Sending response after chain.doFilter in filters

Question

I am new to Servlets. In the book i am reading now it is written, that we need wrappers, because it is late to do anything with response after finishing chain.doFilter() method as response is sent already.

I wrote the following Servlet and Filter:

public class MyServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{
        PrintWriter writer = response.getWriter();
        writer.println("In Servlet");
    }
}

public class MyFilter implements Filter{
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException{
        PrintWriter writer = response.getWriter();
        chain.doFilter(request, response);
        writer.println("After chain");
    }
}

And i see both strings in the browser. My question is: Why do we need wrappers? I still can write to response even after chain.doFilter and i still see result?

Is it because response is sent in two pieces(first in the end of chain.doFilter and second in the end of Filter.doFilter)? So if i had to compress response it would work incorrectly(because first uncompressed part would be sent)?

Answer 1

The book is talking about response headers.

You misunderstood it as response body.

Here are some real world use cases of response wrappers so you can see why we may need them:

• How to add response headers based on Content-type; getting Content-type before the response is committed
• How do delete a HTTP response header?
• How to read and copy the HTTP servlet response output stream content for logging
• How to insert JSF page rendering time and response size into the page itself, at least partially?
• How to configure Tomcat to not encode the session id into the URL when HttpServletResponse.encodeURL() is invoked

For more examples, see this search.