在Java中搭建一個簡單的聊天室，你可以使用套接字編程（Socket Programming）和多線程。以下是一個基本的聊天室實現步驟：

1. 創建服務器端（Server）代碼：

import java.io.*;
import java.net.*;

public class ChatServer {
    private int port;
    private ServerSocket serverSocket;
    private Socket socket;

    public ChatServer(int port) throws IOException {
        this.port = port;
        serverSocket = new ServerSocket(port);
    }

    public void start() throws IOException {
        System.out.println("服務器啟動，等待客戶端連接...");
        socket = serverSocket.accept();
        System.out.println("客戶端已連接：" + socket.getInetAddress());

        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

        String message;
        while ((message = in.readLine()) != null) {
            System.out.println("收到消息：" + message);
            broadcast(message, socket);
        }
    }

    private void broadcast(String message, Socket sender) {
        for (Socket client : clients) {
            if (client != sender) {
                try {
                    PrintWriter out = new PrintWriter(client.getOutputStream(), true);
                    out.println(message);
                } catch (IOException e) {
                    e.printStackTrace();
                    client.close();
                    removeClient(client);
                }
            }
        }
    }

    private void removeClient(Socket client) {
        clients.remove(client);
    }

    public static void main(String[] args) {
        int port = 12345;
        ChatServer server;
        try {
            server = new ChatServer(port);
            server.start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2. 創建客戶端（Client）代碼：

import java.io.*;
import java.net.*;

public class ChatClient {
    private String serverAddress;
    private int port;
    private Socket socket;
    private BufferedReader in;
    private PrintWriter out;

    public ChatClient(String serverAddress, int port) throws IOException {
        this.serverAddress = serverAddress;
        this.port = port;
        socket = new Socket(serverAddress, port);

        in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        out = new PrintWriter(socket.getOutputStream(), true);
    }

    public void sendMessage(String message) {
        out.println(message);
    }

    public void receiveMessage() {
        try {
            String message = in.readLine();
            System.out.println("收到消息：" + message);
        } catch (IOException e) {
            e.printStackTrace();
            removeClient();
        }
    }

    public void removeClient() {
        try {
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        String serverAddress = "localhost";
        int port = 12345;
        ChatClient client;
        try {
            client = new ChatClient(serverAddress, port);
            BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
            String message;
            while ((message = in.readLine()) != null) {
                client.sendMessage(message);
                client.receiveMessage();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3. 運行服務器端和客戶端代碼：

首先運行ChatServer類，然后運行ChatClient類。現在你可以嘗試在客戶端發送消息，服務器端會收到并廣播給所有連接的客戶端。

注意：這個示例僅適用于一個簡單的聊天室，沒有實現用戶身份驗證、私聊功能等高級功能。你可以根據自己的需求對代碼進行修改和擴展。