Capture generated dynamic content at server side

If the request is idempotent (such as GET requests are), then just use java.net.URL to get an InputStream of the JSP output. E.g.

InputStream input = new URL("http://localhost/context/page.jsp").openStream();

If the request is not idempotent (such as POST requests are), then you need to create a Filter which wraps the ServletResponse with a custom implementation of the PrintWriter with the five write() methods been overridden wherein you copy output into some buffer/builder which you store in the session or a temporary folder at local disk file system so that it can be accessed afterwards in the subsequent requests. E.g.

package mypackage;

import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.Writer;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

public class CopyResponseFilter implements Filter {

    public void init(FilterConfig config) throws ServletException {
        // NOOP.
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) 
        throws IOException, ServletException
    {
        // Set character encoding for better world domination.
        response.setCharacterEncoding("UTF-8");

        // Create copy writer.
        HttpServletResponse httpResponse = (HttpServletResponse) response;
        CopyWriter copyWriter = new CopyWriter(new OutputStreamWriter(
            httpResponse.getOutputStream(), httpResponse.getCharacterEncoding()));

        // Filter request with response which is wrapped with new writer.
        chain.doFilter(request, wrapResponse(httpResponse, copyWriter));

        // Store the copy writer afterwards in session so that it's available in next request.
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        httpRequest.getSession().setAttribute("copyWriter", copyWriter);
    }

    public void destroy() {
        // NOOP.
    }

    private static HttpServletResponse wrapResponse
        (final HttpServletResponse response, final PrintWriter writer)
    {
        return new HttpServletResponseWrapper(response) {
            public PrintWriter getWriter() throws IOException {
                return writer;
            }
        };
    }

}

class CopyWriter extends PrintWriter {

    StringBuilder copy = new StringBuilder();

    public CopyWriter(Writer out) {
        super(out);
    }

    public void write(int c) {
        copy.append((char) c); // It is actually a char, not an int.
        super.write(c);
        super.flush();
    }

    public void write(char[] chars) {
        copy.append(chars);
        super.write(chars);
        super.flush();
    }

    public void write(char[] chars, int offset, int length) {
        copy.append(chars, offset, length);
        super.write(chars, offset, length);
        super.flush();
    }

    public void write(String string) {
        copy.append(string);
        super.write(string);
        super.flush();
    }

    public void write(String string, int offset, int length) {
        copy.append(string, offset, length);
        super.write(string, offset, length);
        super.flush();
    }

    public String getCopy() {
        return copy.toString();
    }

}

You can access the final output in any servlet of the subsequent request (note that you cannot access it in any servlet of the current request, because it’s already too late to do something with it) by just accessing the CopyWriter in the session:

CopyWriter copyWriter = (CopyWriter) request.getSession().getAttribute("copyWriter");
String outputOfPreviousRequest = copyWriter.getCopy();

Note that you should map this filter on an url-pattern covering the JSP pages of interest and thus not on /* or so, otherwise it would run on static files (css, js, images, etc) which are included in the same JSP as well.

Also note that multiple requests inside the same session would override each other, it’s up to you to distinguish between those requests by using a proper url-pattern or another way of storing it in session, e.g. in flavor of a Map<URL, CopyWriter> or so.

Hope this helps.