How can I get the HTTP status code out of a ServletResponse in a ServletFilter?

I'm trying to report on every HTTP status code returned from my webapp. However the status code does not appear to be accessible via the ServletResponse, or even if I cast it to a HttpServletResponse. Is there a way to get access to this value within a ServletFilter?

First, you need to save the status code in an accessible place. The best to wrap the response with your implementation and keep it there:

public class StatusExposingServletResponse extends HttpServletResponseWrapper {

	private int httpStatus;
	
	public StatusExposingServletResponse(HttpServletResponse response) {
		super(response);
	}

	@Override
	public void sendError(int sc) throws IOException {
		httpStatus = sc;
		super.sendError(sc);
	}

	@Override
	public void sendError(int sc, String msg) throws IOException {
	    httpStatus = sc;
	    super.sendError(sc, msg);
	}


	@Override
	public void setStatus(int sc) {
		httpStatus = sc;
		super.setStatus(sc);
	}

	public int getStatus() {
		return httpStatus;
	}

}

In order to use this wrapper, you need to add a servlet filter, were you can do your reporting:

public class StatusReportingFilter implements Filter {

	public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
		StatusExposingServletResponse response = new StatusExposingServletResponse((HttpServletResponse)res);
		chain.doFilter(req, response);
		int status = response.getStatus();
		// report
	}

	public void init(FilterConfig config) throws ServletException {
		//empty
	}

	public void destroy() {
		// empty
	}

}

Since Servlet 3.0, there's a HttpServletResponse#getStatus().

So, if there's room for upgrading, upgrade to Servlet 3.0 (Tomcat 7, Glassfish 3, JBoss AS 6, etc) and you don't need a wrapper.

chain.doFilter(request, response);
int status = ((HttpServletResponse) response).getStatus();

Also need to include a wrapper for #sendRedirect, and it would be better to initialize status to '200' rather than '0'

private int httpStatus = SC_OK;

...

@Override
public void sendRedirect(String location) throws IOException {
    httpStatus = SC_MOVED_TEMPORARILY;
    super.sendRedirect(location);
}

One thing missing from David's answer above is that you should also override the other form of sendError:

@Override
public void sendError(int sc, String msg) throws IOException {
	httpStatus = sc;
	super.sendError(sc, msg);
}

In addition to David's answer, you'll also want to override the reset method:

@Override
public void reset() {
    super.reset();
    this.httpStatus = SC_OK;
}

... as well as the deprecated setStatus(int, String)

@Override
public void setStatus(int status, String string) {
    super.setStatus(status, string);
    this.httpStatus = status;
}

Write an HttpServletResponseWrapper and override all the setStatus(), sendError(), and sendRedirect() methods to log everything. Write a Filter that swaps your wrapper in for the response object on every request.

If you are stuck with an older container then a alternate solution to David Rabinowitz that uses the actual status code (in case it changes after it is set using the wrapper) is:

public class StatusExposingServletResponse extends HttpServletResponseWrapper {

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

    @Override
    public void sendError(int sc) throws IOException {
        super.sendError(sc);
    }

    @Override
    public void sendError(int sc, String msg) throws IOException {
        super.sendError(sc, msg);
    }

    @Override
    public void setStatus(int sc) {
        super.setStatus(sc);
    }

    public int getStatus() {
        try {
            ServletResponse object = super.getResponse();

            // call the private method 'getResponse'
            Method method1 = object.getClass().getMethod("getResponse");
            Object servletResponse = method1.invoke(object, new Object[] {});

            // call the parents private method 'getResponse'
            Method method2 = servletResponse.getClass().getMethod("getResponse");
            Object parentResponse = method2.invoke(servletResponse, new Object[] {});

            // call the parents private method 'getResponse'
            Method method3 = parentResponse.getClass().getMethod("getStatus");
            int httpStatus = (Integer) method3.invoke(parentResponse, new Object[] {});

            return httpStatus;
        }
        catch (Exception e) {
            e.printStackTrace();
            return HttpServletResponse.SC_ACCEPTED;
        }
    }

    public String getMessage() {
        try {
            ServletResponse object = super.getResponse();

            // call the private method 'getResponse'
            Method method1 = object.getClass().getMethod("getResponse");
            Object servletResponse = method1.invoke(object, new Object[] {});

            // call the parents private method 'getResponse'
            Method method2 = servletResponse.getClass().getMethod("getResponse");
            Object parentResponse = method2.invoke(servletResponse, new Object[] {});

            // call the parents private method 'getResponse'
            Method method3 = parentResponse.getClass().getMethod("getReason");
            String httpStatusMessage = (String) method3.invoke(parentResponse, new Object[] {});

            if (httpStatusMessage == null) {
                int status = getStatus();
                java.lang.reflect.Field[] fields = HttpServletResponse.class.getFields();

                for (java.lang.reflect.Field field : fields) {
                    if (status == field.getInt(servletResponse)) {
                        httpStatusMessage = field.getName();
                        httpStatusMessage = httpStatusMessage.replace("SC_", "");
                        if (!"OK".equals(httpStatusMessage)) {
                            httpStatusMessage = httpStatusMessage.toLowerCase();
                            httpStatusMessage = httpStatusMessage.replace("_", " ");
                            httpStatusMessage = capitalizeFirstLetters(httpStatusMessage);
                        }
                        
                        break;
                    }
                }
            }

            return httpStatusMessage;
        }
        catch (Exception e) {
            e.printStackTrace();
            return "";
        }
    }

    private static String capitalizeFirstLetters(String s) {

        for (int i = 0; i < s.length(); i++) {
            if (i == 0) {
                // Capitalize the first letter of the string.
                s = String.format("%s%s", Character.toUpperCase(s.charAt(0)), s.substring(1));
            }

            if (!Character.isLetterOrDigit(s.charAt(i))) {
                if (i + 1 < s.length()) {
                    s = String.format("%s%s%s", s.subSequence(0, i + 1), 
                            Character.toUpperCase(s.charAt(i + 1)), 
                            s.substring(i + 2));
                }
            }
        }

        return s;

    }

    @Override
    public String toString() {
        return this.getMessage() + " " + this.getStatus();
    }

}

Warning: lots of assumptions of the class hierarchy when using sneaky reflection and introspection to get to private data values.