CharArrayWriterResponse.java

import java.io.CharArrayWriter;
import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

public class CharArrayWriterResponse extends HttpServletResponseWrapper {

  private final CharArrayWriter charArray = new CharArrayWriter();

  public CharArrayWriterResponse(HttpServletResponse response) {
    super(response);
  }

  @Override
  public PrintWriter getWriter() throws IOException {
    return new PrintWriter(charArray);
  }

  public String getOutput() {
    return charArray.toString();
  }
}

ServletUsingCustomResponse.java

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@SuppressWarnings("serial")
public class ServletUsingCustomResponse extends HttpServlet {

  @Override
  protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    String template = req.getParameter("tmpl");

    CharArrayWriterResponse customResponse = new CharArrayWriterResponse(resp);
    req.getRequestDispatcher(template).forward(req, customResponse);

    System.out.println(String.format("The output of %s is %s", template, customResponse.getOutput()));
  }
}