Change ContentType or CharacterEncoding in Java Filter ONLY IF ContentType === JSON

Question

I'm trying to ensure that all JSON responses from a Jersey based java application have a UTF-8 character encoding parameter appended to their ContentType header.

So if it's a JSON response, I would like the response header for the Content-Type to be

Content-Type: application/json;charset=UTF-8

EDIT: I know I can do this on a case by case basis, but I'd like to do it globally, so it affects all content responses that have a content type of "application/json".

If I just try and set the character encoding in my filter regardless of the content type, it works fine. But I only want to set the character encoding if the ContentType is "application/json". I find that the response.getContentType() method always returns null unless I call chain.doFilter first. But if I try and change the Character Encoding after this, it seems to always get overwritten.

import java.io.IOException;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.ws.rs.core.MediaType;

public class EnsureJsonResponseIsUtf8Filter implements Filter
{
    private class SimpleWrapper extends HttpServletResponseWrapper
    {
        public SimpleWrapper(HttpServletResponse response)
        {
            super(response);
        }

        @Override
        public String getCharacterEncoding()
        {
            return "UTF-8";
        }
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException
    {
        chain.doFilter(request, response);

        if (response.getContentType() != null && response.getContentType().contains(MediaType.APPLICATION_JSON))
        {
            response.setCharacterEncoding("UTF-8");
            chain.doFilter(request, new SimpleWrapper((HttpServletResponse) response));
        }
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException
    {
    }

    @Override
    public void destroy()
    {
    }
}

I've seen other similar questions, but none them seem to have this issue. I've tried registering my filter as the first, and last filter with no luck.

Answer 1

This also may be doable using a ClientFilter, which I've just come across a StackOverflow post for a similar purpose:

https://stackoverflow.com/a/7464585/26510

Answer 2

This is not going to work in this way.

When you call chain.doFilter(request, response); your headers are already flushed and you can't reset them later on.

What you can do is actually a quick and dirty trick:

public void doFilter(...) {
    HttpServletResponse resp = new HttpServletResponseWrapper(response) {
    public void setContentType(String ct) {
        if(ct!=null && ct.toLowerCase().startsWith("application/json")) {
            super.setContentType("application/json;charset=UTF-8");
        } else {
            super.setContentType(ct);
        }
   }
}

// Set content type manually to override any potential defaults,
// See if you need it at all
response.setContentType("application/json;charset=UTF-8");

chain.doFilter(request, resp); // Inject our response!
}

EDIT: ct.toUpperCase().startsWith("application/json") changed to ct.toLowerCase().startsWith("application/json").

Answer 3

Using this answer as reference the solution to your question is to re-encode the JSON text as shown here:

public void doFilter(...) {
    final CharResponseWrapper wrappedResponse =
            new CharResponseWrapper((HttpServletResponse) response);

    chain.doFilter(request, wrappedResponse);

    final String content = wrappedResponse.toString();

    final String type = wrappedResponse.getContentType();
    if (type != null && type.contains(MediaType.APPLICATION_JSON)) {
        // Re-encode the JSON response as UTF-8.
        response.setCharacterEncoding("UTF-8");
        final OutputStream out = response.getOutputStream();
        out.write(content.getBytes("UTF-8"));
        out.close();
    }
    else {
        // Otherwise just write it as-is.
        final PrintWriter out = response.getWriter();
        out.write(content);
        out.close();
    }
}