Support application/x-protobuf errors

[rwinch]

Spring 4.1 added support for protobuf. However, if an error occurs when the Accept header is set to "application/x-protobuf", then the response that is sent is a HTTP 406 Not Acceptable. For example, if a user requests a resource that requires authentication and no credentials are provided, then instead of an HTTP 401, the following response is sent:

HTTP/1.1 406 Not Acceptable
Server: Apache-Coyote/1.1
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0
X-Frame-Options: DENY
WWW-Authenticate: Basic realm="Realm"
Content-Length: 0
Date: Wed, 15 Apr 2015 13:43:21 GMT

instead the error should look something like:

HTTP/1.1 401 Unauthorized
Server: Apache-Coyote/1.1
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0
X-Frame-Options: DENY
WWW-Authenticate: Basic realm="Realm"
X-Protobuf-Schema: customer.proto
X-Protobuf-Message: demo.Error
Content-Type: application/x-protobuf;charset=UTF-8
Content-Length: 160
Date: Wed, 15 Apr 2015 14:23:33 GMT

...

This happens because BasicErrorController is creating an ResponseEntity<Map<String, Object>>. When ProtobufHttpMessageConverter attempts to write the body, it cannot because ProtobufHttpMessageConverter can only write protobuf Message objects (see ProtobufHttpMessageConverter supports method).

Spring Boot should provide a mechanism to ensure that the proper error is delivered to the client. A workaround for users is to create something like this:

package demo;

option java_package = "demo";
option java_outer_classname = "Data";

message MapFieldEntry {
    required string key = 1;
    required string value = 2;
}

message Error {
    repeated MapFieldEntry errors = 1;
}

Generate the respective Java classes. Then create a controller:

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package demo;

import java.util.Map;

import javax.servlet.http.HttpServletRequest;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.web.ErrorAttributes;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.context.request.RequestAttributes;
import org.springframework.web.context.request.ServletRequestAttributes;

import demo.Data.Error.Builder;
import demo.Data.MapFieldEntry;

/**
 * @author Rob Winch
 */
@Controller
public class ErrorController {
    private final ErrorAttributes errorAttributes;

    @Autowired
    public ErrorController(ErrorAttributes errorAttributes) {
        this.errorAttributes = errorAttributes;
    }

    @RequestMapping(value = "/error", produces = "application/x-protobuf")
    @ResponseBody
    public ResponseEntity<Data.Error> error(HttpServletRequest request) {
        Map<String, Object> body = getErrorAttributes(request, getTraceParameter(request));

        Builder errorsBuilder = Data.Error.newBuilder();
        for(Map.Entry<String, Object> error : body.entrySet()) {
            demo.Data.MapFieldEntry.Builder entryBuilder = MapFieldEntry
                    .newBuilder()
                    .setKey(error.getKey())
                    .setValue(String.valueOf(error.getValue()));
            errorsBuilder.addErrors(entryBuilder.build());
        }
        Data.Error errors = errorsBuilder.build();
        HttpStatus status = getStatus(request);
        return new ResponseEntity<Data.Error>(errors, status);
    }

    private boolean getTraceParameter(HttpServletRequest request) {
        String parameter = request.getParameter("trace");
        if (parameter == null) {
            return false;
        }
        return !"false".equals(parameter.toLowerCase());
    }

    private Map<String, Object> getErrorAttributes(HttpServletRequest request,
            boolean includeStackTrace) {
        RequestAttributes requestAttributes = new ServletRequestAttributes(request);
        return this.errorAttributes.getErrorAttributes(requestAttributes,
                includeStackTrace);
    }

    private HttpStatus getStatus(HttpServletRequest request) {
        Integer statusCode = (Integer) request
                .getAttribute("javax.servlet.error.status_code");
        if (statusCode != null) {
            try {
                return HttpStatus.valueOf(statusCode);
            }
            catch (Exception ex) {
            }
        }
        return HttpStatus.INTERNAL_SERVER_ERROR;
    }
}

Related Links:

• StackOverflow: Basic auth must be preemptive with protobuf converter

[mwillema]

With the release of Protobuf 3 this should be easier since it supports map data types: https://developers.google.com/protocol-buffers/docs/proto3#maps

No need for a custom Protobuf message anymore (at least for this version)

[bclozel]

Protobuf 3 is supported by Spring Framework as of 5.0 - and we've got a similar issue there.