代理服务器实现联网功能的java代码，可以用浏览器通过代理服务器上网，可通过dns服务器解析域名，可以识别http和url协议

以下是一个简单的代理服务器的Java代码示例，实现了通过代理服务器上网、DNS解析域名、识别HTTP和URL协议：

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

public class ProxyServer {
    private int port; // 代理服务器监听端口号

    public ProxyServer(int port) {
        this.port = port;
    }

    public void start() throws IOException {
        ServerSocket serverSocket = new ServerSocket(port);
        System.out.println("Proxy server started on port " + port);

        while (true) {
            Socket clientSocket = serverSocket.accept();
            System.out.println("New client connected from " + clientSocket.getInetAddress());

            // 为每个客户端连接创建一个新的线程处理请求
            Thread requestThread = new Thread(() -> {
                try {
                    // 从客户端读取请求
                    BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
                    String requestLine = in.readLine();
                    String[] requestParts = requestLine.split(" ");
                    String method = requestParts[0];
                    String url = requestParts[1];

                    // 解析目标主机和端口号
                    int port = 80; // 默认端口号为80
                    String host = null;
                    if (url.startsWith("http://")) {
                        host = url.substring(7);
                        int slashIndex = host.indexOf('/');
                        if (slashIndex != -1) {
                            host = host.substring(0, slashIndex);
                        }
                    } else {
                        host = url;
                    }
                    int colonIndex = host.indexOf(':');
                    if (colonIndex != -1) {
                        try {
                            port = Integer.parseInt(host.substring(colonIndex + 1));
                        } catch (NumberFormatException e) {
                            // 忽略端口号解析错误
                        }
                        host = host.substring(0, colonIndex);
                    }

                    // 解析HTTP头部，将其转发给目标服务器
                    String line;
                    StringBuilder headers = new StringBuilder();
                    while ((line = in.readLine()) != null && line.length() > 0) {
                        headers.append(line).append("\r\n");
                    }
                    headers.append("\r\n");

                    // 连接目标服务器
                    Socket serverSocket = new Socket(host, port);
                    OutputStream serverOut = serverSocket.getOutputStream();

                    // 将HTTP头部转发给目标服务器
                    serverOut.write(headers.toString().getBytes());

                    // 将客户端发送的请求体转发给目标服务器
                    if (method.equals("POST")) {
                        int contentLength = 0;
                        while ((line = in.readLine()) != null && line.length() > 0) {
                            if (line.startsWith("Content-Length:")) {
                                try {
                                    contentLength = Integer.parseInt(line.substring(15).trim());
                                } catch (NumberFormatException e) {
                                    // 忽略Content-Length解析错误
                                }
                            }
                            serverOut.write(line.getBytes());
                            serverOut.write("\r\n".getBytes());
                        }
                        serverOut.write("\r\n".getBytes());
                        if (contentLength > 0) {
                            char[] buffer = new char[contentLength];
                            in.read(buffer);
                            serverOut.write(new String(buffer).getBytes());
                        }
                    }

                    // 从目标服务器读取响应，并转发给客户端
                    InputStream serverIn = serverSocket.getInputStream();
                    OutputStream clientOut = clientSocket.getOutputStream();
                    byte[] buffer = new byte[1024];
                    int bytesRead;
                    while ((bytesRead = serverIn.read(buffer)) != -1) {
                        clientOut.write(buffer, 0, bytesRead);
                    }

                    // 关闭连接
                    serverSocket.close();
                    clientSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            });
            requestThread.start();
        }
    }

    public static void main(String[] args) throws IOException {
        ProxyServer proxyServer = new ProxyServer(8080);
        proxyServer.start();
    }
}

要使用这个代理服务器，只需在浏览器中设置HTTP代理为`localhost:8080`即可。注意，这个代理服务器只实现了最基本的功能，还有很多细节需要处理，例如HTTPS协议、缓存、安全认证等等。