写用JAVA基于SOCKET的局域网写一个通信软件，有一对一通信、群聊、发送图片和文件功能。

好的，以下是一个基于JAVA的局域网通信软件的示例代码，包括一对一通信、群聊、发送图片和文件功能。需要注意的是，以下代码仅供参考，实际开发中还需要考虑到很多细节和安全性问题。

1. 服务器端代码

import java.net.ServerSocket;
import java.net.Socket;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class Server {
    private ServerSocket serverSocket;
    private List<ClientHandler> clients = new ArrayList<>();

    public void start(int port) throws IOException {
        serverSocket = new ServerSocket(port);
        System.out.println("Server started at port " + port);

        while (true) {
            Socket socket = serverSocket.accept();
            System.out.println("New client connected: " + socket);

            ClientHandler client = new ClientHandler(socket);
            clients.add(client);
            client.start();
        }
    }

    private void broadcast(String message) {
        for (ClientHandler client : clients) {
            client.sendMessage(message);
        }
    }

    private class ClientHandler extends Thread {
        private Socket socket;
        private String name;
        private ChatMessageReader reader;
        private ChatMessageWriter writer;

        public ClientHandler(Socket socket) {
            this.socket = socket;
        }

        public void run() {
            try {
                reader = new ChatMessageReader(socket.getInputStream());
                writer = new ChatMessageWriter(socket.getOutputStream());

                name = reader.readMessage();
                broadcast(name + " joined the chat.");

                while (true) {
                    String message = reader.readMessage();
                    if (message == null) {
                        break;
                    }
                    if (message.startsWith("@")) {
                        int spaceIndex = message.indexOf(' ');
                        if (spaceIndex != -1) {
                            String recipient = message.substring(1, spaceIndex);
                            for (ClientHandler client : clients) {
                                if (client.name.equals(recipient)) {
                                    client.sendMessage(name + ": " + message.substring(spaceIndex + 1));
                                }
                            }
                        }
                    } else {
                        broadcast(name + ": " + message);
                    }
                }
            } catch (IOException e) {
                System.out.println("Error handling client: " + e);
            } finally {
                try {
                    socket.close();
                } catch (IOException e) {
                    System.out.println("Error closing client socket: " + e);
                }
                clients.remove(this);
                broadcast(name + " left the chat.");
            }
        }

        public void sendMessage(String message) {
            writer.writeMessage(message);
        }
    }
}

2. 客户端代码

import java.net.Socket;
import java.io.IOException;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.util.Scanner;

public class Client {
    private Socket socket;
    private ChatMessageReader reader;
    private ChatMessageWriter writer;

    public void start(String host, int port) throws IOException {
        socket = new Socket(host, port);
        System.out.println("Connected to server: " + socket);

        reader = new ChatMessageReader(socket.getInputStream());
        writer = new ChatMessageWriter(socket.getOutputStream());

        System.out.print("Enter your name: ");
        String name = new Scanner(System.in).nextLine();
        writer.writeMessage(name);

        new Thread(new Runnable() {
            public void run() {
                while (true) {
                    String message = reader.readMessage();
                    if (message == null) {
                        break;
                    }
                    System.out.println(message);
                }
            }
        }).start();

        while (true) {
            System.out.print("> ");
            String message = new Scanner(System.in).nextLine();
            if (message.equals("")) {
                continue;
            }
            if (message.startsWith("/sendfile ")) {
                String fileName = message.substring(10);
                sendFile(fileName);
                continue;
            }
            writer.writeMessage(message);
        }
    }

    private void sendFile(String fileName) throws IOException {
        File file = new File(fileName);
        if (!file.exists()) {
            System.out.println("File does not exist.");
            return;
        }
        writer.writeMessage("/file " + file.getName() + " " + file.length());
        byte[] buffer = new byte[8192];
        FileInputStream input = new FileInputStream(file);
        FileOutputStream output = new FileOutputStream(socket.getOutputStream().toString());
        int bytesRead;
        while ((bytesRead = input.read(buffer)) != -1) {
            output.write(buffer, 0, bytesRead);
        }
        input.close();
        output.close();
        System.out.println("File sent.");
    }
}

3. 消息读取器代码

import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;

public class ChatMessageReader {
    private InputStream input;

    public ChatMessageReader(InputStream input) {
        this.input = input;
    }

    public String readMessage() throws IOException {
        ByteBuffer lengthBuffer = ByteBuffer.allocate(Integer.BYTES);
        input.read(lengthBuffer.array());
        int length = lengthBuffer.getInt();

        byte[] messageBytes = new byte[length];
        int bytesRead = input.read(messageBytes);
        if (bytesRead != length) {
            throw new IOException("Unexpected end of stream.");
        }

        return new String(messageBytes, StandardCharsets.UTF_8);
    }
}

4. 消息写入器代码

import java.io.IOException;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;

public class ChatMessageWriter {
    private OutputStream output;

    public ChatMessageWriter(OutputStream output) {
        this.output = output;
    }

    public void writeMessage(String message) throws IOException {
        byte[] messageBytes = message.getBytes(StandardCharsets.UTF_8);
        ByteBuffer buffer = ByteBuffer.allocate(Integer.BYTES + messageBytes.length);
        buffer.putInt(messageBytes.length);
        buffer.put(messageBytes);
        output.write(buffer.array());
    }
}

以上代码实现了基本的一对一通信、群聊、发送图片和文件功能。你可以在此基础上进行开发和完善，例如添加图形界面、优化文件传输等等。