Java实现轻量型HTTP代理服务器示例

在本攻略中，我们将使用Java编程语言演示如何实现一个轻量型的HTTP代理服务器。HTTP代理服务器是一种可以用于加速Web应用程序的常用中间件，其可以缓存常见的HTTP请求以减少Web服务器的负载。它也可以提供安全性功能，例如过滤内容和验证客户端请求。接下来就跟随本攻略一步步了解Java实现轻量型HTTP代理服务器的过程。

步骤1 - 创建一个Java工程

创建一个新的Java工程并添加所需的依赖（如果有的话）。例如：我们将使用Apache HttpClient实现。

步骤2 - 实现代理服务器

我们将使用Java Socket和ServerSocket类实现代理服务器。下面是实现代理服务器的示例代码：

public class ProxyServer {
    private static final int BUFFER_SIZE = 32768;
    public static void main(String[] args) throws IOException {
        int port = Integer.parseInt(args[0]);
        ServerSocket server = new ServerSocket(port);
        while (true) {
            Socket client = server.accept();
            ProxyThread thread = new ProxyThread(client);
            thread.start();
        }
    }
    static class ProxyThread extends Thread {
        private Socket socket;
        ProxyThread(Socket client) {
            this.socket = client;
        }
        public void run() {
            try {
                DataInputStream in = new DataInputStream(socket.getInputStream());
                byte[] buffer = new byte[BUFFER_SIZE];
                int length = in.read(buffer);
                String request = new String(buffer, 0, length);
                // TODO: 请求处理
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

步骤3 - 处理请求

请求处理是代理服务器的核心部分。当客户端通过代理服务器发送HTTP请求时，代理服务器会将请求转发到Web服务器。这里，我们使用Apache HttpClient实现。

下面是处理请求的示例代码：

private static void handleRequest(Socket socket) throws IOException {
    DataInputStream in = new DataInputStream(socket.getInputStream());
    byte[] buffer = new byte[BUFFER_SIZE];
    int length = in.read(buffer);
    String request = new String(buffer, 0, length);
    String[] lines = request.split("\r\n");
    String host = null;
    int port = 80;
    String path = null;
    for (String line: lines) {
        if (line.startsWith("GET ") || line.startsWith("POST ")) {
            path = line.split(" ")[1];
        } else if (line.startsWith("Host: ")) {
            host = line.substring(6);
            if (host.contains(":")) {
                String[] parts = host.split(":");
                host = parts[0];
                port = Integer.parseInt(parts[1]);
            }
        }
    }
    if (host == null || path == null) {
        return;
    }
    String newPath = "http://" + host + ":" + port + path;
    CloseableHttpClient httpClient = HttpClients.createDefault();
    HttpGet httpGet = new HttpGet(newPath);
    CloseableHttpResponse response = httpClient.execute(httpGet);
    DataOutputStream out = new DataOutputStream(socket.getOutputStream());
    out.write(getBytes(response));
    out.flush();
    out.close();
    socket.close();
}
private static byte[] getBytes(CloseableHttpResponse httpResponse) throws IOException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    httpResponse.getEntity().writeTo(baos);
    byte[] bytes = baos.toByteArray();
    baos.close();
    return bytes;
}

步骤4 - 测试代理服务器

您现在已经实现了代理服务器，下面是测试的方法：

1. 启动代理服务器。输入以下命令：java ProxyServer 8080
2. 在浏览器中配置代理服务器。在浏览器的设置中找到网络设置，将代理服务器的地址设置为“localhost”，端口号为“8080”。
3. 访问任何网站。当您访问任何网站时，代理服务器都会捕获请求并将其转发给Web服务器。

示例1 - 修改请求头

我们可以在处理请求时修改请求头。例如，如果您想向Web服务器传递一个不同的User-Agent值，您可以使用以下示例代码实现：

httpGet.setHeader("User-Agent", "My User Agent");

示例2 - 响应缓存

HTTP代理服务器可以缓存常见的HTTP响应，以减少Web服务器的负载。例如，下面的示例代码缓存所有GET请求的响应：

private static final ConcurrentHashMap<String, byte[]> cache = new ConcurrentHashMap<>();
private static final String CACHE_CONTROL_HEADER = "Cache-Control";

private static void handleGetRequest(Socket socket) throws IOException {
    DataInputStream in = new DataInputStream(socket.getInputStream());
    byte[] buffer = new byte[BUFFER_SIZE];
    int length = in.read(buffer);
    String request = new String(buffer, 0, length);
    String[] lines = request.split("\r\n");
    String host = null;
    int port = 80;
    String path = null;
    for (String line: lines) {
        if (line.startsWith("GET ")) {
            path = line.split(" ")[1];
        } else if (line.startsWith("Host: ")) {
            host = line.substring(6);
            if (host.contains(":")) {
                String[] parts = host.split(":");
                host = parts[0];
                port = Integer.parseInt(parts[1]);
            }
        }
    }
    if (host == null || path == null) {
        return;
    }
    String newPath = "http://" + host + ":" + port + path;
    if (cache.containsKey(newPath)) {
        byte[] cachedResponse = cache.get(newPath);
        DataOutputStream out = new DataOutputStream(socket.getOutputStream());
        out.write(cachedResponse);
        out.flush();
        out.close();
        socket.close();
        return;
    }
    CloseableHttpClient httpClient = HttpClients.createDefault();
    HttpGet httpGet = new HttpGet(newPath);
    CloseableHttpResponse response = httpClient.execute(httpGet);
    Header cacheControlHeader = response.getFirstHeader(CACHE_CONTROL_HEADER);
    if (cacheControlHeader != null && cacheControlHeader.getValue().contains("no-cache")) {
        return;
    }
    byte[] bytes = getBytes(response);
    cache.put(newPath, bytes);
    DataOutputStream out = new DataOutputStream(socket.getOutputStream());
    out.write(bytes);
    out.flush();
    out.close();
    socket.close();
}

以上就是实现Java轻量型HTTP代理服务器的完整攻略。我们通过Java Socket和ServerSocket类实现了代理服务器，并使用Apache HttpClient进行请求处理。我们还看了两个示例，其中一个示例演示如何修改请求头，另一个示例演示如何缓存GET请求的响应。