To execute a query against a schema, build a new
GraphQL object with the appropriate arguments and then
The result of a query is an
ExecutionResult which is the query data and/or a list of errors.
More complex query examples can be found in the StarWars query tests
Each graphql field type has a
graphql.schema.DataFetcher associated with it. Other graphql implementations often call this
type of code *resolvers**.
Often you can rely on
graphql.schema.PropertyDataFetcher to examine Java POJO objects to
provide field values from them. If your don’t specify a data fetcher on a field, this is what will be used.
However you will need to fetch your top level domain objects via your own custom data fetchers. This might involve making a database call or contacting another system over HTTP say.
graphql-java is not opinionated about how you get your domain data objects, that is very much your concern. It is also not
opinionated on user authorisation to that data. You should push all that logic into your business logic layer code.
A data fetcher might look like this:
DataFetcher is passed a
graphql.schema.DataFetchingEnvironment object which contains what field is being fetched, what
arguments have been supplied to the field and other information such as the field’s parent object, the query root object or the query
In the above example, the execution will wait for the data fetcher to return before moving on. You can make execution of
DataFetcher asynchronous by returning a
CompletionStage to data, that is explained more further down this page.
If an exception happens during the data fetcher call, then the execution strategy by default will make a
graphql.ExceptionWhileDataFetching error and add it to the list of errors on the result. Remember graphql allows
partial results with errors.
Here is the code for the standard behaviour.
If the exception you throw is itself a
GraphqlError then it will transfer the message and custom extensions attributes from that exception
ExceptionWhileDataFetching object. This allows you to place your own custom attributes into the graphql error that is sent back
to the caller.
For example imagine your data fetcher threw this exception. The
fizz attributes would be included in the resultant
You can change this behaviour by creating your own
graphql.execution.DataFetcherExceptionHandler exception handling code and
giving that to the execution strategy.
For example the code above records the underlying exception and stack trace. Some people may prefer not to see that in the output error list. So you can use this mechanism to change that behaviour.
It is also possible to return both data and multiple errors in a
DataFetcher implementation by returning
graphql.execution.DataFetcherResult either directly or wrapped in a
CompletableFuture instance for asynchronous
execution. This is a useful when your
DataFetcher may need to retrieve data from multiple sources or from another
In this example, the
DataFetcher retrieves a user from another GraphQL resource and returns its data and errors.
The most common way to call graphql is over HTTP and to expect a JSON response back. So you need to turn an
graphql.ExecutionResult into a JSON payload.
A common way to do that is use a JSON serialisation library like Jackson or GSON. However exactly how they interpret the
data result is particular to them. For example
nulls are important in graphql results and hence you must set up the json mappers
to include them.
To ensure you get a JSON result that conforms 100% to the graphql spec, you should call
toSpecification on the result and then
send that back as JSON.
This will ensure that the result follows the specification outlined in http://facebook.github.io/graphql/#sec-Response
A good starting point to learn more about mutating data in graphql is http://graphql.org/learn/queries/#mutations
In essence you need to define a
GraphQLObjectType that takes arguments as input. Those arguments are what you can use to mutate your data store
via the data fetcher invoked.
The mutation is invoked via a query like :
You need to send in arguments during that mutation operation, in this case for the variables for
You would create types like this to handle this mutation :
Notice that the input arguments are of type
GraphQLInputObjectType. This is important. Input arguments can ONLY be of that type
and you cannot use output types such as
GraphQLObjectType. Scalars types are consider both input and output types.
The data fetcher here is responsible for executing the mutation and returning some sensible output values.
Notice how it calls a data store to mutate the backing database and then returns a
Review object that can be used as the output values
to the caller.
graphql-java uses fully asynchronous execution techniques when it executes queries. You can get the
CompleteableFuture to results by calling
executeAsync() like this
The use of
CompletableFuture allows you to compose actions and functions that will be applied when the execution completes. The final
.join() waits for the execution to happen.
In fact under the covers, the graphql-java engine uses asynchronous execution and makes the
.execute() method appear synchronous by
calling join for you. So the following code is in fact the same.
graphql.schema.DataFetcher returns a
CompletableFuture<T> object then this will be composed into the overall asynchronous
query execution. This means you can fire off a number of field fetching requests in parallel. Exactly what
threading strategy you use is up to your data fetcher code.
The following code uses the standard Java
java.util.concurrent.ForkJoinPool.commonPool() thread executor to supply values in another
The code above is written in long form. With Java 8 lambdas it can be written more succinctly as follows
The graphql-java engine ensures that all the
CompletableFuture objects are composed together to provide an execution result
that follows the graphql specification.
There is a helpful shortcut in graphql-java to create asynchronous data fetchers.
graphql.schema.AsyncDataFetcher.async(DataFetcher<T>) to wrap a
DataFetcher. This can be used with static imports to produce more readable code.
A class derived from
graphql.execution.ExecutionStrategy is used to run a query or mutation. A number of different
strategies are provided with graphql-java and if you are really keen you can even write your own.
You can wire in what execution strategy to use when you create the
In fact the code above is equivalent to the default settings and is a very sensible choice of execution strategies for most cases.
By default the “query” execution strategy is
graphql.execution.AsyncExecutionStrategy which will dispatch
each field as
CompleteableFuture objects and not care which ones complete first. This strategy allows for the most
The data fetchers invoked can themselves return
CompletionStage values and this will create
fully asynchronous behaviour.
So imagine a query as follows
AsyncExecutionStrategy is free to dispatch the enemies field at the same time as the friends field. It does not
have to do enemies first followed by friends, which would be less efficient.
It will however assemble the results in order. The query result will follow the graphql specification and return object values assembled in query field order. Only the execution of data fetching is free to be in any order.
This behaviour is allowed in the graphql specification and in fact is actively encouraged http://facebook.github.io/graphql/#sec-Query for read only queries.
specification <http://facebook.github.io/graphql/#sec-Normal-evaluation>_ for details.
The graphql specification says that mutations MUST be executed serially and in the order in which the query fields occur.
graphql.execution.AsyncSerialExecutionStrategy is used by default for mutations and will ensure that each
field is completed before it processes the next one and so forth. You can still return
in the mutation data fetchers, however they will be executed serially and will be completed before the next
mutation field data fetcher is dispatched.
Graphql subscriptions allows you to create stateful subscriptions to graphql data. You uses
as your execution strategy as it has the support for the reactive-streams APIs.
See http://www.reactive-streams.org/ for more information on the reactive
Also see the page on subscriptions for more details on how to write a subscription based graphql service.
graphql-java engine executes a query it must be parsed and validated, and this process can be somewhat time consuming.
To avoid the need for re-parse/validate the
GraphQL.Builder allows an instance of
PreparsedDocumentProvider to reuse
Please note that this does not cache the result of the query, only the parsed
Caffeine <https://github.com/ben-manes/caffeine>_ used as it is a high quality caching solution. The cache instance should be thread safe and shared.
PreparsedDocumentProvideris a functional interface with only a get method and we can therefore pass a method reference that matches the signature into the builder.
In order to achieve high cache hit ration it is recommended that field arguments are passed in as variables instead of directly in the query.
The following query:
Should be rewritten as:
The query is now reused regardless of variable values provided.