<form action="${pageContext.request.contextPath}/Servlet1" method="post">
        <input type="hidden" name="username" value="<h1>你好i好<h1>">
        <input type="submit" value="提交">
    </form>

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String value = request.getParameter("username");
        response.getWriter().write(value);
    }

class MyResponse extends HttpServletResponseWrapper {
    private ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
    private PrintWriter printWriter ;
    private HttpServletResponse response;
    public MyResponse(HttpServletResponse response) {
        super(response);
        this.response = response;
    }
    @Override
    public ServletOutputStream getOutputStream() throws IOException {
        //这个的ServletOutputSteam对象调用write()方法的时候，把数据是写在byteArrayOutputSteam上的
        return new MyServletOutputStream(byteArrayOutputStream);
    }
    @Override
    public PrintWriter getWriter() throws IOException {
        printWriter = new PrintWriter(new OutputStreamWriter(byteArrayOutputStream, this.response.getCharacterEncoding()));
        return printWriter;
    }
    public byte[] getBuffer() {
        try {
            //防止数据在缓存中，要刷新一下！
            if (printWriter != null) {
                printWriter.close();
            }
            if (byteArrayOutputStream != null) {
                byteArrayOutputStream.flush();
                return byteArrayOutputStream.toByteArray();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}
//增强ServletOutputSteam，让writer方法不把数据直接返回给浏览器
class MyServletOutputStream extends ServletOutputStream {
    private ByteArrayOutputStream byteArrayOutputStream;
    public MyServletOutputStream(ByteArrayOutputStream byteArrayOutputStream) {
        this.byteArrayOutputStream = byteArrayOutputStream;
    }
    //当调用write()方法的时候，其实是把数据写byteArrayOutputSteam上
    @Override
    public void write(int b) throws IOException {
        this.byteArrayOutputStream.write(b);
    }
}

  public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        //定义一个Map集合，key为页面的地址，value为内存的缓存
        Map<String, byte[]> map = new HashMap<>();
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        //得到客户端想要请求的资源
        String uri = request.getRequestURI();
        byte[] bytes = map.get(uri);
        //如果有缓存，直接返回给浏览器就行了，就不用执行目标资源了
        if (bytes != null) {
            response.getOutputStream().write(bytes);
            return ;
        }
        //如果没有缓存，就让目标执行
        MyResponse myResponse = new MyResponse(response);
        chain.doFilter(request, myResponse);
        //得到目标资源想要发送给浏览器的数据
        byte[] b = myResponse.getBuffer();
        //把数据存到集合中
        map.put(uri, b);
        //把数据返回给浏览器
        response.getOutputStream().write(b);
    }