改成在Filter中实现

将代理功能实现在Filter中也是一种常见的做法。下面是一个示例代码，展示了如何在Filter中使用AsyncHttpClient和异步Servlet实现代理功能：

public class ProxyFilter implements Filter {
    private static final Logger LOGGER = LoggerFactory.getLogger(ProxyFilter.class);
    private static final String TARGET_URI_ATTRIBUTE = "targetUri";
    private static final String PROXY_HOST_ATTRIBUTE = "proxyHost";
    private static final String PROXY_PORT_ATTRIBUTE = "proxyPort";

    private AsyncHttpClient asyncHttpClient;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        asyncHttpClient = new DefaultAsyncHttpClient();
    }

    @Override
    public void destroy() {
        asyncHttpClient.close();
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        final HttpServletRequest request = (HttpServletRequest) servletRequest;
        final HttpServletResponse response = (HttpServletResponse) servletResponse;
        final AsyncContext asyncContext = request.startAsync();

        final String targetUri = request.getHeader(TARGET_URI_ATTRIBUTE);
        final String proxyHost = request.getHeader(PROXY_HOST_ATTRIBUTE);
        final String proxyPort = request.getHeader(PROXY_PORT_ATTRIBUTE);

        asyncHttpClient.prepareGet(targetUri)
                .setProxyServer(new ProxyServer.Builder(proxyHost, Integer.parseInt(proxyPort)).build())
                .execute(new AsyncCompletionHandler<Response>() {
                    @Override
                    public Response onCompleted(Response response) throws Exception {
                        InputStream inputStream = response.getResponseBodyAsStream();
                        OutputStream outputStream = asyncContext.getResponse().getOutputStream();
                        try {
                            IOUtils.copy(inputStream, outputStream);
                            asyncContext.complete();
                        } catch (IOException e) {
                            LOGGER.error("Failed to write response", e);
                        } finally {
                            IOUtils.closeQuietly(inputStream);
                            IOUtils.closeQuietly(outputStream);
                        }
                        return response;
                    }

                    @Override
                    public void onThrowable(Throwable t) {
                        LOGGER.error("Failed to execute request", t);
                        asyncContext.complete();
                    }
                });
    }
}

在这个示例中，我们将代理逻辑放在了Filter中。当有客户端请求到达时，Filter会获取目标服务器的地址和代理服务器的地址，并使用AsyncHttpClient发送HTTP请求。当目标服务器返回响应时，Filter会将响应写入异步响应中，并完成异步上下文。

值得注意的是，在这个示例中，我们使用了Apache Commons IO库中的IOUtils类来简化输入输出流的操作。这个库中的方法可以帮助我们更方便地处理流操作，避免了手动处理流时可能出现的错误。