Getting syntax error on Mono.onErrorResume() while using a HttpGraphQLClient

Question

I am using java 17, spring-boot: 3.0.5 and maven in my project. I am calling a GraphQL endpoint using HttpGraphQlClient. Below is my expected code snippet:

Mono<Entity> entity = this.graphQlClient.documentName("getDetails").variables(Map.of("xx", "xx"))
                        .retrieve("data")
                        .toEntity(Entity.class)
                        .onErrorResume(FieldAccessException.class, ex -> {
                        ClientGraphQlResponse response = ex.getResponse();
            // ...
            ClientResponseField field = ex.getField();
            // ...
                        });

However, I am getting syntax error at onErrorResume(....). Below is the error I am getting:

Cannot resolve method 'onErrorResume(Class, )'

Below are the suggestions I get:

Candidates for method call this.graphQlClient.documentName("getDetails").variables(Map.of("xx", "xx")) .retrieve("data") .toEntity(Entity.class).onErrorResume(FieldAccessException.class, ex -> { }) are:

• Mono onErrorResume(Function<? super Throwable, ? extends Mono<? extends TransactionEntity>>)
• Mono onErrorResume(Class, Function<? super FieldAccessException, ? extends Mono<? extends TransactionEntity>>)
• Mono onErrorResume(Predicate<? super Throwable>, Function<? super Throwable, ? extends Mono<? extends TransactionEntity>>)

Spring documention has the same code block given as example: Spring-graphql-retrieve-method

I would like to know how to fix this error? How to handle error using retrieve method ?

I want to handle error thrown from graphQL endpoint in an effective manner. Eventhough, we can use .execute() method to have more control on the GraphQL response, I wanted to use .retrieve() since it is provided in the library.

Answer 1

The onErrorResume operator means that you will handle errors of a certain type and provide a fallback (see the Javadoc here).

In your case you should return a fallback value of type Entity.

Mono<Entity> entity = this.graphQlClient.documentName("getDetails")
        .variables(Map.of("xx", "xx"))
        .retrieve("data")
        .toEntity(Entity.class)
        .onErrorResume(FieldAccessException.class, ex -> {
            return Mono.just(new Entity("defaultValue"));
        });

You are getting a compiler error because the compiler expects onErrorResume to return a type Mono<Entity>.