">logs</Property>
+    </properties>
+
+    <appenders>
+        <Console name="Console" target="SYSTEM_OUT">
+            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
+        </Console>
+
+        <File name="LogFile" fileName="${basePath}/app.log">
+            <PatternLayout pattern="%d{yyyy-mm-dd HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
+        </File>
+    </appenders>
+
+    <loggers>
+        <root level="debug">
+            <appender-ref ref="Console"/>
+            <appender-ref ref="LogFile"/>
+        </root>
+    </loggers>
+</configuration>

src/logging_testing_6/ChatServer.java

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+package logging_testing_6;
+
+
+import logging_testing_6.auth.AuthService;
+import logging_testing_6.auth.AuthServiceImpl;
+import logging_testing_6.gui.Message;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+//import org.slf4j.Logger;
+//import org.slf4j.LoggerFactory;
+
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.util.*;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public class ChatServer {
+
+    //    private static final String USER_CONNECTED_PATTERN = "/userconn";
+//    private static final String USER_DISCONN_PATTERN = "/userdissconn";
+    private static final Pattern AUTH_PATTERN = Pattern.compile("^/auth (\\w+) (\\w+)$");
+
+    private AuthService authService = new AuthServiceImpl();
+
+    private Map<String, ClientHandler> clientHandlerMap = Collections.synchronizedMap(new HashMap<>());
+    private ExecutorService executorService;
+
+    //    private static final Logger logger = LoggerFactory.getLogger(ChatServer.class);
+    private static final Logger log = LogManager.getLogger(ChatServer.class);
+
+    public static void main(String[] args) {
+        ChatServer chatServer = new ChatServer();
+        chatServer.start(7777);
+    }
+
+    public void start(int port) {
+        executorService = Executors.newCachedThreadPool();
+        try (ServerSocket serverSocket = new ServerSocket(port)) {
+            log.info("Server started!");
+            while (true) {
+                Socket socket = serverSocket.accept();
+                DataInputStream inp = new DataInputStream(socket.getInputStream());
+                DataOutputStream out = new DataOutputStream(socket.getOutputStream());
+                log.info("New client connected!");
+
+                try {
+                    String authMessage = inp.readUTF();
+                    Matcher matcher = AUTH_PATTERN.matcher(authMessage);
+                    if (matcher.matches()) {
+                        String username = matcher.group(1);
+                        String password = matcher.group(2);
+                        if (authService.authUser(username, password)) {
+                            clientHandlerMap.put(username, new ClientHandler(username, socket, this));
+                            out.writeUTF("/auth successful");
+                            out.flush();
+                            broadcastUserConnected();
+
+                            log.info("Authorization for user {} %s successful", username);
+                        } else {
+                            log.info("Authorization for user {} failed", username);
+                            out.writeUTF("/auth fails");
+                            out.flush();
+                            socket.close();
+                        }
+                    } else {
+                        log.info("Incorrect authorization message {}", authMessage);
+                        out.writeUTF("/auth fails");
+                        out.flush();
+                        socket.close();
+                    }
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
+            }
+        } catch (IOException e) {
+            e.printStackTrace();
+        } finally {
+//            authService.close();
+            executorService.shutdown();
+        }
+    }
+
+    public void sendMessage(String userTo, String userFrom, String msg) throws IOException {
+        ClientHandler userToClientHandler = clientHandlerMap.get(userTo);
+        if (userToClientHandler != null) {
+            userToClientHandler.sendMessage(userFrom, msg);
+        } else {
+            log.debug("User {} not found. Message from {} is lost.", userTo, userFrom);
+        }
+    }
+
+    public ExecutorService getExecutorService() {
+        return executorService;
+    }
+
+    public void sendMessage(Message message) throws IOException {
+        String userTo = message.getUserTo();
+        String userFrom = message.getUserFrom();
+        String msg = message.getText();
+        ClientHandler userToClientHandler = clientHandlerMap.get(userTo);
+        if (userToClientHandler != null) {
+            userToClientHandler.sendMessage(userFrom, msg);
+        } else {
+            log.debug("User {} not found. Message from {} is lost.", userTo, userFrom);
+        }
+    }
+
+    public List<String> getUserList() {
+        return new ArrayList<>(clientHandlerMap.keySet());
+    }
+
+    public void unsubscribeClient(ClientHandler clientHandler) {
+        clientHandlerMap.remove(clientHandler.getUsername());
+        broadcastUserDisconnected();
+    }
+
+    public void broadcastUserConnected() {
+        // TODO сообщать о том, что конкретный пользователь подключился
+    }
+
+    public void broadcastUserDisconnected() {
+        // TODO сообщать о том, что конкретный пользователь отключился
+    }
+}

src/logging_testing_6/ClientHandler.java

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+package logging_testing_6;
+
+import logging_testing_6.gui.Message;
+import logging_testing_6.history.HistoryKeeper;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.net.Socket;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+
+public class ClientHandler {
+
+    private static final Pattern MESSAGE_PATTERN = Pattern.compile("^/w (\\w+) (.+)", Pattern.MULTILINE);
+    private static final String MESSAGE_SEND_PATTERN = "/w %s %s";
+    private static final Logger log = LogManager.getLogger(ClientHandler.class);
+
+
+//    private final Thread handleThread;
+    private final DataInputStream inp;
+    private final DataOutputStream out;
+    private final ChatServer server;
+    private final String username;
+    private final Socket socket;
+
+    public ClientHandler(String username, Socket socket, ChatServer server) throws IOException {
+        this.username = username;
+        this.socket = socket;
+        this.server = server;
+        this.inp = new DataInputStream(socket.getInputStream());
+        this.out = new DataOutputStream(socket.getOutputStream());
+
+        server.getExecutorService().submit(() -> {
+            try {
+                while (!Thread.currentThread().isInterrupted()) {
+                    String msg = inp.readUTF();
+                    log.info("Message from user {}: {}", username, msg);
+
+                    Matcher matcher = MESSAGE_PATTERN.matcher(msg);
+                    if (matcher.matches()) {
+                        String userTo = matcher.group(1);
+                        String messageText = matcher.group(2);
+                        Message message = new Message(username, userTo, messageText);
+                        server.sendMessage(message);
+
+                        // Добавляем сообщения в файл истории отправителя
+                        HistoryKeeper.saveMassageToHistoryFrom(message);
+                        // Добавляем сообщения в файл истории получателя
+                        HistoryKeeper.saveMassageToHistoryTo(message);
+                    }
+                }
+            } catch (IOException e) {
+                e.printStackTrace();
+            } finally {
+                log.info("Client {} disconnected", username);
+                try {
+                    socket.close();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
+                server.unsubscribeClient(ClientHandler.this);
+            }
+        });
+    }
+
+    public void sendMessage(String userTo, String msg) throws IOException {
+        out.writeUTF(String.format(MESSAGE_SEND_PATTERN, userTo, msg));
+    }
+
+    public String getUsername() {
+        return username;
+    }
+}

src/logging_testing_6/auth/AuthService.java

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+package logging_testing_6.auth;
+
+public interface AuthService {
+
+    boolean authUser(String username, String password);
+}

src/logging_testing_6/auth/AuthServiceImpl.java

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+package logging_testing_6.auth;
+
+import logging_testing_6.dbconn.JdbcExample;
+
+public class AuthServiceImpl implements AuthService {
+
+    @Override
+    public boolean authUser(String username, String password) {
+        String pwd = JdbcExample.readFromDB(username);
+
+        return pwd != null && pwd.equals(password);
+    }
+}

src/logging_testing_6/dbconn/JdbcExample.java

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+package logging_testing_6.dbconn;
+
+import java.sql.*;
+
+public class JdbcExample {
+    private static final String URL = "jdbc:sqlite:database.db";
+
+    static {
+        try {
+            Class.forName("org.sqlite.JDBC");
+        } catch (ClassNotFoundException e) {
+            e.printStackTrace();
+        }
+    }
+
+    public static String readFromDB(String clientName) {
+        String password = null;
+        try (Connection connection = DriverManager.getConnection(URL)) {
+            PreparedStatement preparedStatement = connection.prepareStatement("SELECT u.name, p.password FROM users u INNER JOIN passwords p on u.id = p.id WHERE u.name = ?");
+            preparedStatement.setString(1, clientName);
+            ResultSet resultSet = preparedStatement.executeQuery();
+            while (resultSet.next()) {
+//                String name = resultSet.getString("name");
+                password = resultSet.getString("password");
+            }
+        } catch (SQLException e) {
+            e.printStackTrace();
+        }
+        System.out.println("returned password: " + password);
+        return password;
+    }
+
+    public static void changeNick(String oldName, String newName) {
+        try (Connection connection = DriverManager.getConnection(URL)) {
+            PreparedStatement preparedStatement = connection.prepareStatement("UPDATE users SET name = ? WHERE name = ?;");
+            preparedStatement.setString(1, newName);
+            preparedStatement.setString(2, oldName);
+            preparedStatement.executeUpdate();
+            System.out.println("Name " + oldName + " changed to: " + newName);
+        } catch (SQLException e) {
+            e.printStackTrace();
+        }
+    }
+}

src/logging_testing_6/gui/App.java

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+package logging_testing_6.gui;
+
+import javax.swing.*;
+
+public class App {
+
+    private static MainWindow mainWindow;
+
+    public static void main(String[] args) {
+        SwingUtilities.invokeLater(new Runnable() {
+            @Override
+            public void run() {
+                mainWindow = new MainWindow();
+            }
+        });
+    }
+}

src/logging_testing_6/gui/AuthException.java

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+package logging_testing_6.gui;
+
+public class AuthException extends RuntimeException {
+}