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GraphqL Java
There is a newer version: 230521-nf-execution
package graphql.execution;


import graphql.GraphQLError;
import graphql.Internal;

/**
 * This will check that a value is non-null when the type definition says it must be and, it will throw {@link NonNullableFieldWasNullException}
 * if this is not the case.
 *
 * See: https://spec.graphql.org/October2021/#sec-Errors-and-Non-Nullability
 */
@Internal
public class NonNullableFieldValidator {

    private final ExecutionContext executionContext;
    private final ExecutionStepInfo executionStepInfo;

    public NonNullableFieldValidator(ExecutionContext executionContext, ExecutionStepInfo executionStepInfo) {
        this.executionContext = executionContext;
        this.executionStepInfo = executionStepInfo;
    }

    /**
     * Called to check that a value is non null if the type requires it to be non null
     *
     * @param parameters the execution strategy parameters
     * @param result the result to check
     * @param     the type of the result
     *
     * @return the result back
     *
     * @throws NonNullableFieldWasNullException if the value is null but the type requires it to be non null
     */
    public  T validate(ExecutionStrategyParameters parameters, T result) throws NonNullableFieldWasNullException {
        if (result == null) {
            if (executionStepInfo.isNonNullType()) {
                // see https://spec.graphql.org/October2021/#sec-Errors-and-Non-Nullability
                //
                //    > If the field returns null because of an error which has already been added to the "errors" list in the response,
                //    > the "errors" list must not be further affected. That is, only one error should be added to the errors list per field.
                //
                // We interpret this to cover the null field path only.  So here we use the variant of addError() that checks
                // for the current path already.
                //
                // Other places in the code base use the addError() that does not care about previous errors on that path being there.
                //
                // We will do this until the spec makes this more explicit.
                //
                final ResultPath path = parameters.getPath();

                NonNullableFieldWasNullException nonNullException = new NonNullableFieldWasNullException(executionStepInfo, path);
                final GraphQLError error = new NonNullableFieldWasNullError(nonNullException);
                if(parameters.getDeferredCallContext() != null) {
                    parameters.getDeferredCallContext().addError(error);
                } else {
                    executionContext.addError(error, path);
                }
                throw nonNullException;
            }
        }
        return result;
    }

}