Java基础教程（240）使用Filter之修改响应：掌控响应流，Java Filter中动态修改响应体的终极指南

在Java Web开发中，Filter是拦截请求和响应的关键组件。虽然修改请求相对常见，但动态修改响应内容却是许多开发者面临的挑战。本文将深入分析Filter中修改响应体的技术原理，并提供实用示例。

一、Filter与响应修改机制

Filter通过包装HttpServletResponse对象实现响应修改。核心在于自定义ResponseWrapper，覆盖getWriter()和getOutputStream()方法，捕获所有写入操作。

public class CustomResponseWrapper extends HttpServletResponseWrapper {
    private CharArrayWriter output;

    public CustomResponseWrapper(HttpServletResponse response) {
        super(response);
        output = new CharArrayWriter();
    }

    @Override
    public PrintWriter getWriter() {
        return new PrintWriter(output);
    }

    public String getContent() {
        return output.toString();
    }
}

二、完整示例：Gzip压缩过滤器

以下示例演示如何实现响应内容的Gzip压缩：

@WebFilter("/*")
public class GzipFilter implements Filter {
    
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, 
                       FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;
        
        if (supportsGzip(req)) {
            GzipResponseWrapper wrappedResponse = new GzipResponseWrapper(resp);
            chain.doFilter(request, wrappedResponse);
            wrappedResponse.finish(); // 完成压缩并写入响应
        } else {
            chain.doFilter(request, response);
        }
    }
    
    private boolean supportsGzip(HttpServletRequest request) {
        String encoding = request.getHeader("Accept-Encoding");
        return encoding != null && encoding.contains("gzip");
    }
}

class GzipResponseWrapper extends HttpServletResponseWrapper {
    private GzipServletOutputStream gzipStream;
    private PrintWriter writer;
    
    public GzipResponseWrapper(HttpServletResponse response) throws IOException {
        super(response);
        gzipStream = new GzipServletOutputStream(response.getOutputStream());
    }
    
    @Override
    public ServletOutputStream getOutputStream() throws IOException {
        return gzipStream;
    }
    
    @Override
    public PrintWriter getWriter() throws IOException {
        if (writer == null) {
            writer = new PrintWriter(new OutputStreamWriter(gzipStream, 
                         getResponse().getCharacterEncoding()));
        }
        return writer;
    }
    
    public void finish() throws IOException {
        if (writer != null) {
            writer.close();
        }
        gzipStream.finish();
    }
}