Suppress calls to sendError to prevent servlet containers from sending error pages to the client

UnhandledExceptionFilter.java

package servletutils;

import java.io.IOException;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.filter.OncePerRequestFilter;

import com.google.common.base.Preconditions;

/**
 * 1) Catches and logs unhandled exceptions
 * 2) flushes the response buffer to prevent any Servlet3-spec error-page redirects by the servlet container
 */
public class UnhandledExceptionHandlerFilter extends OncePerRequestFilter {

    public interface ExceptionHandler {
        void handle(HttpServletRequest request, HttpServletResponse response, Integer sc, Throwable throwable);
    }

    private static class Slf4jLoggingExceptionHandler implements ExceptionHandler {
        private final Logger logger = LoggerFactory.getLogger(UnhandledExceptionHandlerFilter.class);

        @Override
        public void handle(HttpServletRequest request, HttpServletResponse response, Integer sc, Throwable throwable) {
            if (throwable != null) {
                logger.error(String.format("Sending error response status %s for %s because of", sc, request.getRequestURI()),
                    throwable);
            } else {
                logger.warn(String.format("Sending error response status %s for %s", sc, request.getRequestURI()), new RuntimeException("stacktrace"));
            }
        }
    }

    private final ExceptionHandler exceptionHandler;

    public UnhandledExceptionHandlerFilter() {
        this(new Slf4jLoggingExceptionHandler());
    }

    public UnhandledExceptionHandlerFilter(ExceptionHandler exceptionHandler) {
        Preconditions.checkNotNull(exceptionHandler);
        this.exceptionHandler = exceptionHandler;
    }

    // suppress calls to sendError() and just setStatus() instead
    // do NOT use sendError() otherwise per servlet spec the container will send an html error page
    @SuppressWarnings({ "unused", "unchecked", "deprecation" })
    private class StatusCodeCaptureWrapper extends HttpServletResponseWrapper {

        private Integer statusCode;
        private HttpServletRequest request;
        private HttpServletResponse response;

        public StatusCodeCaptureWrapper(HttpServletRequest request, HttpServletResponse response) {
            super(response);
            this.request = request;
            this.response = response;
        }

        public Integer getStatusCode() {
            return statusCode;
        }

        @Override
        public void sendError(int sc) throws IOException {
            // do NOT use sendError() otherwise per servlet spec the container will send an html error page
            this.setStatus(sc);
            exceptionHandler.handle(request, response, sc, (Throwable) request.getAttribute("javax.servlet.error.exception"));
        }

        @Override
        public void sendError(int sc, String msg) throws IOException {
            // do NOT use sendError() otherwise per servlet spec the container will send an html error page
            this.setStatus(sc, msg);
            exceptionHandler.handle(request, response, sc, (Throwable) request.getAttribute("javax.servlet.error.exception"));
        }

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

        @Override
        public void setStatus(int sc, String sm) {
            this.statusCode = sc;
            super.setStatus(sc, sm);
        }
    }

    @Override
    public void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws IOException,
        ServletException {
        StatusCodeCaptureWrapper responseWrapper = new StatusCodeCaptureWrapper(request, response);
        Throwable exception = null;

        try {
            chain.doFilter(request, responseWrapper);
        } catch (ServletException e) {
            exception = e.getRootCause();
        } catch (Throwable e) { // NOSONAR this is an UnhandledExceptionHandler - we need to catch this
            exception = e;
        }

        if (exception != null && !"ClientAbortException".equals(exception.getClass().getSimpleName())) {
            ensureErrorStatusCodeSet(responseWrapper);
            response.setStatus(responseWrapper.getStatusCode());
            exceptionHandler.handle(request, response, responseWrapper.getStatusCode(), exception);
        }

        // flush to prevent servlet container to add anymore  headers or content
        response.flushBuffer();
    }

    private void ensureErrorStatusCodeSet(StatusCodeCaptureWrapper responseWrapper) {
        if (responseWrapper.getStatusCode() == null) {
            responseWrapper.setStatus(500);
        }
    }

    @Override
    public void destroy() {
        // noop
    }
}