How do I create a JSONP filter with /**/ prepended to the callback?

Question

I am upgrading a Spring Boot application to version 2.0 and Spring Framework to version 5.1.

The application currently uses Spring's built in JSONP support using AbstractJsonpResponseBodyAdvice.

@ControllerAdvice
public class JsonpControllerAdvice extends AbstractJsonpResponseBodyAdvice {

    public JsonpControllerAdvice() {
        super("jsonp");
    }
}

However, JSONP support was deprecated in version 5.0.7 and removed in version 5.1 RC1. In addition, it's not feasible to switch to CORS at this time.

A final caveat is that the JavaScript callback method must begin with /**/. For example (truncated):

/**/jQuery1720351297557893959_1567180700293(...)

I've tried using jsonp-filter but I am unable to configure the callback to include /**/.

How do I create a custom Spring Boot JSONP filter with /**/ prepended to the callback?

---

Note: My example is similar to Spring Boot: Remove /**/ before JSONP callback function name. But I can't remove the /**/ because the existing frontend code expects it in the callback.

Answer 1

While you can't use jsonp-filter, you can define a simple filter based on it. For example:

@Component
public class JsonPFilter implements Filter {
    @Override public void doFilter(ServletRequest request, ServletResponse response,
                                   FilterChain chain) throws IOException, ServletException {
        String callback = null;

        if (request instanceof HttpServletRequest) {
            HttpServletRequest httpServletRequest = (HttpServletRequest) request;
            callback = httpServletRequest.getParameter("jsonp");
        }

        if (callback != null) {
            OutputStream out = response.getOutputStream();
            out.write(String.format("/**/%s(", callback).getBytes());
            chain.doFilter(request, response);
            out.write(new JsonPResponseWrapper((HttpServletResponse) response).getData());
            out.write(")".getBytes());
            out.close();
        } else {
            chain.doFilter(request, response);
        }
    }

    private static class JsonPResponseWrapper extends HttpServletResponseWrapper {
        private JsonPResponseWrapper(HttpServletResponse response) {
            super(response);
        }

        private byte[] getData() {
            return new ByteArrayOutputStream().toByteArray();
        }
    }
}