Sending protobuf as JSON in spring-boot

Question

I´m using protobufs with this concrete definition.

message Hash {
    string category = 1;
    repeated KVPair content = 2;
}

message KVPair {
    string key = 1;
    string value = 2;
}

I want to send this as JSON with my spring-boot application. I added this package to my gradle dependencies:

compile group: 'com.google.protobuf', name: 'protobuf-java', version: '3.6.1'

When i try to output Hash generated object with this code:

@RestController
@RequestMapping("/api/crm/")
public class KVController {

    private final KVService kvService;

    public KVController(KVService kvService) {
        this.kvService = kvService;
    }

    @GetMapping("kv/{category}")
    public Hash getHash(@PathVariable String category) {
        Hash hash = kvService.retrieve(category);
        return hash;
    }
}

It throws this ultimate exception:

Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Direct self-reference leading to cycle (through reference chain: com.blaazha.crm.proto.Hash["unknownFields"]->com.google.protobuf.UnknownFieldSet["defaultInstanceForType"]) at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.BeanPropertyWriter._handleSelfReference(BeanPropertyWriter.java:944) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:721) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1396) ~[jackson-databind-2.9.6.jar:2.9.6] at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:913) ~[jackson-databind-2.9.6.jar:2.9.6] at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:286) ~[spring-web-4.3.18.RELEASE.jar:4.3.18.RELEASE] ... 58 common frames omitted

kvService only returns data from redis. It parses Hash data type (https://redis.io/topics/data-types) to Hash object defined in proto. Where Hash->category is main key of hash and values in hash redis datatype are converted to KVPair defined in proto. I cannot show all source code, because it calls other systems and source code is very long.

kvService returns valid Hash object, but exception happens when I return this Hash object and spring tries convert it to JSON.

important dependencies in my build.gradle:

def versions = [
        logback: '1.2.3',
        owner: '1.0.10',
        jackson: '2.9.6',

        guava: '25.1-jre',
        guice: '4.2.0',
        grpc: '1.9.1',
        protoc: '3.5.1',

        redis: '2.9.0',
]

dependencies {

compile group: 'ch.qos.logback', name: 'logback-classic', version: versions.logback
compile group: 'org.aeonbits.owner', name: 'owner', version: versions.owner

compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: versions.jackson
compile group: 'com.fasterxml.jackson.core', name: 'jackson-annotations', version: versions.jackson
compile group: 'com.fasterxml.jackson.dataformat', name: 'jackson-dataformat-yaml', version: versions.jackson

compile group: 'com.google.guava', name: 'guava', version: versions.guava
compile group: 'com.google.inject', name: 'guice', version: versions.guice
compile group: 'io.grpc', name: 'grpc-netty', version: versions.grpc
compile group: 'io.grpc', name: 'grpc-protobuf', version: versions.grpc
compile group: 'io.grpc', name: 'grpc-stub', version: versions.grpc
compile 'org.glassfish:javax.annotation:10.0-b28'


compile group: 'javax.xml.bind', name: 'jaxb-api', version: '2.2.1'
compile group: 'javax.activation', name: 'activation', version: '1.1.1'

compile group: 'redis.clients', name: 'jedis', version: versions.redis

}

As you can see in my protobuf definition isn´t any self-referencing.

Is there any possible way to fix this problem ?

Answer 1

If you're using Spring WebFlux and trying to produces application/json here is what you can do to make it works for all mappings returning protobuf Message type:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

@Override
public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
    configurer.defaultCodecs().jackson2JsonEncoder(
        new Jackson2JsonEncoder(Jackson2ObjectMapperBuilder.json().serializerByType(
                Message.class, new JsonSerializer<Message>() {
                    @Override
                    public void serialize(Message value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
                        String str = JsonFormat.printer().omittingInsignificantWhitespace().print(value);
                        gen.writeRawValue(str);
                    }
                }
        ).build())
    );
}

Answer 2

Class UnknownFieldSet (reached via generated method Hash.getUnknownFields()) contains getter getDefaultInstanceForType() which returns singleton instance of UnknownFieldSet. This singleton instance references itself in getDefaultInstanceForType() and Jackson-databind can't handle this automatically (see edit2 below).

You might want to use JsonFormat from com.google.protobuf:protobuf-java-util which uses canonical encoding instead of Jackson.

Good luck!

EDIT> For Spring there is ProtobufJsonFormatHttpMessageConverter

EDIT2> Of course you could handle this situation using Mix-in Annotations, but IMHO JsonFormat is definitely the way to go...