实现逻辑就是转发请求和响应。 

核心代码

// 启动代理服务器private void startProxyServer() {new Thread(new ProxyServer()).start();}// 代理服务器static class ProxyServer implements Runnable {@Overridepublic void run() {try {// 监听指定的端口int port = 8098; //一般使用49152到65535之间的端口ServerSocket server = new ServerSocket(port);// 当一个ServerSocket关闭并释放其绑定的端口后，操作系统通常会在几分钟内不允许其他Socket再次绑定到该端口。// true:操作系统将允许其他Socket立即绑定到刚刚被释放的端口。server.setReuseAddress(true);// 使用线程池，防止过多线程耗尽资源ExecutorService threadPool = Executors.newFixedThreadPool(50);while (true) {Socket socket = server.accept(); //会一直阻塞，直到有客户端连接进来// new Thread 只是创建一个类的对象实例而已。而真正创建线程的是start()方法。// 这里并没有直接调用start()方法，所以并没创建新线程，而是交给线程池去执行。threadPool.submit(new ProxyClient(socket));}} catch (Exception e) {Log.e("ProxyServer", e.getMessage(), e);}}}// 代理客户端static class ProxyClient implements Runnable {private final Socket proxySocket;//代理Socketprivate Socket targetSocket = null;//目标Socketpublic ProxyClient(Socket socket) {this.proxySocket = socket;}@Overridepublic void run() {try {//客户端请求的报文InputStream req = proxySocket.getInputStream();int read;int contentLength = 0;//body长度String method = null;//请求方法String url = null;//请求地址String protocol = null;//请求协议ByteArrayOutputStream os = new ByteArrayOutputStream();ByteArrayOutputStream reqBack = new ByteArrayOutputStream();//解析，提取请求报文while ((read = req.read()) != -1) {os.write(read);reqBack.write(read);if (read == '\n') {//CONNECT www.xx.com:443/xx/yy HTTP/1.1String line = os.toString("UTF-8");os.reset();//重置，以便再次使用if ("\r\n".equals(line)) {//空行，请求头结束标志break;}StringTokenizer stringTokenizer = new StringTokenizer(line, " ");if (method == null) {//八种请求方法：GET、POST、HEAD、OPTIONS、PUT、PATCH、DELETE、TRACE、CONNECT 方法method = stringTokenizer.nextToken().toLowerCase();//connecturl = stringTokenizer.nextToken();//www.xx.com:443/xx/yyprotocol = stringTokenizer.nextToken().trim();//HTTP/1.1} else {String key = stringTokenizer.nextToken().toLowerCase();if ("content-length:".equals(key)) {String value = stringTokenizer.nextToken().trim();contentLength = Integer.parseInt(value);}}}}if (contentLength > 0) {for (int i = 0; i < contentLength; i++) {reqBack.write(req.read());}}//完整请求报文// String request = reqBack.toString("UTF-8");// System.out.println("请求报文开始");// System.out.print(request);// System.out.println("\r\n请求报文结束");//拼接完整urlif (url != null && !url.startsWith("http")) {url = method.equals("connect") ? "https://" + url : "http://" + url;}URL u = new URL(url);//目标ipString targetHost = u.getHost();//目标端口int targetPort = u.getPort();if (targetPort == -1) {targetPort = 80;}//目标SockettargetSocket = new Socket(targetHost, targetPort);if ("connect".equals(method)) {//https//HTTP/1.1 200 Connection established//报文直接发送给代理SocketOutputStream outputStream = proxySocket.getOutputStream();outputStream.write((protocol + " 200 Connection established\r\n").getBytes(StandardCharsets.UTF_8));outputStream.write("Proxy-agent: ProxyServer/1.0\r\n".getBytes(StandardCharsets.UTF_8));outputStream.write("\r\n".getBytes(StandardCharsets.UTF_8));outputStream.flush();//前者转发给后者，代理Socket转发给目标SocketThread proxy2target = new Thread(new ForwardData(proxySocket, targetSocket));proxy2target.start();//前者转发给后者，目标Socket转发给代理SocketThread target2proxy = new Thread(new ForwardData(targetSocket, proxySocket));target2proxy.start();proxy2target.join();} else {//http//请求报文转发给目标SocketOutputStream outputStream = targetSocket.getOutputStream();outputStream.write(reqBack.toByteArray());outputStream.flush();//前者转发给后者，目标Socket转发给代理SocketThread thread = new Thread(new ForwardData(targetSocket, proxySocket));thread.start();thread.join();}} catch (Exception e) {Log.e("ProxyClient", e.getMessage(), e);} finally {try {if (targetSocket != null) {targetSocket.close();}} catch (IOException e) {Log.e("ProxyClient", e.getMessage(), e);}try {if (proxySocket != null) {proxySocket.close();}} catch (IOException e) {Log.e("ProxyClient", e.getMessage(), e);}}// Log.e("ProxyClient", "结束");}// 转发数据static class ForwardData implements Runnable {private final Socket inputSocket;private final Socket outputSocket;public ForwardData(Socket inputSocket, Socket outputSocket) {this.inputSocket = inputSocket;this.outputSocket = outputSocket;}@Overridepublic void run() {try {InputStream inputStream = inputSocket.getInputStream();OutputStream outputStream = outputSocket.getOutputStream();int read;while ((read = inputStream.read()) != -1) {outputStream.write(read);}} catch (Exception e) {// Log.e("ForwardData", inputSocket + e.getMessage());}}}}

app源码

proxyserver: 代理服务器app

我已打包，打包地址：https://gitee.com/gloweds/proxyserver/raw/master/app/release/app-release.apk

有时会报错，但是这个错误不影响功能。

报错时间线如下：

2023-10-06 11:29:16.478 客户端请求结束（发起http请求）
2023-10-06 11:29:16.555 代理proxySocket的read()报错 Connection reset
2023-10-06 11:29:16.571 关闭两个Socket连接
2023-10-06 11:29:16.571 目标targetSocket的read()报错 Socket closed

上面报错的原因，是因为客户端请求发送报文没有完整发送结束标识-1，
如果客户端完整发送结束标识，上面的两个错误不会发生（Connection reset、Socket closed），但是这个错误不影响功能，可以不用处理。

代理proxySocket的read()报错 Connection reset，是因为客户端未完整发送结束标识-1，而客户端请求都结束了，也成功拿到了响应数据，这时关闭连接就导致代理proxySocket的read()报错 Connection reset。

目标targetSocket的read()报错 Socket closed，因为前面代理proxySocket的read()阻塞未正常发送结束标识，所以targetSocket的read()也阻塞了，关闭两个proxySocket和targetSocket连接后，targetSocket的read()自然报错Socket closed连接被强制关闭了。

https请求流程图：