A walk in GraphQL

Day 7 exercise - Java

Read the instructions on the Day 7 exercise definition

Java 1.8 is required. Please look at here if you do not have it installed on your local environment.

HINTS:

graphql-java provides the GraphQLError interface representing a GraphQL standard error that our exceptions will implement.
When we throw an exception while fetching data, this exception is handled by default by the SimpleDataFetcherExceptionHandler. This handler wraps our exception into an ExceptionWhileDataFetching error and adds this error to the list of errors of the query result. After this process, another handler (DefaultGraphQLErrorHandler), defined by the kickstart graphql-spring-boot library comes into action to handle the returned list of errors.
Apply the following “hint” git patch to get an example of defending and informative error handling strategies: “…/a-walk-in-graphql/lessons/day_07/java/src/main/resources/Java_day_07_Error_Handling.patch”.
- From command line: git apply src/main/resources/Java_day_07_Error_Handling.patch
- From IntelliJ IDEA:
  - Open the “VCS” menu > “Apply Patch..”
  - Select the “…/a-walk-in-graphql/lessons/day_07/java/src/main/resources/Java_day_07_Error_Handling.patch” file patch > OK

The GraphQL Tools library works by processing GraphQL Schema files to build the correct structure and then wires special beans to this structure.
The Spring Boot GraphQL starter automatically finds these schema files, we just need to save these files with the extension “.graphqls” on the classpath.
Query and Mutation objects are root GraphQL objects. They don’t have any associated “data” class. In such cases, the “resolver” classes would implement GraphQLQueryResolver or GraphQLMutationResolver.
Beans Representing Types: Every complex type in the GraphQL server is represented by a Java bean. Fields inside the Java bean will directly map onto fields in the GraphQL response based on the name of the field.
Sometimes, the value of a field is non-trivial to load. This might involve database lookups, complex calculations, or anything else. GraphQL Tools has a concept of a “Field Resolver” that is used for this purpose. The field resolver is any bean in the Spring Context that has the same name as the data bean, with the suffix “Resolver”, and that implements the GraphQLResolver interface. Methods on the field resolver bean follow all of the same rules as on the data bean but are also provided the data bean itself as a first parameter. If a field resolver and the data bean both have methods for the same GraphQL field then the field resolver will take precedence.