Question

After I make a GET request to the following servlet, the text ‘In service method’ is written and can not explain it. why is that?

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

public class MyServlet extends HttpServlet {
   public void service(HttpServletRequest req,
                       HttpServletResponse resp) throws IOException {
      Writer out = resp.getWriter();
      out.write("In Service method");
   }

   public void doGet(HttpServletRequest req, HttpServletResponse resp) 
                                                        throws IOException {
      Writer out = resp.getWriter();
      out.write("In get method");
   }
}

Answer 1

The default implementation of HttpServlet#service() does exactly that job. Here’s an extract of its javadoc:

Receives standard HTTP requests from the public service method and dispatches them to the doXXX methods defined in this class. This method is an HTTP-specific version of the Servlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse) method. There’s no need to override this method.

It delegates to the appropriate servlet method depending on the HTTP method. E.g. when request.getMethod() equals to "GET", then it delegates to doGet(). By the way, have you read the last sentence as well?

In any way, you should basically be performing the very same job yourself.

@Override
public void service(HttpServletRequest req, HttpServletResponse resp) throws IOException {
    Writer out = resp.getWriter();
    out.write("In Service method");

    if ("GET".equalsIgnoreCase(req.getMethod())) {
        doGet(req, resp);
    }
    // Check other methods as well!
}

Or just call the super method so that it can do its job.

@Override
public void service(HttpServletRequest req, HttpServletResponse resp) throws IOException {
    Writer out = resp.getWriter();
    out.write("In Service method");
    super.service(req, resp);
}

Or just don’t override it at all.