寻找使用servlet筛选器将内容插入响应的示例

Question

我在net和stackoverflow中搜索了一个使用servlet过滤器将内容插入响应的示例，但只能找到捕获/压缩输出和/或更改头部的示例。我的目标是在所有HTML响应的结束之前追加一个HTML块。

我正在研究一个解决方案，它扩展HttpServletResponseWrapper以使用我自己的PrintWriter，然后重写它上的write方法。在write方法中，我存储了最后7个字符，以查看它是否等于结束体标记，然后在继续对文档其余部分进行正常的写操作之前，写入HTML块和结束体标记。

作为对评论的回应，我还试图从http://www.oracle.com/technetwork/java/filters-137243.html实现CharResponseWrapper。下面是我的代码：

PrintWriter out = response.getWriter();
CharResponseWrapper wrappedResponse = new CharResponseWrapper(
        (HttpServletResponse)response);

chain.doFilter(wrappedRequest, wrappedResponse);
String s = wrappedResponse.toString();

if (wrappedResponse.getContentType().equals("text/html") &&
        StringUtils.isNotBlank(s)) {
    CharArrayWriter caw = new CharArrayWriter();
    caw.write(s.substring(0, s.indexOf("</body>") - 1));
    caw.write("WTF</body></html>");
    response.setContentLength(caw.toString().length());
    out.write(caw.toString());
}
else {
    out.write(wrappedResponse.toString());
}

out.close();

我也在包装请求，但代码可以工作，不应该影响响应。

Answer 1

我使用的代码库在处理响应时调用getOutputStream方法，而不是getWriter，因此其他答案中包含的示例没有帮助。下面是一个更完整的答案，它同时适用于OutputStream和PrintWriter，如果writer被访问两次，即使错误也是正确的。这是从使用javax.servlet.filter的转储请求和响应这个很棒的示例派生出来的。

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class MyFilter implements Filter
{
    private FilterConfig filterConfig = null;

    private static class ByteArrayServletStream extends ServletOutputStream
    {
        ByteArrayOutputStream baos;

        ByteArrayServletStream(ByteArrayOutputStream baos)
        {
            this.baos = baos;
        }

        public void write(int param) throws IOException
        {
            baos.write(param);
        }
    }

    private static class ByteArrayPrintWriter
    {

        private ByteArrayOutputStream baos = new ByteArrayOutputStream();

        private PrintWriter pw = new PrintWriter(baos);

        private ServletOutputStream sos = new ByteArrayServletStream(baos);

        public PrintWriter getWriter()
        {
            return pw;
        }

        public ServletOutputStream getStream()
        {
            return sos;
        }

        byte[] toByteArray()
        {
            return baos.toByteArray();
        }
    }

    public class CharResponseWrapper extends HttpServletResponseWrapper
    {
        private ByteArrayPrintWriter output;
        private boolean usingWriter;

        public CharResponseWrapper(HttpServletResponse response)
        {
            super(response);
            usingWriter = false;
            output = new ByteArrayPrintWriter();
        }

        public byte[] getByteArray()
        {
            return output.toByteArray();
        }

        @Override
        public ServletOutputStream getOutputStream() throws IOException
        {
            // will error out, if in use
            if (usingWriter) {
                super.getOutputStream();
            }
            usingWriter = true;
            return output.getStream();
        }

        @Override
        public PrintWriter getWriter() throws IOException
        {
            // will error out, if in use
            if (usingWriter) {
                super.getWriter();
            }
            usingWriter = true;
            return output.getWriter();
        }

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

    public void init(FilterConfig filterConfig) throws ServletException
    {
        this.filterConfig = filterConfig;
    }

    public void destroy()
    {
        filterConfig = null;
    }

    public void doFilter(
            ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException
    {
        CharResponseWrapper wrappedResponse = new CharResponseWrapper(
                (HttpServletResponse)response);

        chain.doFilter(request, wrappedResponse);
        byte[] bytes = wrappedResponse.getByteArray();

        if (wrappedResponse.getContentType().contains("text/html")) {
            String out = new String(bytes);
            // DO YOUR REPLACEMENTS HERE
            out = out.replace("</head>", "WTF</head>");
            response.getOutputStream().write(out.getBytes());
        }
        else {
            response.getOutputStream().write(bytes);
        }
    }
}