Need help with creating custom HttpServletResponse

Question

Suddenly stuck on generating custom servlet response. I want to replace servlet response with predefined one:

public class MyCustomResponse extends HttpServletResponseWrapper {
  private String customOutput;
  public MyCustomResponse(String customOutput, HttpServletResponse response) {
    super(response);
    // PrintWriter and Outputstream should stream this variable as output
    this.customOutput = customOutput;
  }

  //
  // Below I need to override something
  //
}

and filter code snipped as follows:

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
  //
  //
  MyCustomResponse customResponse = new MyCustomResponse("Hello world!", (HttpServletResponse) response);
  chain.doFilter(request, customResponse);
}

Shame on me, but i'm really stuck on coding this simple task :(

Any help would be appreciated.

UPDATE:

All I want is to implement custom response wrapper which, once it's put into filter chain, would always respond with some predefined text. I know how to write custom data from within doFilter() method, but I want MyCustomResponse to be responsible for that - just instantiate and put in chain. Any well-reasoned responses "You cant do that because..." are also welcome.

Answer 1

As quoted in one of your comments :

"I want my custom response to return a string in response to getWriter or getOutputStream method invocation"

For that, you have to provide your own implementation for getWriter() & getOutputStream() by overriding them.

//---

    private PrintWriter printWriter = null;
    private ServletOutputStream outputStream = null;

    public PrintWriter getWriter( ) throws IOException {

        if (this.outputStream != null) {
            throw new IllegalStateException(
                    "Cannot call getWriter( ) after getOutputStream( )");
        }

        if (this.printWriter == null) {
            // initialize printWriter
        }
        return this.printWriter;
    }

    public ServletOutputStream getOutputStream( ) throws IOException {

        if (this.printWriter != null) {
            throw new IllegalStateException(
                "Cannot call getOutputStream( ) after getWriter( )");
        }

        if (this.outputStream == null) {
            // initialize outputStream
        }
        return this.outputStream;
    }

//---

Answer 2

I am sorry, but

1. it is not clear what is your problem. You code is written, so? It does not work? what exactly does not work?
2. Why do you want to do this? The "right" solution is to pass information as session attribute.
3. I do not believe this can work. Really, you do not call directly the next filter in chain. You are kindly asking the app. server to do this. And you are not expected to replace the servlet request/response by your own. Use method explained above (#2)

Answer 3

Your response wrapper is useless as is, since it only stores a string in the Java object used to model the actual HTTP response.

The actual HTTP response that the client receives is the stream of bytes (resp. characters) sent via the output stream (resp. writer) of the HttpServletResponse object (and the headers, cookies, etc. stored in the HttpServletResponse object). If you want to send a custom output string to the client, just use response.getWriter().print("Hello worlds!").

Passing the response to the rest of the filter chain is questionable, since the rest of the chain will probably want to add its own data to the response stream.

If you want to hard-code the response to send to the client to your custom output, but be able to still pass the response to the chain and ignore whatever the rest of the chain puts in the response, you could try to add the following to your wrapper :

private ServletOutputStream fakeOutputStream = 
    new ServletOutputStream() {

        @Override
        public void write(int b) throws IOException {
            // do nothing. Everything written to this stream is ignored
        }
    }

private PrintWriter fakeWriter = new PrintWriter(fakeOutputStream);

public MyCustomResponse(String customOutput, HttpServletResponse response) {
    super(response);
    response.getWriter().print(customOutput);
}

@Override
public ServletOutputStream getOutputStream() {
    return fakeOutputStream;
}

@Override
public PrintWriter getWriter() {
    return fakeWriter;
}

Answer 4

I don't see the reason of what you want to do, but if you want to use your wrapper, my suggestion would be:

Create your own servlet that uses your wrapper and register it in web.xml, in something like this:

• Extend javax.servlet.GenericServlet and override the service(ServletRequest, ServletResponse) method. Then you use the Template Method pattern to create a service(ServletRequest, ServletResponseWrapper). OR
• Extend javax.servlet.HttpServlet and override service(HttpServletRequest, HttpServletResponse) method. Use the Template Method pattern to create a service(HttpServletRequest, HttpServletResponseWrapper). This will require that you don't use the doGet, doPost, doPut, doTrace methods already provided by HttpServlet but, instead create your own that uses your wrapper.

Hope this helps.