How to write a resolver for a GraphQL subscription using graphql-java and graphql-java-servlet?

Question

I try to setup a Spring Boot based GraphQL service (based on project graphql-spring-boot-starter , https://github.com/graphql-java-kickstart/graphql-spring-boot ), which does not use graphql-java-tools , but builds the RuntimeWiring manually/programmatically. GraphQL queries and mutations work just fine that way. However, I have problems to make subscriptions work.

I've added the following wiring to bind the DataFetcher for the subscription:

RuntimeWiring.Builder runtimeWiringBuilder =
    newRuntimeWiring()
    ...
        .type(TypeRuntimeWiring.newTypeWiring("Subscription")
            .dataFetcher("stockQuotes", gqlService::stockQuotes)
        );

And my resolver function returns correctly an org.reactivestream.Publisher

Publisher<StockPriceUpdate> stockQuotes(DataFetchingEnvironment env) {
    return stockTickerPublisher.getPublisher();
}

However, if I call my subscription from GraphiQL or GraphQL Playground, I get the following error: "No serializer found for class graphql.execution.reactive.CompletionStageMappingPublisher" (stack trace below).

It looks like that GraphQLObjectMapper from lib graphql-java-servlet tries to serialize the result, which is a (wrapped) org.reactivestream.Publisher , directly as a JSON result. That does not make sense, of course. Instead it should subscribe as listener to the publisher and push updates via the WebSocket whenever the Publisher emits the next update.

As far as I understood, graphql-java-servlet already has built-in support for subscriptions via WebSockets. Unfortunately there is no plain-vanilla example available, which does subscriptions without using graphql-tools .

Any ideas how I can get this working? Thanks a lot!

The relevant parts of my application.yml file are those:

server:
  port: 9200

graphql:
  servlet:
    mapping: /graphql
    enabled: true
    corsEnabled: true
    websocket:
      enabled: true
      path: /graphql
    subscriptions:
      websocket:
        path: /graphql

The schema (excerpt):

schema {
  query: Query
  mutation: Mutation
  subscription : Subscription
}

type Subscription {
  stockQuotes: StockPriceUpdate!
}

type StockPriceUpdate {
  dateTime : String
  stockCode : String
  stockPrice : Float
  stockPriceChange : Float
}

The stacktrace:

java.lang.RuntimeException: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class graphql.execution.reactive.CompletionStageMappingPublisher and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.LinkedHashMap["data"])
    at graphql.servlet.GraphQLObjectMapper.serializeResultAsJson(GraphQLObjectMapper.java:95) ~[graphql-java-servlet-7.1.1.jar:na]
    at graphql.servlet.AbstractGraphQLHttpServlet.query(AbstractGraphQLHttpServlet.java:339) ~[graphql-java-servlet-7.1.1.jar:na]
    at graphql.servlet.AbstractGraphQLHttpServlet.lambda$init$4(AbstractGraphQLHttpServlet.java:209) ~[graphql-java-servlet-7.1.1.jar:na]
    at graphql.servlet.AbstractGraphQLHttpServlet.doRequest(AbstractGraphQLHttpServlet.java:306) ~[graphql-java-servlet-7.1.1.jar:na]
    at graphql.servlet.AbstractGraphQLHttpServlet.doRequestAsync(AbstractGraphQLHttpServlet.java:297) ~[graphql-java-servlet-7.1.1.jar:na]
    at graphql.servlet.AbstractGraphQLHttpServlet.doPost(AbstractGraphQLHttpServlet.java:327) ~[graphql-java-servlet-7.1.1.jar:na]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:660) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.CorsFilter.doFilterInternal(CorsFilter.java:96) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.boot.actuate.web.trace.servlet.HttpTraceFilter.doFilterInternal(HttpTraceFilter.java:90) ~[spring-boot-actuator-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.filterAndRecordMetrics(WebMvcMetricsFilter.java:117) ~[spring-boot-actuator-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:106) ~[spring-boot-actuator-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:834) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1417) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[na:1.8.0_192]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[na:1.8.0_192]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at java.lang.Thread.run(Thread.java:748) ~[na:1.8.0_192]
Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class graphql.execution.reactive.CompletionStageMappingPublisher and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.LinkedHashMap["data"])
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:313) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:71) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:33) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:718) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:639) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:33) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3905) ~[jackson-databind-2.9.7.jar:2.9.7]
    at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3219) ~[jackson-databind-2.9.7.jar:2.9.7]
    at graphql.servlet.GraphQLObjectMapper.serializeResultAsJson(GraphQLObjectMapper.java:93) ~[graphql-java-servlet-7.1.1.jar:na]
    ... 57 common frames omitted

Answer 1

Above stack trace comes when you try to use electron based GraphiQL.app which was not updated since Mar 22, 2018 (v.0.7.2). So, to fix an issue and see subscription working:

1. Don't use GraphiQL.app ;

2. Add following dependency to your pom.xml to expose resent /graphiql endpoint from your spring boot micro service (for testing):

        <!-- Graph(i)QL tool: for development -->
        <dependency>
            <groupId>com.graphql-java-kickstart</groupId>
            <artifactId>graphiql-spring-boot-starter</artifactId>
            <version>5.4.1</version>
        </dependency>

3. graphiql by default expects that subscriptions are exposed over /subscriptions endpoint. So:

   • either change graphql.subscriptions.websocket.path from /graphql to /subscriptions ;

   • or customize graphiql in your application.yml (to reflect graphql configuration):

graphiql:
  endpoint:
    subscriptions: /graphql

After restart you can go to http://localhost:9200/graphiql and run subscription, as:

subscription StockCodeSubscription {
    stockQuotes {
        dateTime
        stockCode
        stockPrice
        stockPriceChange
    }
}

Should work!