Why Tomcat returns different headers for HEAD and GET requests to my RESTful API?

Question

My initial purpose was to verify the HTTP chunked transfer. But accidentally found this inconsistency.

The API is designed to return a file to client. I use HEAD and GET methods against it. Different headers are returned.

For GET, I get these headers: (This is what I expected.)

For HEAD, I get these headers:

According to this thread, HEAD and GET SHOULD return identical headers but not necessarily.

My question is:

If Transfer-Encoding: chunked is used because the file is dynamically fed to the client and Tomcat server cannot know its size beforehand, how could Tomcat know the Content-Length when HEAD method is used? Does Tomcat just dry-run the handler and count all the file bytes? Why doesn't it simply return the same Transfer-Encoding: chunked header?

Below is my RESTful API implemented with Spring Web MVC:

@RestController
public class ChunkedTransferAPI {

    @Autowired
    ServletContext servletContext;

    @RequestMapping(value = "bootfile.efi", method = { RequestMethod.GET, RequestMethod.HEAD })
    public void doHttpBoot(HttpServletResponse response) {

        String filename = "/bootfile.efi";
        try {
            ServletOutputStream output = response.getOutputStream();
            InputStream input = servletContext.getResourceAsStream(filename);
            BufferedInputStream bufferedInput = new BufferedInputStream(input);
            int datum = bufferedInput.read();
            while (datum != -1) {
                output.write(datum);
                datum = bufferedInput.read();
            }
            output.flush();
            output.close();

        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

ADD 1

In my code, I didn't explicitly add any headers, then it must be Tomcat that add the Content-Length and Transfer-Encoding headers as it sees fit.

So, what are the rules for Tomcat to decide which headers to send?

ADD 2

Maybe it's related to how Tomcat works. I hope someone can shed some light here. Otherwise, I will debug into the source of Tomcat 8 and share the result. But that may take a while.

Related:

• HTTP HEAD and GET different result

• Content-Length header with HEAD requests?

Answer 1

Does Tomcat just dry-run the handler and count all the file bytes?

Yes, the default implementation of javax.servlet.http.HttpServlet.doHead() does that.

You can look at helper classes NoBodyResponse, NoBodyOutputStream in HttpServlet.java

The DefaultServlet class (the Tomcat servlet that is used to serve static files) is more wise. It is capable of sending the correct Content-Length value, as well as serving GET requests for a subset of the file (the Range header). You can forward your request to that servlet, with

  ServletContext.getNamedDispatcher("default").forward(request, response);